Contents

1 Constellation 11.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 Ancient near East . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.2 Ancient Egyptian star charts and astronomical ceilings . . . . . . . . . . . . . . . . . . . . 21.2.3 Hindu or Indian Constellation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.4 Greece-Roman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.5 Classical Chinese constellations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.6 Early Modern era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 IAU constellations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 Asterisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.5 Dark cloud constellations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.6 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.8 Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.8.1 Mythology, lore, history, and archaeoastronomy . . . . . . . . . . . . . . . . . . . . . . . 61.8.2 Atlases and celestial maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.8.3 Catalogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.9 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Galaxy 92.1 Etymology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3 Observation history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3.1 Milky Way . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3.2 Distinction from other nebulae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.3.3 Modern research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4 Types and morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.1 Ellipticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.2 Spirals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.3 Other morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4.4 Dwarfs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.5 Unusual dynamics and activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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ii CONTENTS

2.5.1 Interacting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.5.2 Starburst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.5.3 Active nucleus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.6 Formation and evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.6.1 Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.6.2 Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.6.3 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.7 Larger-scale structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.8 Multi-wavelength observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.9 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.10 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.11.1 Other references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.12 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.13 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3 Meteor shower 243.1 Historical developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.2 Radiant point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.3 Naming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.4 Origin of meteoroid streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.5 Dynamical evolution of meteoroid streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.6 Famous meteor showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.6.1 Perseid and Leonid meteor showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.6.2 Other meteor showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.7 Extraterrestrial meteor showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.8 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.10 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4 Nebula 314.1 Observational history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.2 Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.3 Types of nebulae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.3.1 Classical types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3.2 Diffuse nebulae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3.3 Planetary nebulae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.3.4 Supernova remnants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.4 Notable named nebulae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.4.1 Nebula catalogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.5 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

CONTENTS iii

4.7 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

5 Star 375.1 Observation history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385.2 Designations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.3 Units of measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.4 Formation and evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.4.1 Protostar formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.4.2 Main sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.4.3 Post–main sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5.5 Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.6 Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.6.1 Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445.6.2 Chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445.6.3 Diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445.6.4 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.6.5 Magnetic field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.6.6 Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465.6.7 Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465.6.8 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.7 Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.7.1 Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.7.2 Magnitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.8 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485.9 Variable stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485.10 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495.11 Nuclear fusion reaction pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505.12 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.14 Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565.15 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565.16 Text and image sources, contributors, and licenses . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.16.1 Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575.16.2 Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615.16.3 Content license . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Chapter 1

Constellation

This article is about the star grouping. For other uses, seeConstellation (disambiguation).In modern astronomy, a constellation is a specific area

The constellation Orion is one of the most recognizable in thenight sky.

of the celestial sphere as defined by the International As-tronomical Union (IAU). These areas had their originsin Western-traditional asterisms from which the constel-lations take their names. There are 88 officially recog-nized constellations, covering the entire sky. When as-tronomers say an object is “in” a given constellation, theymean it is within the boundaries of one of these definedareas of sky.

1.1 Terminology

The Late Latin term constellātiō can be translated as“set of stars”. The term was first used in astrology, ofasterisms that supposedly exerted influence, attested in

The constellation Orion as it can be seen by the naked eye (imageenhanced with lines and text).

Ammianus (4th century). In English the term was usedfrom the 14th century, also in astrology, of conjunctionsof planets. The modern astronomical sense of “area ofthe celestial sphere around a specific asterism” dates tothe mid-16th century.Colloquial usage does not draw a sharp distinction be-tween “constellation” in the sense of an asterism (patternof stars) and “constellation” in the sense of an area sur-rounding an asterism. The modern system of constella-tions used in astronomy employs the latter concept. Forexample, the asterism known as the Big Dipper comprisesthe seven brightest stars in the IAU constellation (area)Ursa Major.The term circumpolar constellation is used for any con-stellation that, from particular latitude on Earth, never

1

2 CHAPTER 1. CONSTELLATION

sets below the horizon. From the North Pole, all con-stellations north of the celestial equator are circumpo-lar constellations. In the northern latitudes, the informalterm equatorial constellation has sometimes been usedfor constellations that lie to the south of the circumpolarconstellations.[1] Depending on the definition, equatorialconstellations can include those that lie entirely betweendeclinations 45° north and 45° south,[2] or those that passoverhead between the tropics of Cancer and Capricorn.They generally include all constellations that intersect thecelestial equator.Usually the only thing the stars in a constellation have incommon is that they appear near each other in the skywhen viewed from the Earth. In space, the stars in a con-stellation can be very far away from each other, becausea bright star very far away is as visible to an observer onthe Earth as a dim star closer to the Earth. Constellationswhose stars are “near” each other include the five centralstars of the Big Dipper in the constellation of Ursa Major.Since stars travel on their own orbits through their galaxy,constellations change slowly and can even disappear. Thistakes tens of thousands of years.[3]

1.2 History

Further information: Former constellations and Historyof the constellations

The current list of 88 constellations recognized by theInternational Astronomical Union since 1922 is basedon the 48 listed by Ptolemy in his Almagest in the 2ndcentury.[4][5] Ptolemy’s catalogue is informed by Eudoxusof Cnidus, a Greek astronomer of the 4th century BCwhointroduced earlier Babylonian astronomy to the Hellenis-tic culture.

1.2.1 Ancient near East

See also: Babylonian star catalogues and MUL.APIN

The oldest catalogues of stars and constellations arefromOld Babylonian astronomy, beginning in theMiddleBronze Age. The numerous Sumerian names in these cat-alogues suggest that they build on older, but otherwiseunattested, Sumerian traditions of the Early Bronze Age.The classical Zodiac is a product of a revision of theOld Babylonian system in later Neo-Babylonian astron-omy 6th century BC. Knowledge of the Neo-Babylonianzodiac is also reflected in the Hebrew Bible. E. W.Bullinger interpreted the creatures appearing in the booksof Ezekiel (and thence in Revelation) as the middle signsof the four quarters of the Zodiac,[6][7] with the Lion asLeo, the Bull is Taurus, the Man representing Aquar-ius and the Eagle standing in for Scorpio.[8] The biblical

Book of Job also makes reference to a number of constel-lations, including ` Ayishעיש “bier”, כסיל Kĕciyl “fool”and כימה Kiymah “heap” (Job 9:9, 38:31-32), renderedas “Arcturus, Orion and Pleiades” by the KJV, but `Ayish“the bier” actually corresponding to Ursa Major.[9] Theterm Mazzaroth ,ַמזָּרוֹת a hapax legomenon in Job 38:32,may be the Hebrew word for the zodiacal constellations.The Greeks adopted the Babylonian system in the 4thcentury BC. A total of twenty Ptolemaic constellationsare directly continued from the Ancient Near East. An-other ten have the same stars but different names.[10]

Ancient Egyptian star chart and decanal clock on the ceiling fromthe tomb of Senenmut

1.2.2 Ancient Egyptian star charts and as-tronomical ceilings

See also: Egyptian_astronomy

In ancient Egypt, the observation of stars such as Siriusin the day and night sky were used from a very ancientperiod, in order to predict the Nile Flood.[11] This prac-tical observation of the stars was also associated with avery complex cosmology that involved various gods andspirits, some of whom were associated with stars andheavenly bodies, such as Sothis/Sopdet, who was likelyassociated with Sirius and Sah who was associated withOrion.[12][13] This cosmology and practice of astronomyeventually led to the Egyptians producing decanal clockson coffin lids[14] and star charts featuring their gods andstar observations on the ceilings of tombs and temples.Over time these became more complex, featuring vari-ous human and anthropomorphic figures representing the

1.2. HISTORY 3

planets, stars and various constellations.[14] This traditionwas later combined with Greek and Babylonian astro-nomical systems under the Ptolemies culminating in theZodiac of Dendera. The first circular zodiac showing allthe constellations we are familiar with, along with Egyp-tian Constellations, Decans and Planets.[15][16]

1.2.3 Hindu or Indian Constellation

Main article: Nakshatra

Nakshatra (Devanagari: nákṣatra) is the term for lunarmansion in Hindu astrology. A nakshatra is one of 27(sometimes also 28) sectors along the ecliptic. Theirnames are related to the most prominent asterisms in therespective sectors.The starting point for the nakshatras is the point on theecliptic directly opposite to the star Spica called Chitrā inSanskrit (other slightly different definitions exist). It iscalled Meshādi or the "start of Aries". The ecliptic is di-vided into each of the nakshatras eastwards starting fromthis point. The number of nakshatras reflects the num-ber of days in a sidereal month (modern value: 27.32days), the width of a nakshatra traversed by the Moonin about one day. Each nakshatra is further subdividedinto four quarters (or padas). These play a role in popu-lar Hindu astrology, where each pada is associated with asyllable, conventionally chosen as the first syllable of thegiven name of a child born when the Moon was in thecorresponding pada.The nakshatras of traditional bhartiya astronomy arebased on a list of 28 asterisms found in the Atharvaveda(AVŚ 19.7) and also in the Shatapatha Brahmana. Thefirst astronomical text that lists them is the Vedanga Jy-otisha.In classical Hindu mythology (Mahabharata, Harivamsa),the creation of the nakshatras is attributed to Daksha.They are personified as daughters of the deity and asmythological wives of Chandra, the Moon god, or al-ternatively the daughters of Kashyapa, the brother ofDaksha.[17]

Each of the nakshatras is governed as 'lord' by one of thenine graha in the following sequence: Ketu (South LunarNode), Shukra (Venus), Ravi or Surya (Sun), Chandra(Moon), Mangala (Mars), Rahu (North Lunar Node),Guru or Brihaspati (Jupiter), Shani (Saturn) and Budha(Mercury). This cycle repeats itself three times to coverall 27 nakshatras. The lord of each nakshatra determinesthe planetary period known as the dasha, which is con-sidered of major importance in forecasting the life pathof the individual in Hindu astrology.In Vedic Sanskrit, the term nákṣatra may refer to anyheavenly body, or to “the stars” collectively. The clas-sical sense of “lunar mansion” is first found in theAtharvaveda, and becomes the primary meaning of the

term in Classical Sanskrit.

1.2.4 Greece-Roman

There is only limited information on indigenous Greekconstellations. Some evidence is found in Hesiod. Greekastronomy essentially adopted the older Babylonian sys-tem in the Hellenistic era, first introduced to Greece byEudoxus of Cnidus in the 4th century BC. The originalwork of Eudoxus is lost, but it survives as a versifica-tion by Aratus, dating to the 3rd century BC. The mostcomplete existing works dealing with the mythical originsof the constellations are by the Hellenistic writer termedpseudo-Eratosthenes and an early Roman writer styledpseudo-Hyginus.The basis of western astronomy as taught during Late An-tiquity and until the Early Modern period is the Almagestby Ptolemy, written in the 2nd century.

1.2.5 Classical Chinese constellations

Main article: Chinese constellationFurther information: Treatise on Astrology of theKaiyuan Era

In classical Chinese astronomy, the northern sky is di-vided geometrically, into five “enclosures” and twenty-eight mansions along the ecliptic, grouped into Four Sym-bols of seven asterisms each. The 28 lunar mansionsare one of the most important and also the most an-cient structures in the Chinese sky, attested from the 5thcentury BC. Parallels to the earliest Babylonian (Sume-rian) star catalogues suggest that the ancient Chinese sys-tem did not arise independently from that of the AncientNear west and East.[18] Classical Chinese astronomy isrecorded in the Han period and appears in the form ofthree schools, which are attributed to astronomers of theZhanguo period. The constellations of the three schoolswere conflated into a single system by Chen Zhuo, an as-tronomer of the 3rd century (Three Kingdoms period).Chen Zhuo’s work has been lost, but information on hissystem of constellations survives in Tang period records,notably by Qutan Xida. The oldest extant Chinese starchart dates to the Tang period and was preserved aspart of the Dunhuang Manuscripts. Native Chinese as-tronomy flourished during the Song Dynasty, and dur-ing the Yuan Dynasty became increasingly influenced bymedieval Islamic astronomy.[19]

1.2.6 Early Modern era

The constellations around the South Pole were not ob-servable from north of the equator, by Babylonians,Greeks, Chinese or Arabs.

4 CHAPTER 1. CONSTELLATION

The modern constellations in this region were definedduring the age of exploration, notably by Dutch naviga-tors Pieter Dirkszoon Keyser and Frederick de Houtmanat the end of sixteenth century. They were depicted byJohann Bayer in his star atlas Uranometria of 1603.[20]Several more were created by Nicolas Louis de Lacaillein his star catalogue, published in 1756.[21]

Some modern proposals for new constellations were notsuccessful; an example is Quadrans, eponymous of theQuadrantid meteors, now divided between Boötes andDraco. The classical constellation of Argo Navis was bro-ken up into several different constellations, for the con-venience of stellar cartographers.By the end of the Ming Dynasty, Xu Guangqi introduced23 asterisms of the southern sky based on the knowledgeof western star charts.[22] These asterisms have since beenincorporated into the traditional Chinese star maps.

1.3 IAU constellations

0 h3 h6 h9 h12 h15 h18 h21 h

+90°

+60°

+30°

0°

-30°

-60°

-90°

Right ascension

Dec

linat

ion

ECLIPTIC

MILKY W

AY

Andromeda

Aquarius

Cassiopeia

Cepheus Cepheus

Phoenix

Pisces♓

Sculptor

Tucana Tucana

Andromeda

Antlia

Apus

Aquarius♒

Aquila

Ara

Aries♈

AurigaBoötes

Caelum

Camelopardalis

Cancer ♋

CanesVenatici

CanisMajor

CanisMinor

Capricornus♑

Carina

Cassiopeia

Centaurus

Cepheus Cepheus

Cetus

Chamaeleon

Circinus

Columba

ComaBerenices

CoronaAustrina

CoronaBorealis

CorvusCrater

Crux

Cygnus

Delphinus

Dorado

Draco

Equuleus

Eridanus

Fornax

Gemini ♊

Grus

Hercules

Horologium

Hydra

Hydrus

Indus

Lacerta

Leo ♌

LeoMinor

LepusLibra

♎

Lupus

Lynx

Lyra

Mensa

Microscopium

Monoceros

Musca

Norma

Octans

Ophiuchus

Orion

Pavo

Pegasus

Perseus

PhoenixPictor

Pisces♓

PiscisAustrinus

Puppis

Pyxis

Reticulum

Sagitta

Sagittarius♐ Scorpius

♏ Sculptor

Scutum

Serpens(Caput)

Serpens(Cauda) Sextans

Taurus

♉

Telescopium

Triangulum

TriangulumAustrale

Tucana Tucana

UrsaMajor

UrsaMinor

Vela

Virgo ♍

Volans

Vulpecula

Andromeda

Cassiopeia

Cepheus Cepheus

Phoenix

Pisces♓

Sculptor

Tucana Tucana

Antlia

Apus

Aquarius♒

Aquila

Ara

Aries♈

AurigaBoötes

Caelum

Camelopardalis

Cancer ♋

CanesVenatici

CanisMajor

CanisMinor

Capricornus♑

Carina

Centaurus

Cepheus Cepheus

Cetus

Chamaeleon

Circinus

Columba

ComaBerenices

CoronaAustrina

CoronaBorealis

CorvusCrater

Crux

Cygnus

Delphinus

Dorado

Draco

Equuleus

Eridanus

Fornax

Gemini ♊

Grus

Hercules

Horologium

Hydra

Hydrus

Indus

Lacerta

Leo ♌

LeoMinor

LepusLibra

♎

Lupus

Lynx

Lyra

Mensa

Microscopium

Monoceros

Musca

Norma

Octans

Ophiuchus

Orion

Pavo

Pegasus

Perseus

Pictor

PiscisAustrinus

Puppis

Pyxis

Reticulum

Sagitta

Sagittarius♐ Scorpius

♏ Sculptor

Scutum

Serpens(Caput)

Serpens(Cauda) Sextans

Taurus

♉

Telescopium

Triangulum

TriangulumAustrale

Tucana Tucana

UrsaMajor

UrsaMinor

Vela

Virgo ♍

Volans

Vulpecula

Family: Zodiac Ursa Major Perseus Hercules Orion Heavenly Waters Bayer La CailleOrigin/year: Ancient (Ptolemy) 1592 1603 1613 1692 1763

Equirectangular plot of declination vs right ascension of themodern constellations with a dotted line denoting the ecliptic.Constellations are colour-coded by family and year established.(detailed view)

Main article: 88 modern constellations

In 1922, Henry Norris Russell aided the IAU (Inter-national Astronomical Union) in dividing the celestialsphere into 88 official constellations.[23] Where possible,these modern constellations usually share the names oftheir Graeco-Roman predecessors, such as Orion, Leo orScorpius. The aim of this system is area-mapping, i.e. thedivision of the celestial sphere into contiguous fields.[24]Out of the 88modern constellations, 36 lie predominantlyin the northern sky, and the other 52 predominantly in thesouthern.In 1930, the boundaries between the 88 constellationswere devised by Eugène Delporte along vertical and hor-izontal lines of right ascension and declination.[25] How-ever, the data he used originated back to epoch B1875.0,which was when Benjamin A. Gould first made his pro-posal to designate boundaries for the celestial sphere, asuggestion upon which Delporte would base his work.The consequence of this early date is that due to the

precession of the equinoxes, the borders on a modern starmap, such as epoch J2000, are already somewhat skewedand no longer perfectly vertical or horizontal.[26] This ef-fect will increase over the years and centuries to come.

1.4 Asterisms

Much of the dark space between stars, as seen in the sky of theimage above, is due to the human eye’s low light sensitivity. Otherimages (like the Hubble Deep Field – not pictured) detect far morestars.

Main article: Asterism (astronomy)

The stars of the main asterism within a constellation areusually givenGreek letters in their order of brightness, theso-called Bayer designation introduced by Johann Bayerin 1603. A total of 1,564 stars are so identified, out ofapproximately 10,000 stars visible to the naked eye.[27]

The brightest stars, usually the stars that make up theconstellation’s eponymous asterism, also retain propernames, often from Arabic. For example, the “Little Dip-per” asterism of the constellation UrsaMinor has ten starswith Bayer designation, α UMi to π UMi. Of these tenstars, six have a proper name, viz. Polaris (α UMi),Kochab (β UMi), Pherkad (γ UMi), Yildun (δ UMi),Ahfa al Farkadain (ζ UMi) and Anwar al Farkadain (ηUMi).The stars within an asterism rarely have any substantial as-trophysical relationship to each other, and their apparentproximity when viewed from Earth disguises the fact thatthey are far apart, some being much farther from Earththan others. However, there are some exceptions: manyof the stars in the constellation of Ursa Major (includ-ing most of the Big Dipper) are genuinely close to oneanother, travel through the galaxy with similar velocities,and are likely to have formed together as part of a clusterthat is slowly dispersing. These stars form the UrsaMajormoving group.3D anaglyph 3D red cyan glasses are recommendedto view this image correctly.

1.6. SEE ALSO 5

Distances of stars in the Orion constellation (left) varygreatly. By contrast, the stars comprising the Big Dipper(right) are actually close to each other. The other starsof Ursa major have different distances.

1.5 Dark cloud constellations

Further information: Great Rift (astronomy)

The Great Rift, a series of dark patches in theMilkyWay,is more visible and striking in the southern hemispherethan in the northern. It vividly stands out when condi-tions are otherwise so dark that the Milky Way’s centralregion casts shadows on the ground. Some cultures havediscerned shapes in these patches and have given namesto these “dark cloud constellations.” Members of the Incacivilization identified various dark areas or dark nebulaein the Milky Way as animals, and associated their ap-pearance with the seasonal rains.[28] Australian Aborig-inal astronomy also describes dark cloud constellations,

the most famous being the “emu in the sky” whose headis formed by the Coalsack.

• The Emu in the sky—a constellation defined by darkclouds rather than by stars. The head of the emu isthe Coalsackwith the SouthernCross directly above.Scorpius is to the left.

1.6 See also• Apophenia

• Astrological sign

• 88 modern constellations – a list of the current con-stellations.

• 88 modern constellations by area

• 88 modern constellations in different languages

• Lists of stars by constellation

• Former constellations – a list of former constella-tions.

• Chinese constellations

• List of Nakshatras – sectors along the Moon’s eclip-tic

• Asterism (astronomy)

• Pareidolia

• Planisphere

1.7 References[1] Steele, Joel Dorman (1884). “The story of the stars: New

desscriptive astronomy”. Science series. American BookCompany. p. 220

[2] Harbord, John Bradley; Goodwin, H. B. (1897).“Glossary of navigation: a vade mecum for practical nav-igators” (3rd ed.). Griffin. p. 142

[3] “Do Constellations Ever Break Apart or Change?".NASA. Retrieved November 27, 2014.

[4] International Astronomical Union. “The Constellations”.

[5] Ian Ridpath. “Constellation names, abbreviations andsizes”.

[6] E.W. Bullinger, The Witness of the Stars

[7] D. James Kennedy, The Real Meaning of the Zodiac.

[8] Richard Hinckley Allen, Star Names: Their Lore andMeaning, Vol. 1 (New York: Dover Publications, 1899,p. 213-215.) argued for Scorpio having previously beencalled Eagle.

6 CHAPTER 1. CONSTELLATION

[9] Gesenius, Hebrew Lexicon

[10] The Origin of the Greek Constellations, by Bradley E.Schaefer. Scientific American, November 2006.

[11] Carolyn A. Krebs. Groundbreaking Scientific Experiments,Inventions, and Discoveries of the Ancient World (Green-wood Publishing Group, 2003, p. 272)

[12] Wilkinson, Richard H. (2003). The complete gods andgoddesses of ancient Egypt. London: Thames & Hudson.pp. 167–168, 211. ISBN 0-500-05120-8.

[13] D.M. Murdock. Christ in Egypt: The Horus-Jesus Connec-tion (Stellar House Publishing, 2008, p. 201.)

[14] Marshall Clagett Ancient Egyptian Science: Calendars,clocks and astronomy (American Philosophical Society,1995, p. 111.)

[15] John H. Rogers, "Origins of the ancient constellations: I.The Mesopotamian traditions", Journal of the British As-tronomical Association 108 (1998) 9–28

[16] Zodiac of Dendera, epitome. (Exhibition, Leic. square).J. Haddon, 1825.

[17] Mythology of the Hindus, by Charles Coleman p.131

[18] Xiaochun Sun, Jacob Kistemaker, The Chinese sky duringthe Han, vol. 38 of Sinica Leidensia, BRILL, 1997, ISBN978-90-04-10737-3, p. 18, note 9.

[19] Xiaochun Sun, Jacob Kistemaker, The Chinese sky duringthe Han, vol. 38 of Sinica Leidensia, BRILL, 1997, ISBN978-90-04-10737-3, chapter 2, 15-36.

[20] Ian Ridpath. “Bayer’s southern star chart”.

[21] Ian Ridpath. “Lacaille’s southern planisphere”.

[22] Sun, Xiaochun (1997). Helaine Selin, ed. Encyclopae-dia of the History of Science, Technology, and Medicine inNon-Western Cultures. Kluwer Academic Publishers. p.910. ISBN 0-7923-4066-3.

[23] “The original names and abbreviations for constellationsfrom 1922.”. Retrieved 2010-01-31.

[24] “The IAU on constellations”. Retrieved 2010-01-31.

[25] “Constellation boundaries.”. Retrieved 2011-05-24.

[26] A.C. Davenhall & S.K. Leggett, “A Catalogue of Con-stellation Boundary Data”, (Centre de Donneés as-tronomiques de Strasbourg, February 1990).

[27] The Bright Star Catalogue lists 9,110 objects of the nightsky which are visible to the naked eye (apparent magni-tude of 6.5 or brighter). 9,096 of these are stars, all ofthem well within our galaxy.

[28] The Incan View of the Night Sky

1.8 Further reading

1.8.1 Mythology, lore, history, and ar-chaeoastronomy

• Allen, Richard Hinckley. (1899) Star-Names AndTheir Meanings, G. E. Stechert, New York, NewYork, U.S.A., hardcover; reprint 1963 as StarNames: Their Lore and Meaning, Dover Publica-tions, Inc., Mineola, New York, U.S.A., ISBN 978-0-486-21079-7 softcover.

• Olcott, William Tyler. (1911); Star Lore of All Ages,G. P. Putnam’s Sons, NewYork, NewYork, U.S.A.,hardcover; reprint 2004 as Star Lore: Myths, Leg-ends, and Facts, Dover Publications, Inc., Mineola,New York, U.S.A., ISBN 978-0-486-43581-7 soft-cover.

• Kelley, David H. and Milone, Eugene F. (2004)Exploring Ancient Skies: An Encyclopedic Surveyof Archaeoastronomy, Springer, ISBN 978-0-387-95310-6 hardcover.

• Ridpath, Ian. (1989) Star Tales, Lutterworth Press,ISBN 0-7188-2695-7 hardcover.

• Staal, Julius D. W. (1988) The New Patterns in theSky: Myths and Legends of the Stars, McDonald&Woodward Publishing Co., ISBN 0-939923-10-6hardcover, ISBN 0-939923-04-1 softcover.

• John H. Rogers, "Origins of the Ancient Contella-tions: I. The Mesopotamian Traditions", Journal ofthe British Astronomical Association 108 (1998) 9–28.

• John H. Rogers, "Origins of the Ancient Contella-tions: II. The Mediterranean Traditions", Journal ofthe British Astronomical Association 108 (1998) 79–89.

1.8.2 Atlases and celestial maps

General & Nonspecialized – Entire Celestial Heavens:

• Becvar, Antonin. Atlas Coeli. Published as Atlasof the Heavens, Sky Publishing Corporation, Cam-bridge, Massachusetts, U.S.A.; with coordinate gridtransparency overlay.

• Norton, Arthur Philip. (1910) Norton’s Star Atlas,20th Edition 2003 as Norton’s Star Atlas and Ref-erence Handbook, edited by Ridpath, Ian, Pi Press,ISBN 978-0-13-145164-3, hardcover.

• National Geographic Society. (1957, 1970, 2001,2007) The Heavens (1970), Cartographic Division

1.8. FURTHER READING 7

Celestial map, signs of the Zodiac and lunar mansions.

of the National Geographic Society (NGS), Wash-ington, D.C., U.S.A., two sided large map chart de-picting the constellations of the heavens; as specialsupplement to the August 1970 issue ofNational Ge-ographic. Forerunner map as A Map of The Heav-ens, as special supplement to the December 1957issue. Current version 2001 (Tirion), with 2007reprint.

• Sinnott, Roger W. and Perryman, Michael A.C.(1997) Millennium Star Atlas, Epoch 2000.0,Sky Publishing Corporation, Cambridge, Mas-sachusetts, U.S.A., and European Space Agency(ESA), ESTEC, Noordwijk, The Netherlands. Sub-title: “An All-Sky Atlas Comprising One MillionStars to Visual Magnitude Eleven from the Hippar-cos and Tycho Catalogues and Ten Thousand Non-stellar Objects”. 3 volumes, hardcover, in hard-cover slipcase, set ISBN 0-933346-84-0. Vol. 1,0–8 Hours (Right Ascension), ISBN 0-933346-81-6 hardcover; Vol. 2, 8–16 Hours, ISBN 0-933346-82-4 hardcover; Vol. 3, 16–24 Hours, ISBN 0-933346-83-2 hardcover. Softcover version avail-able. Supplemental separate purchasable coordinategrid transparent overlays.

• Tirion, Wil; et al. (1987) Uranometria 2000.0,Willmann-Bell, Inc., Richmond, Virginia, U.S.A., 3volumes, hardcover. Vol. 1 (1987): “The NorthernHemisphere to −6°", by Wil Tirion, Barry Rappa-port, and George Lovi, ISBN 0-943396-14-X hard-cover, printed boards (blue). Vol. 2 (1988): “TheSouthern Hemisphere to +6°", by Wil Tirion, BarryRappaport and George Lovi, ISBN 0-943396-15-8hardcover, printed boards (red). Vol. 3 (1993) asa separate added work: The Deep Sky Field Guideto Uranometria 2000.0, by Murray Cragin, JamesLucyk, and Barry Rappaport, ISBN 0-943396-38-7

hardcover, printed boards (gray). 2nd Edition 2001(black or dark background) as collective set of 3 vol-umes – Vol. 1: Uranometria 2000.0 Deep Sky Atlas,by Wil Tirion, Barry Rappaport, and Will Remak-lus, ISBN 978-0-943396-71-2 hardcover, printedboards (blue edging); Vol. 2: Uranometria 2000.0Deep Sky Atlas, byWil Tirion, Barry Rappaport, andWill Remaklus, ISBN 978-0-943396-72-9 hard-cover, printed boards (green edging); Vol. 3: Ura-nometria 2000.0 Deep Sky Field Guide by MurrayCragin and Emil Bonanno, ISBN 978-0-943396-73-6, hardcover, printed boards (teal green).

• Tirion, Wil and Sinnott, Roger W. (1998) Sky Atlas2000.0, various editions. 2nd Deluxe Edition, Cam-bridge University Press, Cambridge, England (UK).

Northern Celestial Hemisphere & North Circumpolar Re-gion:

• Becvar, Antonin. (1962) Atlas Borealis1950.0, Czechoslovak Academy of Sciences(Ceskoslovenske Akademie Ved), Praha,Czechoslovakia, 1st Edition, elephant foliohardcover, with small transparency overlaycoordinate grid square and separate paper mag-nitude legend ruler. 2nd Edition 1972 and1978 reprint, Czechoslovak Academy of Sci-ences (Ceskoslovenske Akademie Ved), Prague,Czechoslovakia, and Sky Publishing Corporation,Cambridge, Massachusetts, U.S.A., ISBN 0-933346-01-8 oversize folio softcover spiral bound,with transparency overlay coordinate grid ruler.

Equatorial, Ecliptic, & Zodiacal Celestial Sky:

• Becvar, Antonin. (1958) Atlas Eclipticalis1950.0, Czechoslovak Academy of Sciences(Ceskoslovenske Akademie Ved), Praha,Czechoslovakia, 1st Edition, elephant folio hard-cover, with small transparency overlay coordinategrid square and separate paper magnitude legendruler. 2nd Edition 1974, Czechoslovak Academy ofSciences (Ceskoslovenske Akademie Ved), Prague,Czechoslovakia, and Sky Publishing Corporation,Cambridge, Massachusetts, U.S.A., oversize foliosoftcover spiral bound, with transparency overlaycoordinate grid ruler.

Southern Celestial Hemisphere & South Circumpolar Re-gion:

• Becvar, Antonin. Atlas Australis 1950.0, Czechoslo-vak Academy of Sciences (CeskoslovenskeAkademie Ved), Praha, Czechoslovakia, 1stEdition, elephant folio hardcover, with smalltransparency overlay coordinate grid squareand separate paper magnitude legend ruler.

8 CHAPTER 1. CONSTELLATION

2nd Edition, Czechoslovak Academy of Sci-ences (Ceskoslovenske Akademie Ved), Prague,Czechoslovakia, and Sky Publishing Corporation,Cambridge, Massachusetts, U.S.A., oversize foliosoftcover spiral bound, with transparency overlaycoordinate grid ruler.

1.8.3 Catalogs

• Becvar, Antonin. (1959) Atlas Coeli II Katalog1950.0, Praha, 1960 Prague. Published 1964 as At-las of the Heavens - II Catalogue 1950.0, Sky Pub-lishing Corporation, Cambridge, Massachusetts,U.S.A.

• Hirshfeld, Alan and Sinnott, Roger W. (1982) SkyCatalogue 2000.0, Cambridge University Press andSky Publishing Corporation, 1st Edition, 2 vol-umes. LCCN 81017975 both vols., and LCCN83240310 vol. 1. “Volume 1: Stars to Magni-tude 8.0”, ISBN 0-521-24710-1 (Cambridge) and0-933346-35-2 (Sky) hardcover, ISBN 0-933346-34-4 (Sky) softcover. Vol. 2 (1985) - “Volume2: Double Stars, Variable Stars, and NonstellarObjects”, ISBN 0-521-25818-9 (Cambridge) hard-cover, ISBN 0-521-27721-3 (Cambridge) softcover.2nd Edition (1991) with additional third authorFrangois Ochsenbein, 2 volumes, LCCN 91026764.Vol. 1: ISBN 0-521-41743-0 (Cambridge) hard-cover (black binding); ISBN 0-521-42736-3 (Cam-bridge) softcover (red lettering with Hans Vehren-berg astrophoto). Vol. 2 (1999): ISBN 0-521-27721-3 (Cambridge) softcover and 0-933346-38-7(Sky) softcover - reprint of 1985 edition (blue letter-ing with Hans Vehrenberg astrophoto).

• Yale University Observatory. (1908, et al.)Catalogue of Bright Stars, New Haven, Connecticut,U.S.A. Referred to commonly as “Bright Star Cat-alogue”. Various editions with various authors his-torically, the longest term revising author as (Ellen)Dorrit Hoffleit. 1st Edition 1908. 2nd Edition 1940by Frank Schlesinger and Louise F. Jenkins. 3rdEdition (1964), 4th Edition, 5th Edition (1991), and6th Edition (pending posthumous) by Hoffleit.

1.9 External links

• IAU: The Constellations, including high qualitymaps.

• Star Tales origins and mythology of the constella-tions (Ian Ridpath)

• Celestia free 3D realtime space-simulation(OpenGL)

• Stellarium realtime sky rendering program(OpenGL)

• Strasbourg Astronomical Data Center Files on offi-cial IAU constellation boundaries

• Interactive Sky Charts (Java applets allowing navi-gation through the entire sky with variable star de-tail, optional constellation lines)

• Studies ofOccidental Constellations and Star Namesto the Classical Period: An Annotated Bibliography

• Table of Constellations

• Online Text: Hyginus, Astronomica translated byMary Grant Greco-Roman constellation myths

• Neave Planetarium Adobe Flash interactive webbrowser planetarium and stardome with realisticmovement of stars and the planets.

• Audio - Cain/Gay (2009) Astronomy Cast Constel-lations

• Constellation Guide Constellation facts, myths, starsand deep sky objects.

• The Greek Star-Map by Gavin White

Chapter 2

Galaxy

This article is about the astronomical structure. For themobile devices, see Samsung Galaxy. For other uses,see Galaxy (disambiguation).

NGC 4414, a typical spiral galaxy in the constellation ComaBerenices, is about 55,000 light-years in diameter and approxi-mately 60 million light-years away from Earth.

A galaxy is a gravitationally bound system consisting ofstars, stellar remnants, interstellar gas and dust, and darkmatter.[1][2] The word galaxy is derived from the Greekgalaxias (γαλαξίας), literally “milky”, a reference to theMilkyWay. Examples of galaxies range from dwarfs withjust a few thousand (103) stars to giants with one hun-dred trillion (1014) stars,[3] each orbiting their galaxy’sown center of mass. Galaxies can be categorized ac-cording to their visual morphology, including elliptical,[4]spiral, and irregular.[5]Many galaxies are believed to haveblack holes at their active center. The Milky Way’s cen-tral black hole, known as Sagittarius A*, has a mass fourmillion times that of our Sun.[6]

There are approximately 170 billion (1.7 × 1011) galaxiesin the observable universe.[7] Most are 1,000 to 100,000parsecs in diameter and usually separated by distances onthe order of millions of parsecs (or megaparsecs). Thespace between galaxies is filled with a tenuous gas withan average density less than one atom per cubic meter.The majority of galaxies are gravitationally organized

into associations known as galaxy groups, clusters, andsuperclusters. At the largest scale, these associations aregenerally arranged into sheets and filaments, which aresurrounded by immense voids.[8]

2.1 Etymology

The word galaxy derives from the Greek term for ourown galaxy, galaxias (γαλαξίας, “milky one”), or kyklosgalaktikos (“milky circle”)[9] due to its appearance as a“milky” band of light in the sky. In Greek mythology,Zeus places his son born by a mortal woman, the infantHeracles, on Hera's breast while she is asleep so that thebaby will drink her divine milk and will thus become im-mortal. Hera wakes up while breastfeeding and then real-izes she is nursing an unknown baby: she pushes the babyaway and a jet of her milk sprays the night sky, producingthe faint band of light known as the Milky Way.[10][11]

In the astronomical literature, the capitalized word“Galaxy” is often used to refer to our galaxy, the MilkyWay, to distinguish it from the other galaxies in ouruniverse. The English termMilkyWay can be traced backto a story by Chaucer c. 1380:

“See yonder, lo, the GalaxyëWhich men clepeth the Milky Wey,For hit is whyt.”—Geoffrey Chaucer, The House of Fame[9]

When William Herschel constructed his catalog of deepsky objects in 1786, he used the term spiral nebula forcertain objects such as M31. These would later be recog-nized as conglomerations of stars, when the true distanceto these objects began to be appreciated, and they wouldlater be termed island universes. However, the word Uni-verse was understood to mean the entirety of existence,so this expression fell into disuse and the objects insteadbecame known as galaxies.[12]

9

10 CHAPTER 2. GALAXY

2.2 Nomenclature

Tens of thousands of galaxies have been catalogued,but only a few have well-established names, such asthe Andromeda Galaxy, the Magellanic clouds, theWhirlpool Galaxy and the Sombrero Galaxy. As-tronomers work with numbers from certain catalogues,such as the Messier catalogue, the NGC (New Gen-eral Catalogue), the IC (Index Catalogue), the CGCG(Catalogue of Galaxies and of Clusters of Galaxies), theMCG (Morphological Catalogue of Galaxies) and UGC(Uppsala General Catalogue ofGalaxies). All of the well-known galaxies appear in one or more of these cataloguesbut each time under a different number. For example,the Messier 109, a spiral system which has the number109 in the catalogue of Messier also codes NCG3992,UGC6937, CGCG269-023, MCG+09-20-044 and PGC37617.Because it is customary in science to assign names tomost of the studied objects, even to the smallest ones, theBelgian astrophysicist Gerard Bodifee and the classicistMichel Berger started a new catalogue (CNG-Catalogueof Named Galaxies)[13] in which a thousand well-knowngalaxies are given meaningful, descriptive names in Latin(or Latinized Greek)[14] in accordance with the binomialnomenclature that one uses in other sciences such as bi-ology, anatomy, paleontology and in other fields of as-tronomy such as the geography of Mars. One of the ar-guments to do so is that these impressive objects deservebetter than uninspired codes. For instance, Bodifee andBerger propose the informal, descriptive name Callimor-phus Ursae Majoris for the well-formed barred galaxyMessier 109 in Ursa Major.

2.3 Observation history

The realization that we live in a galaxy, and that ours isone among many, parallels major discoveries that weremade about the MilkyWay and other nebulae in the nightsky.

2.3.1 Milky Way

Main article: Milky Way

The Greek philosopher Democritus (450–370 BC) pro-posed that the bright band on the night sky known asthe Milky Way might consist of distant stars.[15] Aristotle(384–322 BC), however, believed the Milky Way to becaused by “the ignition of the fiery exhalation of somestars that were large, numerous and close together” andthat the “ignition takes place in the upper part of theatmosphere, in the region of the World that is contin-uous with the heavenly motions.”[16] The Neoplatonistphilosopher Olympiodorus the Younger (c. 495–570

AD) was critical of this view, arguing that if the MilkyWay is sublunary (situated between Earth and the Moon)it should appear different at different times and places onEarth, and that it should have parallax, which it does not.In his view, the Milky Way is celestial.[17]

According to Mohani Mohamed, the Arabian astronomerAlhazen (965–1037) made the first attempt at observ-ing and measuring the Milky Way’s parallax,[18] and hethus “determined that because the Milky Way had noparallax, it must be remote from the Earth, not belong-ing to the atmosphere.”[19] The Persian astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be“a collection of countless fragments of the nature of neb-ulous stars.”[20][21] The Andalusian astronomer Ibn Baj-jah (“Avempace”, d. 1138) proposed that the Milky Waywas made up of many stars that almost touch one anotherand appear to be a continuous image due to the effect ofrefraction from sublunary material,[16][22] citing his ob-servation of the conjunction of Jupiter and Mars as evi-dence of this occurring when two objects are near.[16] Inthe 14th century, the Syrian-born Ibn Qayyim proposedtheMilkyWay galaxy to be “a myriad of tiny stars packedtogether in the sphere of the fixed stars.”[23]

The shape of the Milky Way as deduced from star counts byWilliam Herschel in 1785; the solar system was assumed to benear the center.

Actual proof of the Milky Way consisting of many starscame in 1610 when the Italian astronomer Galileo Galileiused a telescope to study the Milky Way and discoveredthat it is composed of a huge number of faint stars.[24][25]In 1750 the English astronomer ThomasWright, in hisAnoriginal theory or new hypothesis of the Universe, specu-lated (correctly) that the galaxy might be a rotating bodyof a huge number of stars held together by gravitationalforces, akin to the solar system but on amuch larger scale.The resulting disk of stars can be seen as a band on thesky from our perspective inside the disk.[26][27] In a trea-tise in 1755, Immanuel Kant elaborated on Wright’s ideaabout the structure of the Milky Way.[28]

The first project to describe the shape of the Milky Wayand the position of the Sun was undertaken by WilliamHerschel in 1785 by counting the number of stars in dif-ferent regions of the sky. He produced a diagram of theshape of the galaxy with the solar system close to the cen-ter.[29][30] Using a refined approach, Kapteyn in 1920 ar-rived at the picture of a small (diameter about 15 kilo-parsecs) ellipsoid galaxy with the Sun close to the center.A different method by Harlow Shapley based on the cat-aloguing of globular clusters led to a radically differentpicture: a flat disk with diameter approximately 70 kilo-

2.3. OBSERVATION HISTORY 11

parsecs and the Sun far from the center.[27] Both anal-yses failed to take into account the absorption of lightby interstellar dust present in the galactic plane, but af-ter Robert Julius Trumpler quantified this effect in 1930by studying open clusters, the present picture of our hostgalaxy, the Milky Way, emerged.[31]

A fish-eye mosaic of the Milky Way arching at a high inclinationacross the night sky, shot from a dark-sky location in Chile

2.3.2 Distinction from other nebulae

A few galaxies outside the Milky Way are visible in thenight sky to the unaided eye. In the 10th century, thePersian astronomer Al-Sufi made the earliest recordedidentification of the Andromeda Galaxy, describing it asa “small cloud”.[32] In 964, Al-Sufi identified the LargeMagellanic Cloud in his Book of Fixed Stars; it wasnot seen by Europeans until Magellan's voyage in the16th century.[33][34] The Andromeda Galaxy was inde-pendently noted by Simon Marius in 1612.[32]

In 1750, Thomas Wright speculated (correctly) that theMilky Way was a flattened disk of stars, and that someof the nebulae visible in the night sky might be sepa-rateMilkyWays.[27][35] In 1755, Immanuel Kant used theterm “island Universe” to describe these distant nebulae.

Photograph of the “Great Andromeda Nebula” from 1899, lateridentified as the Andromeda Galaxy

Toward the end of the 18th century, Charles Messiercompiled a catalog containing the 109 brightest ce-lestial objects having nebulous appearance. Subse-quently, William Herschel assembled a catalog of 5,000nebulae.[27] In 1845, Lord Rosse constructed a new tele-scope and was able to distinguish between elliptical andspiral nebulae. He also managed to make out individual

point sources in some of these nebulae, lending credenceto Kant’s earlier conjecture.[36]

In 1912, Vesto Sliphermade spectrographic studies of thebrightest spiral nebulae to determine their composition.Slipher discovered that spiral nebulae have high red shifts,indicating that they are moving away from the MilkyWay at a rate exceeding the Milky Way’s escape veloc-ity. Thus, they are not gravitationally bound to the MilkyWay, and are unlikely to be a part of the galaxy.[37][38]

In 1917, Heber Curtis observed nova S Andromedaewithin the “Great Andromeda Nebula” (as the An-dromeda Galaxy, Messier object M31, was then known).Searching the photographic record, he found 11 morenovae. Curtis noticed that these novae were, on average,10 magnitudes fainter than those that occurred within ourgalaxy. As a result he was able to come up with a distanceestimate of 150,000 parsecs. He became a proponent ofthe so-called “island universes” hypothesis, which holdsthat spiral nebulae are actually independent galaxies.[39]

In 1920 the so-called Great Debate took place betweenHarlow Shapley and Heber Curtis, concerning the natureof the Milky Way, spiral nebulae, and the dimensions ofthe Universe. To support his claim that the Great An-dromeda Nebula was an external galaxy, Curtis noted theappearance of dark lanes resembling the dust clouds intheMilkyWay, as well as the significant Doppler shift.[40]

In 1922, the Estonian astronomer Ernst Öpik gave adistance determination that supported the theory thatthe Andromeda Nebula is indeed a distant extra-galacticobject.[41] Using the new 100 inch Mt. Wilson telescope,Edwin Hubble was able to resolve the outer parts of somespiral nebulae as collections of individual stars and iden-tified some Cepheid variables, thus allowing him to esti-mate the distance to the nebulae: they were far too distantto be part of theMilkyWay.[42] In 1936 Hubble produceda classification of galactic morphology that is used to thisday.[43]

2.3.3 Modern research

Distance

Velocity

A

B

Rotation curve of a typical spiral galaxy: predicted based on thevisible matter (A) and observed (B). The distance is from thegalactic core.

12 CHAPTER 2. GALAXY

In 1944, Hendrik van de Hulst predicted that microwaveradiation with wavelength of 21 cm would be detectablefrom interstellar atomic hydrogen gas;[44] and in 1951 itwas observed. This radiation is not affected by dust ab-sorption, and so its Doppler shift can be used to map themotion of the gas in our galaxy. These observations led tothe hypothesis of a rotating bar structure in the center ofour galaxy.[45] With improved radio telescopes, hydrogengas could also be traced in other galaxies. In the 1970s,Vera Rubin uncovered a discrepancy between observedgalactic rotation speed and that predicted by the visiblemass of stars and gas. Today, the galaxy rotation problemis thought to be explained by the presence of large quan-tities of unseen dark matter.[46][47] A concept known asthe universal rotation curve of spirals, moreover, showsthat the problem is ubiquitous in these objects.Beginning in the 1990s, the Hubble Space Telescopeyielded improved observations. Among other things,Hubble data helped establish that the missing dark matterin our galaxy cannot solely consist of inherently faint andsmall stars.[48] The Hubble Deep Field, an extremely longexposure of a relatively empty part of the sky, providedevidence that there are about 125 billion (1.25×1011)galaxies in the Universe.[49] Improved technology in de-tecting the spectra invisible to humans (radio telescopes,infrared cameras, and x-ray telescopes) allow detectionof other galaxies that are not detected by Hubble. Partic-ularly, galaxy surveys in the Zone of Avoidance (the re-gion of the sky blocked by the Milky Way) have revealeda number of new galaxies.[50]

2.4 Types and morphology

Main article: Galaxy morphological classificationGalaxies come in three main types: ellipticals, spirals,

Types of galaxies according to the Hubble classification scheme:an E indicates a type of elliptical galaxy; an S is a spiral; and SBis a barred-spiral galaxy.[note 1]

and irregulars. A slightly more extensive description ofgalaxy types based on their appearance is given by theHubble sequence. Since the Hubble sequence is entirelybased upon visual morphological type, it maymiss certainimportant characteristics of galaxies such as star forma-tion rate in starburst galaxies and activity in the cores of

active galaxies.[5]

2.4.1 Ellipticals

Main article: Elliptical galaxy

The Hubble classification system rates elliptical galax-ies on the basis of their ellipticity, ranging from E0,being nearly spherical, up to E7, which is highly elon-gated. These galaxies have an ellipsoidal profile, givingthem an elliptical appearance regardless of the viewingangle. Their appearance shows little structure and theytypically have relatively little interstellar matter. Conse-quently these galaxies also have a low portion of openclusters and a reduced rate of new star formation. Insteadthey are dominated by generally older, more evolved starsthat are orbiting the common center of gravity in randomdirections. The stars contain low abundances of heavyelements because star formation ceases after the initialburst. In this sense they have some similarity to the muchsmaller globular clusters.[51]

The largest galaxies are giant ellipticals. Many ellipti-cal galaxies are believed to form due to the interactionof galaxies, resulting in a collision and merger. Theycan grow to enormous sizes (compared to spiral galax-ies, for example), and giant elliptical galaxies are oftenfound near the core of large galaxy clusters.[52] Starburstgalaxies are the result of such a galactic collision that canresult in the formation of an elliptical galaxy.[51]

2.4.2 Spirals

Main articles: Spiral galaxy and Barred spiral galaxySpiral galaxies resemble spiraling pinwheels. Though the

The Pinwheel Galaxy, NGC 5457.

stars and other visible material contained in such a galaxylie mostly on a plane, the majority of mass in spiral galax-ies exists in a roughly spherical halo of dark matter thatextends beyond the visible component, as demonstratedby the universal rotation curve concept.[53]

2.4. TYPES AND MORPHOLOGY 13

Spiral galaxies consist of a rotating disk of stars and in-terstellar medium, along with a central bulge of generallyolder stars. Extending outward from the bulge are rela-tively bright arms. In the Hubble classification scheme,spiral galaxies are listed as type S, followed by a letter (a,b, or c) that indicates the degree of tightness of the spi-ral arms and the size of the central bulge. An Sa galaxyhas tightly wound, poorly defined arms and possesses arelatively large core region. At the other extreme, anSc galaxy has open, well-defined arms and a small coreregion.[54] A galaxy with poorly defined arms is some-times referred to as a flocculent spiral galaxy; in contrastto the grand design spiral galaxy that has prominent andwell-defined spiral arms.[55]

It appears the reason that some spiral galaxies are fat andbulging while some are flat discs is because of how fastthey rotate.[56]

NGC 1300, an example of a barred spiral galaxy.

In spiral galaxies, the spiral arms do have the shape of ap-proximate logarithmic spirals, a pattern that can be the-oretically shown to result from a disturbance in a uni-formly rotating mass of stars. Like the stars, the spiralarms rotate around the center, but they do so with con-stant angular velocity. The spiral arms are thought to beareas of high-density matter, or "density waves".[57] Asstars move through an arm, the space velocity of eachstellar system is modified by the gravitational force of thehigher density. (The velocity returns to normal after thestars depart on the other side of the arm.) This effect isakin to a “wave” of slowdowns moving along a highwayfull of moving cars. The arms are visible because thehigh density facilitates star formation, and therefore theyharbor many bright and young stars.[58]

Hoag’s Object, an example of a ring galaxy

NGC 5866, an example of a lenticular galaxy

A majority of spiral galaxies, including our own MilkyWay galaxy, have a linear, bar-shaped band of stars thatextends outward to either side of the core, then mergesinto the spiral arm structure.[59] In the Hubble classifica-tion scheme, these are designated by an SB, followed bya lower-case letter (a, b or c) that indicates the form ofthe spiral arms (in the same manner as the categoriza-tion of normal spiral galaxies). Bars are thought to betemporary structures that can occur as a result of a den-sity wave radiating outward from the core, or else due toa tidal interaction with another galaxy.[60] Many barredspiral galaxies are active, possibly as a result of gas beingchanneled into the core along the arms.[61]

Our own galaxy, the Milky Way, is a large disk-shapedbarred-spiral galaxy[62] about 30 kiloparsecs in diameterand a kiloparsec thick. It contains about two hundred bil-lion (2×1011)[63] stars and has a total mass of about sixhundred billion (6×1011) times the mass of the Sun.[64]

2.4.3 Other morphologies

Peculiar galaxies are galactic formations that develop

14 CHAPTER 2. GALAXY

unusual properties due to tidal interactions with othergalaxies. An example of this is the ring galaxy, whichpossesses a ring-like structure of stars and interstellarmedium surrounding a bare core. A ring galaxy is thoughtto occur when a smaller galaxy passes through the core ofa spiral galaxy.[65] Such an event may have affected theAndromeda Galaxy, as it displays a multi-ring-like struc-ture when viewed in infrared radiation.[66]

A lenticular galaxy is an intermediate form that has prop-erties of both elliptical and spiral galaxies. These are cat-egorized as Hubble type S0, and they possess ill-definedspiral arms with an elliptical halo of stars.[67] (Barredlenticular galaxies receive Hubble classification SB0.)In addition to the classifications mentioned above, thereare a number of galaxies that can not be readily classifiedinto an elliptical or spiral morphology. These are cate-gorized as irregular galaxies. An Irr-I galaxy has somestructure but does not align cleanly with the Hubble clas-sification scheme. Irr-II galaxies do not possess any struc-ture that resembles a Hubble classification, and may havebeen disrupted.[68] Nearby examples of (dwarf) irregulargalaxies include the Magellanic Clouds.

2.4.4 Dwarfs

Main article: Dwarf galaxy

Despite the prominence of large elliptical and spiralgalaxies, most galaxies in the Universe are dwarf galaxies.These galaxies are relatively small when compared withother galactic formations, being about one hundredth thesize of the MilkyWay, containing only a few billion stars.Ultra-compact dwarf galaxies have recently been discov-ered that are only 100 parsecs across.[69]

Many dwarf galaxies may orbit a single larger galaxy; theMilkyWay has at least a dozen such satellites, with an es-timated 300–500 yet to be discovered.[70] Dwarf galax-ies may also be classified as elliptical, spiral, or irregular.Since small dwarf ellipticals bear little resemblance tolarge ellipticals, they are often called dwarf spheroidalgalaxies instead.A study of 27 Milky Way neighbors found that in alldwarf galaxies, the central mass is approximately 10 mil-lion solar masses, regardless of whether the galaxy hasthousands or millions of stars. This has led to the sug-gestion that galaxies are largely formed by dark matter,and that the minimum size may indicate a form of warmdark matter incapable of gravitational coalescence on asmaller scale.[71]

2.5 Unusual dynamics and activi-ties

2.5.1 Interacting

Main article: Interacting galaxyInteractions between galaxies are relatively frequent, and

The Antennae Galaxies are undergoing a collision that will resultin their eventual merger.

they can play an important role in galactic evolution. Nearmisses between galaxies result in warping distortions dueto tidal interactions, and may cause some exchange of gasand dust.[72][73] Collisions occur when two galaxies passdirectly through each other and have sufficient relativemomentum not to merge. The stars of interacting galax-ies will usually not collide, but the gas and dust withinthe two forms will interact, sometimes triggering star for-mation. A collision can severely distort the shape of thegalaxies, forming bars, rings or tail-like structures.[72][73]

At the extreme of interactions are galactic mergers. Inthis case the relative momentum of the two galaxies isinsufficient to allow the galaxies to pass through eachother. Instead, they gradually merge to form a single,larger galaxy. Mergers can result in significant changesto morphology, as compared to the original galaxies. Inthe case where one of the galaxies is much more massive,however, the result is known as cannibalism. In this casethe larger galaxy will remain relatively undisturbed by themerger, while the smaller galaxy is torn apart. The MilkyWay galaxy is currently in the process of cannibalizing theSagittarius Dwarf Elliptical Galaxy and the Canis MajorDwarf Galaxy.[72][73]

2.5.2 Starburst

Main article: Starburst galaxyStars are created within galaxies from a reserve of coldgas that forms into giant molecular clouds. Some galaxieshave been observed to form stars at an exceptional rate,known as a starburst. Should they continue to do so, how-

2.6. FORMATION AND EVOLUTION 15

M82, a starburst galaxy that has ten times the star formation ofa “normal” galaxy.[74]

ever, they would consume their reserve of gas in a timeframe lower than the lifespan of the galaxy. Hence star-burst activity usually lasts for only about tenmillion years,a relatively brief period in the history of a galaxy. Star-burst galaxies were more common during the early his-tory of the Universe,[75] and, at present, still contributean estimated 15% to the total star production rate.[76]

Starburst galaxies are characterized by dusty concentra-tions of gas and the appearance of newly formed stars, in-cluding massive stars that ionize the surrounding cloudsto create H II regions.[77] These massive stars producesupernova explosions, resulting in expanding remnantsthat interact powerfully with the surrounding gas. Theseoutbursts trigger a chain reaction of star building thatspreads throughout the gaseous region. Only when theavailable gas is nearly consumed or dispersed does thestarburst activity come to an end.[75]

Starbursts are often associated with merging or interact-ing galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a closeencounter with the larger M81. Irregular galaxies oftenexhibit spaced knots of starburst activity.[78]

2.5.3 Active nucleus

Main article: Active galactic nucleusA portion of the observable galaxies are classified as ac-tive. That is, a significant portion of the total energy out-put from the galaxy is emitted by a source other than thestars, dust and interstellar medium.The standard model for an active galactic nucleus is basedupon an accretion disc that forms around a supermassiveblack hole (SMBH) at the core region. The radiation froman active galactic nucleus results from the gravitationalenergy of matter as it falls toward the black hole fromthe disc.[79] In about 10% of these objects, a diamet-rically opposed pair of energetic jets ejects particlesfrom the core at velocities close to the speed of light.

A jet of particles is being emitted from the core of the ellipticalradio galaxy M87.

The mechanism for producing these jets is still not wellunderstood.[80]

Active galaxies that emit high-energy radiation in theform of x-rays are classified as Seyfert galaxies orquasars, depending on the luminosity. Blazars are be-lieved to be an active galaxy with a relativistic jet that ispointed in the direction of Earth. A radio galaxy emitsradio frequencies from relativistic jets. A unified modelof these types of active galaxies explains their differencesbased on the viewing angle of the observer.[80]

Possibly related to active galactic nuclei (as well asstarburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-typegalaxies is dominated by weakly ionized elements.[81] Ap-proximately one-third of nearby galaxies are classified ascontaining LINER nuclei.[79][81][82]

2.6 Formation and evolution

Main article: Galaxy formation and evolution

Galactic formation and evolution is an active area of re-search in astrophysics.

2.6.1 Formation

Current cosmological models of the early Universe arebased on the Big Bang theory. About 300,000 years afterthis event, atoms of hydrogen and helium began to form,in an event called recombination. Nearly all the hydro-gen was neutral (non-ionized) and readily absorbed light,and no stars had yet formed. As a result this period has

16 CHAPTER 2. GALAXY

Artist’s impression of a protocluster forming in the earlyUniverse.[83]

been called the "Dark Ages". It was from density fluc-tuations (or anisotropic irregularities) in this primordialmatter that larger structures began to appear. As a re-sult, masses of baryonic matter started to condense withincold dark matter halos.[84][85] These primordial structureswould eventually become the galaxies we see today.

Artist’s impression of a young galaxy accreting material.

Evidence for the early appearance of galaxies was foundin 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, correspondingto just 750 million years after the Big Bang and mak-ing it the most distant and primordial galaxy yet seen.[86]While some scientists have claimed other objects (such asAbell 1835 IR1916) have higher redshifts (and thereforeare seen in an earlier stage of the Universe’s evolution),IOK-1’s age and composition have been more reliably es-tablished. However, in December 2012, astronomers re-ported that the UDFj-39546284 galaxy is the most dis-tant galaxy known and has a redshift value of 11.9. Thegalaxy, estimated to have existed around “380 millionyears”[87] after the Big Bang (which was about 13.8 bil-lion years ago),[88] is about 13.42 billion light years away.The existence of such early protogalaxies suggests thatthey must have grown in the so-called “Dark Ages”.[84]

The detailed process by which early galaxies formed is anopen question in astrophysics. Theories can be divided

into two categories: top-down and bottom-up. In top-down theories (such as the Eggen–Lynden-Bell–Sandage[ELS] model), protogalaxies form in a large-scale si-multaneous collapse lasting about one hundred millionyears.[89] In bottom-up theories (such as the Searle-Zinn[SZ] model), small structures such as globular clustersform first, and then a number of such bodies accrete toform a larger galaxy.[90]

Once protogalaxies began to form and contract, the firsthalo stars (called Population III stars) appeared withinthem. These were composed almost entirely of hydro-gen and helium, and may have been massive. If so, thesehuge stars would have quickly consumed their supply offuel and became supernovae, releasing heavy elementsinto the interstellar medium.[91] This first generation ofstars re-ionized the surrounding neutral hydrogen, creat-ing expanding bubbles of space through which light couldreadily travel.[92]

2.6.2 Evolution

Within a billion years of a galaxy’s formation, key struc-tures begin to appear. Globular clusters, the central su-permassive black hole, and a galactic bulge of metal-poorPopulation II stars form. The creation of a supermassiveblack hole appears to play a key role in actively regulat-ing the growth of galaxies by limiting the total amountof additional matter added.[93] During this early epoch,galaxies undergo a major burst of star formation.[94]

During the following two billion years, the accumu-lated matter settles into a galactic disc.[95] A galaxy willcontinue to absorb infalling material from high-velocityclouds and dwarf galaxies throughout its life.[96] Thismat-ter is mostly hydrogen and helium. The cycle of stellarbirth and death slowly increases the abundance of heavyelements, eventually allowing the formation of planets.[97]

Hubble eXtreme Deep Field (XDF)

XDF view field compared to the angular size of theMoon. Several thousand galaxies, each consisting ofbillions of stars, are in this small view.

2.7. LARGER-SCALE STRUCTURES 17

XDF (2012) view: Each light speck is a galaxy, some ofwhich are as old as 13.2 billion years[98] – the observableuniverse is estimated to contain 200 billion galaxies.

XDF image shows (from left) fully mature galaxies,nearly mature galaxies (from 5 to 9 billion years ago),protogalaxies, blazing with young stars (beyond 9 billionyears).

The evolution of galaxies can be significantly affectedby interactions and collisions. Mergers of galaxies werecommon during the early epoch, and the majority ofgalaxies were peculiar in morphology.[99] Given the dis-tances between the stars, the great majority of stellar sys-tems in colliding galaxies will be unaffected. However,gravitational stripping of the interstellar gas and dust thatmakes up the spiral arms produces a long train of starsknown as tidal tails. Examples of these formations canbe seen in NGC 4676[100] or the Antennae Galaxies.[101]

The Milky Way galaxy and the nearby AndromedaGalaxy are moving toward each other at about 130 km/s,and—depending upon the lateral movements—the twomight collide in about five to six billion years. Althoughthe Milky Way has never collided with a galaxy as largeas Andromeda before, evidence of past collisions of theMilky Way with smaller dwarf galaxies is increasing.[102]

Such large-scale interactions are rare. As time passes,mergers of two systems of equal size become less com-mon. Most bright galaxies have remained fundamentallyunchanged for the last few billion years, and the net rateof star formation probably also peaked approximately tenbillion years ago.[103]

2.6.3 Future trends

Spiral galaxies, like the Milky Way, produce new genera-tions of stars as long as they have dense molecular clouds

of interstellar hydrogen in their spiral arms.[104] Ellipti-cal galaxies are largely devoid of this gas, and so formfew new stars.[105] The supply of star-forming material isfinite; once stars have converted the available supply ofhydrogen into heavier elements, new star formation willcome to an end.[106]

The current era of star formation is expected to continuefor up to one hundred billion years, and then the “stel-lar age” will wind down after about ten trillion to onehundred trillion years (1013–1014 years), as the small-est, longest-lived stars in our astrosphere, tiny red dwarfs,begin to fade. At the end of the stellar age, galaxieswill be composed of compact objects: brown dwarfs,white dwarfs that are cooling or cold ("black dwarfs"),neutron stars, and black holes. Eventually, as a result ofgravitational relaxation, all stars will either fall into cen-tral supermassive black holes or be flung into intergalacticspace as a result of collisions.[106][107]

2.7 Larger-scale structures

Main articles: Observable universe § Large-scale struc-ture, Galaxy filament and Galaxy groups and clusters

Deep sky surveys show that galaxies are often found ingroups and clusters. Solitary galaxies that have not sig-nificantly interacted with another galaxy of comparablemass during the past billion years are relatively scarce.Only about 5% of the galaxies surveyed have been foundto be truly isolated; however, these isolated formationsmay have interacted and even merged with other galaxiesin the past, and may still be orbited by smaller, satellitegalaxies. Isolated galaxies[note 2] can produce stars at ahigher rate than normal, as their gas is not being strippedby other nearby galaxies.[108]

On the largest scale, the Universe is continually expand-ing, resulting in an average increase in the separation be-tween individual galaxies (see Hubble’s law). Associa-tions of galaxies can overcome this expansion on a localscale through their mutual gravitational attraction. Theseassociations formed early in the Universe, as clumpsof dark matter pulled their respective galaxies together.Nearby groups later merged to form larger-scale clus-ters. This on-going merger process (as well as an in-flux of infalling gas) heats the inter-galactic gas withina cluster to very high temperatures, reaching 30–100megakelvins.[109] About 70–80% of the mass in a clus-ter is in the form of dark matter, with 10–30% consistingof this heated gas and the remaining few percent of thematter in the form of galaxies.[110]

18 CHAPTER 2. GALAXY

Seyfert’s Sextet is an example of a compact galaxy group.

Most galaxies in the Universe are gravitationally boundto a number of other galaxies. These form a fractal-like hierarchical distribution of clustered structures, withthe smallest such associations being termed groups. Agroup of galaxies is the most common type of galac-tic cluster, and these formations contain a majority ofthe galaxies (as well as most of the baryonic mass) inthe Universe.[111][112] To remain gravitationally bound tosuch a group, each member galaxy must have a suffi-ciently low velocity to prevent it from escaping (see Virialtheorem). If there is insufficient kinetic energy, however,the group may evolve into a smaller number of galaxiesthrough mergers.[113]

Structures consisting of anywhere from hundreds to thou-sands of galaxies bound together by gravity are calledclusters.[114] Clusters of galaxies are often dominated by asingle giant elliptical galaxy, known as the brightest clus-ter galaxy, which, over time, tidally destroys its satellitegalaxies and adds their mass to its own.[115]

Superclusters contain tens of thousands of galaxies,which are found in clusters, groups and sometimesindividually. At the supercluster scale, galaxies arearranged into sheets and filaments surrounding vastempty voids.[116] Above this scale, the Universe ap-pears to be the same in all directions (isotropic andhomogeneous).[117]

The Milky Way galaxy is a member of an associationnamed the Local Group, a relatively small group of galax-ies that has a diameter of approximately one megaparsec.The Milky Way and the Andromeda Galaxy are the twobrightest galaxies within the group; many of the othermember galaxies are dwarf companions of these twogalaxies.[118] The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, ex-tended structure of groups and clusters of galaxies cen-tered on the Virgo Cluster.[119] And the Virgo Superclus-ter itself is a part of the Pisces-Cetus Supercluster Com-plex, a giant galaxy filament.

2.8 Multi-wavelength observation

See also: Observational astronomy

This ultraviolet image of Andromeda shows blue regionscontaining young, massive stars.

The peak radiation of most stars lies in the visible spec-trum, so the observation of the stars that form galaxies hasbeen a major component of optical astronomy. It is alsoa favorable portion of the spectrum for observing ionizedH II regions, and for examining the distribution of dustyarms.The dust present in the interstellar medium is opaque tovisual light. It is more transparent to far-infrared, whichcan be used to observe the interior regions of giant molec-ular clouds and galactic cores in great detail.[120] Infraredis also used to observe distant, red-shifted galaxies thatwere formed much earlier in the history of the Universe.Water vapor and carbon dioxide absorb a number of use-ful portions of the infrared spectrum, so high-altitude orspace-based telescopes are used for infrared astronomy.The first non-visual study of galaxies, particularly ac-tive galaxies, was made using radio frequencies. The at-mosphere is nearly transparent to radio between 5 MHzand 30 GHz. (The ionosphere blocks signals below thisrange.)[121] Large radio interferometers have been used tomap the active jets emitted from active nuclei. Radio tele-scopes can also be used to observe neutral hydrogen (via21 cm radiation), including, potentially, the non-ionizedmatter in the early Universe that later collapsed to formgalaxies.[122]

Ultraviolet and X-ray telescopes can observe highly en-ergetic galactic phenomena. An ultraviolet flare was ob-served when a star in a distant galaxy was torn apart fromthe tidal forces of a black hole.[123] The distribution of hotgas in galactic clusters can be mapped by X-rays. The ex-istence of supermassive black holes at the cores of galax-ies was confirmed through X-ray astronomy.[124]

2.11. REFERENCES 19

2.9 See also

• Dark galaxy

• Galactic orientation

• Galaxy formation and evolution

• Illustris project

• List of galaxies

• List of nearest galaxies

• Luminous infrared galaxy

• Supermassive black hole

• Timeline of knowledge about galaxies, clusters ofgalaxies, and large-scale structure

2.10 Notes[1] Galaxies to the left side of the Hubble classification

scheme are sometimes referred to as “early-type”, whilethose to the right are “late-type”.

[2] The term “field galaxy” is sometimes used to mean an iso-lated galaxy, although the same term is also used to de-scribe galaxies that do not belong to a cluster but may bea member of a group of galaxies.

2.11 References[1] Sparke & Gallagher III 2000, p. i

[2] Hupp, E.; Roy, S.; Watzke, M. (August 12, 2006).“NASA Finds Direct Proof of Dark Matter”. NASA. Re-trieved April 17, 2007.

[3] Uson, J. M.; Boughn, S. P.; Kuhn, J. R. (1990). “The cen-tral galaxy in Abell 2029 – An old supergiant”. Science250 (4980): 539–540. Bibcode:1990Sci...250..539U.doi:10.1126/science.250.4980.539.

[4] Hoover, A. (June 16, 2003). “UF Astronomers: UniverseSlightly Simpler Than Expected”. Hubble News Desk.Retrieved March 4, 2011. Based upon:

• Graham, A. W.; Guzman, R. (2003). “HSTPhotometry of Dwarf Elliptical Galaxies inComa, and an Explanation for the AllegedStructural Dichotomy between Dwarf andBright Elliptical Galaxies”. AstronomicalJournal 125 (6): 2936–2950. arXiv:astro-ph/0303391. Bibcode:2003AJ....125.2936G.doi:10.1086/374992.

[5] Jarrett, T. H. “Near-Infrared Galaxy Morphology Atlas”.California Institute of Technology. Retrieved January 9,2007.

[6] Finley, D.; Aguilar, D. (November 2, 2005).“Astronomers Get Closest LookYet AtMilkyWay’sMys-terious Core”. National Radio Astronomy Observatory.Retrieved August 10, 2006.

[7] Gott III, J. R. et al. (2005). “A Map of theUniverse”. Astrophysical Journal 624 (2): 463–484.arXiv:astro-ph/0310571. Bibcode:2005ApJ...624..463G.doi:10.1086/428890.

[8] “Galaxy Clusters and Large-Scale Structure”. Universityof Cambridge. Retrieved January 15, 2007.

[9] Harper, D. “galaxy”. Online Etymology Dictionary. Re-trieved November 11, 2011.

[10] Waller & Hodge 2003, p. 91

[11] Koneãn˘, Lubomír. “Emblematics, Agriculture, andMythography in The Origin of theMilkyWay”. Academyof Sciences of the Czech Republic. Archived from theoriginal on July 20, 2006. Retrieved January 5, 2007.

[12] Rao, J. (September 2, 2005). “Explore the Archer’sRealm”. Space.com. Retrieved January 3, 2007.

[13] Bodifée G. & Berger M. (2010). “CNG-Catalogue ofNamed Galaxies”. Retrieved January 17, 2014.

[14] “Contemporary Latin”. Retrieved January 22, 2014.

[15] Plutarch (2006). The Complete Works Volume 3: Essaysand Miscellanies. Chapter 3: Echo Library. p. 66. ISBN978-1-4068-3224-2.

[16] Montada, J. P. (September 28, 2007). “Ibn Bajja”.Stanford Encyclopedia of Philosophy. Retrieved July 11,2008.

[17] Heidarzadeh 2008, pp. 23–25

[18] Mohamed 2000, pp. 49–50

[19] Bouali, H.-E.; Zghal, M.; Lakhdar, Z. B. (2005).“Popularisation of Optical Phenomena: Establishing theFirst Ibn Al-Haytham Workshop on Photography”. TheEducation and Training in Optics and Photonics Confer-ence. Retrieved July 8, 2008.

[20] O'Connor, John J.; Robertson, Edmund F., “Abu RayhanMuhammad ibn Ahmad al-Biruni”, MacTutor History ofMathematics archive, University of St Andrews.

[21] Al-Biruni 2004, p. 87

[22] Heidarzadeh 2008, p. 25, Table 2.1

[23] Livingston, J. W. (1971). “Ibn Qayyim al-Jawziyyah:A Fourteenth Century Defense against Astrological Div-ination and Alchemical Transmutation”. Journal ofthe American Oriental Society 91 (1): 96–103 [99].doi:10.2307/600445. JSTOR 600445.

[24] Galileo Galilei, Sidereus Nuncius (Venice, (Italy): ThomasBaglioni, 1610), pages 15 and 16.English translation: Galileo Galilei with Edward StaffordCarlos, trans., The Sidereal Messenger (London, England:Rivingtons, 1880), pages 42 and 43.

20 CHAPTER 2. GALAXY

[25] O'Connor, J. J.; Robertson, E. F. (November 2002).“Galileo Galilei”. University of St. Andrews. RetrievedJanuary 8, 2007.

[26] Thomas Wright, An Original Theory or New Hypothe-sis of the Universe … (London, England: H. Chapelle,1750). From p.48: " … the stars are not infinitely dis-persed and distributed in a promiscuous manner through-out all the mundane space, without order or design, …this phænomenon [is] no other than a certain effect aris-ing from the observer’s situation, … To a spectator placedin an indefinite space, … it [i.e., the Milky Way (ViaLactea)] [is] a vast ring of stars … "On page 73, Wright called theMilkyWay theVortex Mag-nus (the great whirlpool) and estimated its diameter at8.64×1012 miles (13.9×1012 km).

[27] Evans, J. C. (November 24, 1998). “Our Galaxy”. GeorgeMason University. Retrieved January 4, 2007.

[28] Immanuel Kant, Allgemeine Naturgeschichte und Theoriedes Himmels… [Universal Natural History and Theory ofthe Heavens … ], (Koenigsberg and Leipzig, (Germany):Johann Friederich Petersen, 1755).Available in English translation by Ian Johnston at:Vancouver Island University, British Columbia, Canada

[29] William Herschel (1785) “On the Construction of theHeavens,” Philosophical Transactions of the Royal Soci-ety of London, 75 : 213-266. Herschel’s diagram of thegalaxy appears immediately after the article’s last page.See:

• Google Books• The Royal Society of London

[30] Paul 1993, pp. 16–18

[31] Trimble, V. (1999). “Robert Trumpler and the(Non)transparency of Space”. Bulletin of theAmerican Astronomical Society 31 (31): 1479.Bibcode:1999AAS...195.7409T.

[32] Kepple & Sanner 1998, p. 18

[33] “Abd-al-Rahman Al Sufi (December 7, 903 – May 25,986 A.D.)". Observatoire de Paris. Retrieved April 19,2007.

[34] “The Large Magellanic Cloud, LMC”. Observatoire deParis. Retrieved April 19, 2007.

[35] See text quoted from Wright’s An original theory or newhypothesis of the Universe in Dyson, F. (1979). Disturbingthe Universe. Pan Books. p. 245. ISBN 0-330-26324-2.

[36] “Parsonstown | The genius of the Parsons family | WilliamRosse”. parsonstown.info.

[37] Slipher, V. M. (1913). “The radial velocity of the An-dromeda Nebula”. Lowell Observatory Bulletin 1: 56–57.Bibcode:1913LowOB...2...56S.

[38] Slipher, V. M. (1915). “Spectrographic Observa-tions of Nebulae”. Popular Astronomy 23: 21–24.Bibcode:1915PA.....23...21S.

[39] Curtis, H. D. (1988). “Novae in Spiral Nebu-lae and the Island Universe Theory”. Publicationsof the Astronomical Society of the Pacific 100: 6.Bibcode:1988PASP..100....6C. doi:10.1086/132128.

[40] Weaver, H. F. “Robert Julius Trumpler”. US NationalAcademy of Sciences. Retrieved January 5, 2007.

[41] Öpik, E. (1922). “An estimate of the distance of theAndromeda Nebula”. Astrophysical Journal 55: 406.Bibcode:1922ApJ....55..406O. doi:10.1086/142680.

[42] Hubble, E. P. (1929). “A spiral nebula as a stellar sys-tem, Messier 31”. Astrophysical Journal 69: 103–158.Bibcode:1929ApJ....69..103H. doi:10.1086/143167.

[43] Sandage, A. (1989). “Edwin Hubble, 1889–1953”.Journal of the Royal Astronomical Society of Canada83 (6): 351–362. Bibcode:1989JRASC..83..351S. Re-trieved January 8, 2007.

[44] Tenn, J. “Hendrik Christoffel van deHulst”. Sonoma StateUniversity. Retrieved January 5, 2007.

[45] López-Corredoira, M. et al. (2001). “Searching for thein-plane Galactic bar and ring in DENIS”. Astronomy andAstrophysics 373 (1): 139–152. arXiv:astro-ph/0104307.Bibcode:2001A&A...373..139L. doi:10.1051/0004-6361:20010560.

[46] Rubin, V. C. (1983). “Dark matter in spi-ral galaxies”. Scientific American 248 (6):96–106. Bibcode:1983SciAm.248...96R.doi:10.1038/scientificamerican0683-96.

[47] Rubin, V. C. (2000). “One Hundred Years ofRotating Galaxies”. Publications of the Astronom-ical Society of the Pacific 112 (772): 747–750.Bibcode:2000PASP..112..747R. doi:10.1086/316573.

[48] “Hubble Rules Out a Leading Explanation for Dark Mat-ter”. Hubble News Desk. October 17, 1994. RetrievedJanuary 8, 2007.

[49] “How many galaxies are there?". NASA. November 27,2002. Retrieved January 8, 2007.

[50] Kraan-Korteweg, R. C.; Juraszek, S. (2000). “Mappingthe hidden Universe: The galaxy distribution in the Zoneof Avoidance”. Publications of the Astronomical Soci-ety of Australia 17 (1): 6–12. arXiv:astro-ph/9910572.Bibcode:1999astro.ph.10572K. doi:10.1071/AS00006.

[51] Barstow, M. A. (2005). “Elliptical Galaxies”. LeicesterUniversity Physics Department. Retrieved June 8, 2006.

[52] “Galaxies”. Cornell University. October 20, 2005. Re-trieved August 10, 2006.

[53] Williams, M. J.; Bureau, M.; Cappellari, M. (2009).“Kinematic constraints on the stellar and dark mat-ter content of spiral and S0 galaxies”. Monthly No-tices of the Royal Astronomical Society 400 (4): 1665.doi:10.1111/j.1365-2966.2009.15582.x.

[54] Smith, G. (March 6, 2000). “Galaxies— The Spiral Neb-ulae”. University of California, San Diego Center for As-trophysics & Space Sciences. Retrieved November 30,2006.

2.11. REFERENCES 21

[55] Van den Bergh 1998, p. 17

[56] “Fat or flat: Getting galaxies into shape”. phys.org. Febru-ary 2014

[57] Bertin & Lin 1996, pp. 65–85

[58] Belkora 2003, p. 355

[59] Eskridge, P. B.; Frogel, J. A. (1999). “Whatis the True Fraction of Barred Spiral Galax-ies?". Astrophysics and Space Science. 269/270:427–430. Bibcode:1999Ap&SS.269..427E.doi:10.1023/A:1017025820201.

[60] Bournaud, F.; Combes, F. (2002). “Gas accretion on spi-ral galaxies: Bar formation and renewal”. Astronomy andAstrophysics 392 (1): 83–102. arXiv:astro-ph/0206273.Bibcode:2002A&A...392...83B. doi:10.1051/0004-6361:20020920.

[61] Knapen, J. H.; Perez-Ramirez, D.; Laine, S. (2002).“Circumnuclear regions in barred spiral galaxies — II.Relations to host galaxies”. Monthly Notices of the RoyalAstronomical Society 337 (3): 808–828. arXiv:astro-ph/0207258. Bibcode:2002MNRAS.337..808K.doi:10.1046/j.1365-8711.2002.05840.x.

[62] Alard, C. (2001). “Another bar in the Bulge”. Astronomyand Astrophysics Letters 379 (2): L44–L47. arXiv:astro-ph/0110491. Bibcode:2001A&A...379L..44A.doi:10.1051/0004-6361:20011487.

[63] Sanders, R. (January 9, 2006). “Milky Way galaxy iswarped and vibrating like a drum”. UCBerkeley News.Retrieved May 24, 2006.

[64] Bell, G. R.; Levine, S. E. (1997). “Mass of theMilkyWayand Dwarf Spheroidal Stream Membership”. Bulletinof the American Astronomical Society 29 (2): 1384.Bibcode:1997AAS...19110806B.

[65] Gerber, R. A.; Lamb, S. A.; Balsara, D. S. (1994). “RingGalaxy Evolution as a Function of “Intruder” Mass”.Bulletin of the American Astronomical Society 26: 911.Bibcode:1994AAS...184.3204G.

[66] “ISO unveils the hidden rings of Andromeda” (Press re-lease). European Space Agency. October 14, 1998. Re-trieved May 24, 2006.

[67] “Spitzer Reveals What Edwin Hubble Missed”. Harvard-Smithsonian Center for Astrophysics. May 31, 2004. Re-trieved December 6, 2006.

[68] Barstow, M. A. (2005). “Irregular Galaxies”. Universityof Leicester. Retrieved December 5, 2006.

[69] Phillipps, S.; Drinkwater, M. J.; Gregg, M. D.; Jones,J. B. (2001). “Ultracompact Dwarf Galaxies in the For-nax Cluster”. Astrophysical Journal 560 (1): 201–206.arXiv:astro-ph/0106377. Bibcode:2001ApJ...560..201P.doi:10.1086/322517.

[70] Groshong, K. (April 24, 2006). “Strange satellite galaxiesrevealed around Milky Way”. New Scientist. RetrievedJanuary 10, 2007.

[71] Schirber, M. (August 27, 2008). “No Slimming Downfor Dwarf Galaxies”. ScienceNOW. Retrieved August 27,2008.

[72] “Galaxy Interactions”. University of Maryland Depart-ment of Astronomy. Archived from the original on May9, 2006. Retrieved December 19, 2006.

[73] “Interacting Galaxies”. Swinburne University. RetrievedDecember 19, 2006.

[74] “Happy Sweet Sixteen, Hubble Telescope!". NASA.April 24, 2006. Retrieved August 10, 2006.

[75] “Starburst Galaxies”. Harvard-Smithsonian Center forAstrophysics. August 29, 2006. Retrieved August 10,2006.

[76] Kennicutt Jr., R. C. et al. (2005). Demographicsand Host Galaxies of Starbursts. Starbursts: From 30Doradus to Lyman Break Galaxies (Springer): 187.Bibcode:2005sdlb.proc..187K.

[77] Smith, G. (July 13, 2006). “Starbursts & Colliding Galax-ies”. University of California, San Diego Center for As-trophysics & Space Sciences. Retrieved August 10, 2006.

[78] Keel, B. (September 2006). “Starburst Galaxies”.University of Alabama. Retrieved December 11, 2006.

[79] Keel, W. C. (2000). “Introducing Active Galactic Nu-clei”. University of Alabama. Retrieved December 6,2006.

[80] Lochner, J.; Gibb, M. “AMonster in theMiddle”. NASA.Retrieved December 20, 2006.

[81] Heckman, T. M. (1980). “An optical and radio survey ofthe nuclei of bright galaxies — Activity in normal galac-tic nuclei”. Astronomy and Astrophysics 87: 152–164.Bibcode:1980A&A....87..152H.

[82] Ho, L. C.; Filippenko, A. V.; Sargent, W. L. W. (1997).“A Search for “Dwarf” Seyfert Nuclei. V. Demographicsof Nuclear Activity in Nearby Galaxies”. AstrophysicalJournal 487 (2): 568–578. arXiv:astro-ph/9704108.Bibcode:1997ApJ...487..568H. doi:10.1086/304638.

[83] “Construction Secrets of a Galactic Metropolis”. www.eso.org. ESO Press Release. Retrieved October 15, 2014.

[84] “Search for Submillimeter Protogalaxies”. Harvard-Smithsonian Center for Astrophysics. November 18,1999. Retrieved January 10, 2007.

[85] Firmani, C.; Avila-Reese, V. (2003). “Physicalprocesses behind the morphological Hubble se-quence”. Revista Mexicana de Astronomía y As-trofísica 17: 107–120. arXiv:astro-ph/0303543.Bibcode:2003RMxAC..17..107F.

[86] McMahon, R. (2006). “Journey to the birthof the Universe”. Nature 443 (7108): 151–2.Bibcode:2006Natur.443..151M. doi:10.1038/443151a.PMID 16971933.

[87] Wall, Mike (December 12, 2012). “Ancient Galaxy MayBe Most Distant Ever Seen”. Space.com. Retrieved De-cember 12, 2012.

22 CHAPTER 2. GALAXY

[88] “Cosmic Detectives”. The European Space Agency(ESA). April 2, 2013. Retrieved April 15, 2013.

[89] Eggen, O. J.; Lynden-Bell, D.; Sandage, A. R. (1962).“Evidence from the motions of old stars that the Galaxycollapsed”. Reports on Progress in Physics 136: 748.Bibcode:1962ApJ...136..748E. doi:10.1086/147433.

[90] Searle, L.; Zinn, R. (1978). “Compositions ofhalo clusters and the formation of the galactichalo”. Astrophysical Journal 225 (1): 357–379.Bibcode:1978ApJ...225..357S. doi:10.1086/156499.

[91] Heger, A.; Woosley, S. E. (2002). “The Nucle-osynthetic Signature of Population III”. AstrophysicalJournal 567 (1): 532–543. arXiv:astro-ph/0107037.Bibcode:2002ApJ...567..532H. doi:10.1086/338487.

[92] Barkana, R.; Loeb, A. (1999). “In the beginning: the firstsources of light and the reionization of the Universe”.Physics Reports 349 (2): 125–238. arXiv:astro-ph/0010468. Bibcode:2001PhR...349..125B.doi:10.1016/S0370-1573(01)00019-9.

[93] “Simulations Show How Growing Black Holes RegulateGalaxy Formation”. Carnegie Mellon University. Febru-ary 9, 2005. Retrieved January 7, 2007.

[94] Massey, R. (April 21, 2007). “Caught in the act; forminggalaxies captured in the young Universe”. Royal Astro-nomical Society. Retrieved April 20, 2007.

[95] Noguchi, M. (1999). “Early Evolution of Disk Galax-ies: Formation of Bulges in Clumpy Young Galac-tic Disks”. Astrophysical Journal 514 (1): 77–95.arXiv:astro-ph/9806355. Bibcode:1999ApJ...514...77N.doi:10.1086/306932.

[96] Baugh, C.; Frenk, C. (May 1999). “How are galaxiesmade?". PhysicsWeb. Retrieved January 16, 2007.

[97] Gonzalez, G. (1998). The Stellar Metallicity —Planet Connection. Proceedings of a workshopon brown dwarfs and extrasolar planets: 431.Bibcode:1998bdep.conf..431G.

[98] Moskowitz, Clara (September 25, 2012). “HubbleTelescope Reveals Farthest View Into Universe Ever”.Space.com. Retrieved September 26, 2012.

[99] Conselice, C. J. (February 2007). “The Universe’s In-visible Hand”. Scientific American 296 (2): 35–41.doi:10.1038/scientificamerican0207-34.

[100] Ford, H. et al. (April 30, 2002). “Hubble’s New CameraDelivers Breathtaking Views of the Universe”. HubbleNews Desk. Retrieved May 8, 2007.

[101] Struck, C. (1999). “Galaxy Collisions”. PhysicsReports 321: 1. arXiv:astro-ph/9908269.Bibcode:1999PhR...321....1S. doi:10.1016/S0370-1573(99)00030-7.

[102] Wong, J. (April 14, 2000). “Astrophysicist maps out ourown galaxy’s end”. University of Toronto. Archived fromthe original on January 8, 2007. Retrieved January 11,2007.

[103] Panter, B.; Jimenez, R.; Heavens, A. F.; Charlot, S.(2007). “The star formation histories of galaxies in theSloan Digital Sky Survey”. Monthly Notices of the RoyalAstronomical Society 378 (4): 1550–1564. arXiv:astro-ph/0608531. Bibcode:2007MNRAS.378.1550P.doi:10.1111/j.1365-2966.2007.11909.x.

[104] Kennicutt Jr., R. C.; Tamblyn, P.; Congdon, C.E. (1994). “Past and future star formation in diskgalaxies”. Astrophysical Journal 435 (1): 22–36.Bibcode:1994ApJ...435...22K. doi:10.1086/174790.

[105] Knapp, G. R. (1999). Star Formation in EarlyType Galaxies. Astronomical Society of the Pa-cific. Bibcode:1998astro.ph..8266K. ISBN 1-886733-84-8. OCLC 41302839.

[106] Adams, Fred; Laughlin, Greg (July 13, 2006). “The GreatCosmic Battle”. Astronomical Society of the Pacific. Re-trieved January 16, 2007.

[107] Pobojewski, S. (January 21, 1997). “Physics offersglimpse into the dark side of the Universe”. Universityof Michigan. Retrieved January 13, 2007.

[108] McKee, M. (June 7, 2005). “Galactic loners producemore stars”. New Scientist. Retrieved January 15, 2007.

[109] “Groups & Clusters of Galaxies”. NASA/Chandra. Re-trieved January 15, 2007.

[110] Ricker, P. “When Galaxy Clusters Collide”. San DiegoSupercomputer Center. Retrieved August 27, 2008.

[111] Dahlem, M. (November 24, 2006). “Optical and ra-dio survey of Southern Compact Groups of galaxies”.University of Birmingham Astrophysics and Space Re-search Group. Archived from the original on June 13,2007. Retrieved January 15, 2007.

[112] Ponman, T. (February 25, 2005). “Galaxy Systems:Groups”. University of Birmingham Astrophysics andSpace Research Group. Retrieved January 15, 2007.

[113] Girardi, M.; Giuricin, G. (2000). “The ObservationalMass Function of Loose Galaxy Groups”. The Astrophys-ical Journal 540 (1): 45–56. arXiv:astro-ph/0004149.Bibcode:2000ApJ...540...45G. doi:10.1086/309314.

[114] “Hubble Pinpoints Furthest Protocluster of Galaxies EverSeen”. ESA/Hubble Press Release. Retrieved January 22,2015.

[115] Dubinski, J. (1998). “The Origin of the Brightest Clus-ter Galaxies”. Astrophysical Journal 502 (2): 141–149.arXiv:astro-ph/9709102. Bibcode:1998ApJ...502..141D.doi:10.1086/305901.

[116] Bahcall, N. A. (1988). “Large-scale structurein the Universe indicated by galaxy clusters”.Annual Review of Astronomy and Astrophysics 26(1): 631–686. Bibcode:1988ARA&A..26..631B.doi:10.1146/annurev.aa.26.090188.003215.

[117] Mandolesi, N. et al. (1986). “Large-scale homo-geneity of the Universe measured by the microwavebackground”. Letters to Nature 319 (6056): 751–753.Bibcode:1986Natur.319..751M. doi:10.1038/319751a0.

2.13. EXTERNAL LINKS 23

[118] van den Bergh, S. (2000). “Updated Information on theLocal Group”. Publications of the Astronomical Society ofthe Pacific 112 (770): 529–536. arXiv:astro-ph/0001040.Bibcode:2000PASP..112..529V. doi:10.1086/316548.

[119] Tully, R. B. (1982). “The Local Superclus-ter”. Astrophysical Journal 257: 389–422.Bibcode:1982ApJ...257..389T. doi:10.1086/159999.

[120] “Near, Mid&Far Infrared”. IPAC/NASA.Retrieved Jan-uary 2, 2007.

[121] “The Effects of Earth’s Upper Atmosphere on Radio Sig-nals”. NASA. Retrieved August 10, 2006.

[122] “Giant Radio Telescope Imaging Could Make Dark Mat-ter Visible”. ScienceDaily. December 14, 2006. Re-trieved January 2, 2007.

[123] “NASA Telescope Sees Black Hole Munch on a Star”.NASA. December 5, 2006. Retrieved January 2, 2007.

[124] Dunn, R. “An Introduction toX-rayAstronomy”. Instituteof Astronomy X-Ray Group. Retrieved January 2, 2007.

2.11.1 Other references

• “Unveiling the Secret of a Virgo Dwarf Galaxy”.ESO. May 3, 2000. Retrieved January 3, 2007.

2.12 Bibliography

• Al-Biruni (2004). The Book of Instruction in the El-ements of the Art of Astrology. R. Ramsay Wright(transl.). Kessinger Publishing. ISBN 0-7661-9307-1.

• Belkora, L. (2003). Minding the Heavens: the Storyof our Discovery of the Milky Way. CRC Press.ISBN 0-7503-0730-7.

• Bertin, G.; Lin, C.-C. (1996). Spiral Structure inGalaxies: a DensityWave Theory. MIT Press. ISBN0-262-02396-2.

• Binney, J.; Merrifield, M. (1998). Galactic Astron-omy. Princeton University Press. ISBN 0-691-00402-1. OCLC 39108765.

• Dickinson, T. (2004). The Universe and Beyond(4th ed.). Firefly Books. ISBN 1-55297-901-6.OCLC 55596414.

• Heidarzadeh, T. (2008). A History of Physical The-ories of Comets, from Aristotle to Whipple. Springer.ISBN 1-4020-8322-X.

• Ho, Houjun; van den Bosch, Frank; White, Si-mon (2010). Galaxy Formation and Evolution (1ed.). Cambridge University Press. ISBN 978-0521857932.

• Kepple, G. R.; Sanner, G. W. (1998). The Night SkyObserver’s Guide, Volume 1. Willmann-Bell. ISBN0-943396-58-1.

• Merritt, D. (2013). Dynamics and Evolution ofGalactic Nuclei. Princeton University Press. ISBN9781400846122.

• Mohamed, M. (2000). Great Muslim Mathe-maticians. Penerbit UTM. ISBN 983-52-0157-9.OCLC 48759017.

• Paul, E. R. (1993). The Milky Way Galaxy and Sta-tistical Cosmology, 1890–1924. Cambridge Univer-sity Press. ISBN 0-521-35363-7.

• Sparke, L. S.; Gallagher III, J. S. (2000). Galaxiesin the Universe: An Introduction. Cambridge Uni-versity Press. ISBN 0-521-59740-4.

• Van den Bergh, S. (1998). Galaxy Morphology andClassification. Cambridge University Press. ISBN0-521-62335-9.

• Waller, W. H.; Hodge, P. W. (2003). Galaxiesand the Cosmic Frontier. Harvard University Press.ISBN 0-674-01079-5.

2.13 External links•

• Galaxies on In Our Time at the BBC. (listen now)

• Galaxies, SEDS Messier pages

• An Atlas of The Universe

• Galaxies — Information and amateur observations

• The Oldest Galaxy Yet Found

• Galaxy classification project, harnessing the powerof the internet and the human brain

• How many galaxies are in our Universe?

• The most beautiful galaxies on Astronoo

• 3-D Video (01:46) – Over a Million Galaxies of Bil-lions of Stars each – BerkeleyLab/animated.

Chapter 3

Meteor shower

This article is about the falling of meteors. For the TVprogram, see Meteor Shower (TV series).A meteor shower is a celestial event in which a num-

Four hour time lapse exposure of sky

Leonids from space

ber of meteors are observed to radiate, or originate, from

Meteor shower over High Desert (California), with unknown air-borne object in left of frame. (March 7, 2015)

one point in the night sky. These meteors are causedby streams of cosmic debris called meteoroids enteringEarth’s atmosphere at extremely high speeds on paralleltrajectories. Most meteors are smaller than a grain ofsand, so almost all of them disintegrate and never hit theEarth’s surface. Intense or unusual meteor showers areknown as meteor outbursts and meteor storms, whichmay produce greater than 1,000 meteors an hour.[1] TheMeteor Data Center lists about 600 suspected meteorshowers of which about 100 are well established.[2]

3.1 Historical developments

The first great storm in modern times was the Leonids ofNovember 1833. One estimate is over one hundred thou-sand meteors an hour,[3] but another, done as the stormabated, estimated in excess of two hundred thousand me-teors an hour[4] over the entire region of North Amer-ica east of the Rocky Mountains. American DenisonOlmsted (1791−1859) explained the event most accu-rately. After spending the last weeks of 1833 collectinginformation he presented his findings in January 1834 tothe American Journal of Science and Arts, published inJanuary–April 1834,[5] and January 1836.[6] He noted theshower was of short duration and was not seen in Europe,and that the meteors radiated from a point in the con-stellation of Leo and he speculated the meteors had orig-inated from a cloud of particles in space.[7] Work con-tinued, however, coming to understand the annual natureof showers though the occurrences of storms perplexed

24

3.2. RADIANT POINT 25

Diagram from 1872

researchers.[8]

In the 1890s, Irish astronomer George Johnstone Stoney(1826–1911) and British astronomer Arthur MatthewWeld Downing (1850–1917), were the first to attempt tocalculate the position of the dust at Earth’s orbit. Theystudied the dust ejected in 1866 by comet 55P/Tempel-Tuttle in advance of the anticipated Leonid shower re-turn of 1898 and 1899. Meteor storms were anticipated,but the final calculations showed that most of the dustwould be far inside of Earth’s orbit. The same resultswere independently arrived at by Adolf Berberich of theKönigliches Astronomisches Rechen Institut (Royal As-tronomical Computation Institute) in Berlin, Germany.Although the absence of meteor storms that season con-firmed the calculations, the advance of much better com-puting tools was needed to arrive at reliable predictions.In 1981 Donald K. Yeomans of the Jet Propulsion Lab-oratory reviewed the history of meteor showers for theLeonids and the history of the dynamic orbit of CometTempel-Tuttle.[9] A graph [10] from it was adapted and re-published in Sky and Telescope.[11] It showed relative po-

sitions of the Earth and Tempel-Tuttle and marks whereEarth encountered dense dust. This showed that the me-teoroids are mostly behind and outside the path of thecomet, but paths of the Earth through the cloud of parti-cles resulting in powerful storms were very near paths ofnearly no activity.In 1985, E. D. Kondrat'eva and E. A. Reznikov ofKazan State University first correctly identified the yearswhen dust was released which was responsible for severalpast Leonid meteor storms. In 1995, Peter Jenniskenspredicted the 1995 Alpha Monocerotids outburst fromdust trails.[12] In anticipation of the 1999 Leonid storm,Robert H. McNaught,[13] David Asher,[14] and Finland’sEsko Lyytinen were the first to apply this method in theWest.[15][16] In 2006 Jenniskens has published predic-tions for future dust trail encounters covering the next50 years.[17] Jérémie Vaubaillon continues to update pre-dictions based on observartions each year for the Insti-tut de Mécanique Céleste et de Calcul des Éphémérides(IMCCE).[18]

3.2 Radiant point

Meteor shower on chart

Because meteor shower particles are all traveling in par-allel paths, and at the same velocity, they will all appearto an observer below to radiate away from a single pointin the sky. This radiant point is caused by the effect ofperspective, similar to parallel railroad tracks convergingat a single vanishing point on the horizon when viewedfrom the middle of the tracks. Meteor showers are almostalways named after the constellation from which the me-teors appear to originate. This “fixed point” slowly movesacross the sky during the night due to the Earth turning onits axis, the same reason the stars appear to slowly marchacross the sky. The radiant also moves slightly from nightto night against the background stars (radiant drift) dueto the Earth moving in its orbit around the sun. See IMOMeteor Shower Calendar 2007 (International Meteor Or-ganization) for maps of drifting “fixed points.”

26 CHAPTER 3. METEOR SHOWER

When the moving radiant is at the highest point it willreach in the observer’s sky that night, the sun will be justclearing the eastern horizon. For this reason, the bestviewing time for a meteor shower is generally slightly be-fore dawn— a compromise between the maximum num-ber of meteors available for viewing, and the lighteningsky which makes them harder to see.

3.3 Naming

Meteor showers are named after the nearest constella-tion or bright star with a Greek or Roman letter assignedthat is close to the radiant position at the peak of theshower, whereby the grammatical declension of the Latinpossessive form is replaced by “id” or “ids”. Hence, mete-ors radiating from near the star delta Aquarii (declension"-i”) are called delta Aquariids. The International Astro-nomical Union’s Task Group on Meteor Shower Nomen-clature and the IAU’s Meteor Data Center keep track ofmeteor shower nomenclature and which showers are es-tablished.

3.4 Origin of meteoroid streams

Comet Encke's meteoroid trail is the diagonal red glow

A meteor shower is the result of an interaction betweena planet, such as Earth, and streams of debris from acomet. Comets can produce debris by water vapor drag,as demonstrated by Fred Whipple in 1951,[19] and bybreakup. Whipple envisioned comets as “dirty snow-balls,” made up of rock embedded in ice, orbiting the

Meteoroid trail between fragments of Comet 73P

Sun. The “ice” may be water, methane, ammonia, orother volatiles, alone or in combination. The “rock” mayvary in size from that of a dust mote to that of a smallboulder. Dust mote sized solids are orders of magnitudemore common than those the size of sand grains, which,in turn, are similarly more common than those the size ofpebbles, and so on. When the ice warms and sublimates,the vapor can drag along dust, sand, and pebbles.Each time a comet swings by the Sun in its orbit, some ofits ice vaporizes and a certain amount of meteoroids willbe shed. The meteoroids spread out along the entire orbitof the comet to form a meteoroid stream, also known asa “dust trail” (as opposed to a comet’s “dust tail” causedby the very small particles that are quickly blown away bysolar radiation pressure).Recently, Peter Jenniskens[17] has argued that most of ourshort-period meteor showers are not from the normal wa-ter vapor drag of active comets, but the product of in-frequent disintegrations, when large chunks break off amostly dormant comet. Examples are the Quadrantidsand Geminids, which originated from a breakup ofasteroid-looking objects, 2003 EH1 and 3200 Phaethon,respectively, about 500 and 1000 years ago. The frag-ments tend to fall apart quickly into dust, sand, and peb-bles, and spread out along the orbit of the comet to form adense meteoroid stream, which subsequently evolves intoEarth’s path.

3.5 Dynamical evolution of mete-oroid streams

Shortly after Whipple predicted that dust particles trav-elled at low speeds relative to the comet, Milos Plavecwas the first to offer the idea of a dust trail, when he cal-culated howmeteroids, once freed from the comet, woulddrift mostly in front of or behind the comet after com-pleting one orbit. The effect is simple orbital mechanics– the material drifts only a little laterally away from thecomet while drifting ahead or behind the comet becausesome particles make a wider orbit than others.[17] These

3.7. EXTRATERRESTRIAL METEOR SHOWERS 27

dust trails are sometimes observed in comet images takenat mid infrared wavelengths (heat radiation), where dustparticles from the previous return to the Sun are spreadalong the orbit of the comet (see figures).The gravitational pull of the planets determines where thedust trail would pass by Earth orbit, much like a gardenerdirecting a hose to water a distant plant. Most years, thosetrails would miss the Earth altogether, but in some yearsthe Earth is showered by meteors. This effect was firstdemonstrated from observations of the 1995 alphaMono-cerotids,[20][21] and from earlier not widely known identi-fications of past earth storms.Over longer periods of time, the dust trails can evolvein complicated ways. For example, the orbits of somerepeating comets, and meteoroids leaving them, are inresonant orbits with Jupiter or one of the other large plan-ets – so many revolutions of one will equal another num-ber of revolutions of the other. This creates a showercomponent called a filament.A second effect is a close encounter with a planet. Whenthe meteoroids pass by Earth, some are accelerated (mak-ing wider orbits around the Sun), others are decelerated(making shorter orbits), resulting in gaps in the dust trailin the next return (like opening a curtain, with grainspiling up at the beginning and end of the gap). Also,Jupiter’s perturbation can change sections of the dust traildramatically, especially for short period comets, when thegrains approach the big planet at their furthest point alongthe orbit around the Sun, moving most slowly. As a re-sult, the trail has a clumping, a braiding or a tangling ofcrescents, of each individual release of material.The third effect is that of radiation pressure which willpush less massive particles into orbits further from thesun – while more massive objects (responsible for bolidesor fireballs) will tend to be affected less by radiation pres-sure. This makes some dust trail encounters rich in brightmeteors, others rich in faint meteors. Over time, these ef-fects disperse the meteoroids and create a broader stream.Themeteors we see from these streams are part of annualshowers, because Earth encounters those streams everyyear at much the same rate.When the meteoroids collide with other meteoroids in thezodiacal cloud, they lose their stream association and be-come part of the “sporadic meteors” background. Longsince dispersed from any stream or trail, they form iso-lated meteors, not a part of any shower. These randommeteors will not appear to come from the radiant of themain shower.

3.6 Famous meteor showers

3.6.1 Perseid and Leonid meteor showers

The most visible meteor shower in most years are thePerseids, which peak on 12 August of each year at overone meteor per minute. NASA has a useful tool to cal-culate how many meteors per hour are visible from one’sobserving location.The Leonid meteor shower peaks around 17 Novemberof each year. Approximately every 33 years, the Leonidshower produces ameteor storm, peaking at rates of thou-sands of meteors per hour. Leonid storms gave birth tothe term meteor shower when it was first realised, duringthe November 1833 storm, that themeteors radiated fromnear the star Gamma Leonis. The last Leonid stormswere in 1999, 2001 (two), and 2002 (two). Before that,there were storms in 1767, 1799, 1833, 1866, 1867, and1966. When the Leonid shower is not storming it is lessactive than the Perseids.

3.6.2 Other meteor showers

Further information: List of meteor showers

Established meteor showers

Official names are given in the International AstronomicalUnion meteor shower list.[22]

3.7 Extraterrestrial meteor show-ers

Mars meteor by MER Spirit rover

Any other solar system body with a reasonably transpar-ent atmosphere can also have meteor showers. As theMoon is in the neighborhood of Earth it can have the

28 CHAPTER 3. METEOR SHOWER

same showers but have its own phenomena without anatmosphere per se (but kicking up a Sodium tail of theMoon for example).[37] NASA now maintains an ongo-ing database of observed impacts on the moon.[38] main-tained by the Marshall Space Flight Center whether froma shower or not.Many planets and moons have impact craters dating backlarge spans of time. But new craters, perhaps even relatedto meteor showers are possible. Mars, and thus its moons,is known to have meteor showers.[39] These have not beenobserved on other planets as yet but may be presumed toexist. For Mars in particular, although these are differentfrom the ones seen on Earth because the different orbitsof Mars and Earth relative to the orbits of comets. TheMartian atmosphere has less than one percent of the den-sity of Earth’s at ground level, at their upper edges, wheremeteoroids strike, the two are more similar. Because ofthe similar air pressure at altitudes for meteors, the ef-fects are much the same. Only the relatively slower mo-tion of the meteoroids due to increased distance from thesun shouldmarginally decrease meteor brightness. This issomewhat balanced in that the slower descent means thatMartian meteors have more time in which to ablate.[40]

On March 7, 2004, the panoramic camera on Mars Ex-ploration Rover Spirit recorded a streak which is now be-lieved to have been caused by a meteor from a Martianmeteor shower associated with comet 114P/Wiseman-Skiff. A strong display from this shower was expectedon December 20, 2007. Other showers speculated aboutare a “Lambda Geminid” shower associated with theEta Aquariids of Earth (i.e., both associated with Comet1P/Halley), a “Beta Canis Major” shower associated withComet 13P/Olbers, and “Draconids” from 5335 Damo-cles.[41]

Isolated massive impacts have been observed at Jupiter:The 1994 Comet Shoemaker–Levy 9 which formed abrief trail as well, and successive events since then (seeList of Jupiter events.) Meteors or meteor showers havebeen discussed for most of the objects in the solar sys-tem with an atmosphere: Mercury,[42] Venus,[43] Saturn’smoon Titan,[44] Neptune’s moon Triton,[45] and Pluto.[46]

3.8 See also• International Meteor Organization (IMO)

• List of meteor showers

• Radiant The point in the sky from which (to a plan-etary observer) meteors appear to originate.

• Zenith Hourly Rate

• American Meteor Society (AMS)

• North American Meteor Network

• Meteor procession

• Earth-grazing fireball

3.9 References[1] Jenniskens, P. (2006). Meteor Showers and their Parent

Comets. Cambridge University Press. ISBN 978-0-521-85349-1.

[2] Meteor Data Center list of Meteor Showers

[3] Space.com The 1833 Leonid Meteor Shower: A Fright-ening Flurry

[4] Leonid MAC Brief history of the Leonid shower

[5] Olmsted, Denison (1833). “Observations on the Meteorsof November 13th, 1833”. The American journal of sci-ence and arts 25: 363–411. Retrieved 21 May 2013.

[6] Olmsted, Denison (1836). “Facts respecting the MeteoricPhenomena of November 13th, 1834.”. The Americanjournal of science and arts 29 (1): 168–170.

[7] Observing the Leonids Gary W. Kronk

[8] F.W. Russell, Meteor Watch Organizer, by Richard Taibi, May 19, 2013, accessed 21 May 2013

[9] Yeomans, Donald K. (September 1981). “Comet Tempel-Tuttle and the Leonid meteors”. Icarus 47 (3): 492–499. Bibcode:1981Icar...47..492Y. doi:10.1016/0019-1035(81)90198-6{{inconsistent citations}}

[10] http://web.archive.org

[11] Comet 55P/Tempel-Tuttle and the Leonid Meteors(1996,see p. 6)

[12] Article published in 1997, notes prediction in 1995 - Jen-niskens, P.; Betlem, H.; De Lignie, M.; Langbroek, M.(1997). “The Detection of a Dust Trail in the Orbit ofan Earth-threatening Long-Period Comet”. Astrophys-ical Journal 479: 441. Bibcode:1997ApJ...479..441J.doi:10.1086/303853.

[13] Re: (meteorobs) Leonid Storm? By Rob McNaught,

[14] Blast from the Past Armagh Observatory press release1999 April 21st.

[15] Royal Astronomical Society Press Notice Ref. PN 99/27,Issued by: Dr Jacqueline Mitton RAS Press Officer

[16] Voyage through a comet’s trail, The 1998 Leonidssparkled over Canada By BBC Science’s Dr Chris Rileyon board NASA’s Leonid mission

[17] Jenniskens P. (2006). Meteor Showers and their ParentComets. Cambridge University Press, Cambridge, U.K.,790 pp.

[18] IMCCE Prediction page

[19] Whipple F. L. (1951). A Comet Model. II. Physical Re-lations for Comets and Meteors. Astrophys. J. 113, 464

3.9. REFERENCES 29

[20] Jenniskens P., 1997. Meteor steram activity IV. Meteoroutbursts ad the reflex motion of the Sun. Astron. Astro-phys. 317, 953–961.

[21] Jenniskens P., Betlem, H., De Lignie, M., Langbroek, M.(1997). The detection of a dust trail in the orbit of anEarth-threatening long-period comet. Astrohys. J. 479,441–447.

[22] “List of all meteor showers”. International AstronomicalUnion.

[23] Jenniskens, P. (March 2004). “2003 EH1 is the Quad-rantid shower parent comet”. Astronomical Journal127 (5): 3018–3022. Bibcode:2004AJ....127.3018J.doi:10.1086/383213.

[24] Ball, Phillip. Dead comet spawned New Year meteors,Nature online website, ISSN: 1744-7933, doi:10.1038/news031229-5, published online on December 31, 2003.

[25] Haines, Lester, Meteor shower traced to 1490 cometbreak-up: Quadrantid mystery solved, The Register, Jan-uary 8, 2008.

[26] Marco Micheli; Fabrizio Bernardi; David J. Tholen (May16, 2008). “Updated analysis of the dynamical relationbetween asteroid 2003 EH1 and comets C/1490 Y1 andC/1385 U1”. Monthly Notices of the Royal Astronomi-cal Society: Letters 390 (1): L6–L8. arXiv:0805.2452.Bibcode:2008MNRAS.390L...6M. doi:10.1111/j.1745-3933.2008.00510.x.

[27] Sekanina, Zdeněk; Chodas, Paul W. (December 2005).“Origin of the Marsden and Kracht Groups of SunskirtingComets. I. Association with Comet 96P/Machholz and ItsInterplanetary Complex”. Astrophysical Journal Supple-ment Series 161 (2): 551. Bibcode:2005ApJS..161..551S.doi:10.1086/497374.

[28] Jenniskens, P.; Vaubaillon, J. (2010). “Minor Planet2002 EX12 (=169P/NEAT) and the Alpha Capri-cornid Shower”. Astronomical Journal 139 (5): 1822–1830. Bibcode:2010AJ....139.1822J. doi:10.1088/0004-6256/139/5/1822.

[29] Jenniskens, P.; Vaubaillon, J. (2008). “MinorPlanet 2008 ED69 and the Kappa Cygnid MeteorShower”. Astronomical Journal 136 (2): 725–730.Bibcode:2008AJ....136..725J. doi:10.1088/0004-6256/136/2/725.

[30] Jenniskens, Peter; Vaubaillon, Jérémie (2007). “AnUnusual Meteor Shower on 1 September 2007”.Eos, Transactions, American Geophysical Union 88(32): 317–318. Bibcode:2007EOSTr..88..317J.doi:10.1029/2007EO320001.

[31] Porubčan, V.; Kornoš, L.; Williams, I.P. (2006).“The Taurid complex meteor showers and aster-oids”. Contributions of the Astronomical ObservatorySkalnaté Pleso 36: 103–117. arXiv:0905.1639.Bibcode:2006CoSka..36..103P.

[32] Jenniskens, P.; Vaubaillon, J. (2007). “3D/Bielaand the Andromedids: Fragmenting versus SublimatingComets”. The Astronomical Journal 134 (3): 1037.doi:10.1086/519074.

[33] Jenniskens, P.; Betlem, H.; De Lignie, M.; Langbroek,M. (1997). “The Detection of a Dust Trail in the Orbitof an Earth-threatening Long-Period Comet”. Astrophys-ical Journal 479: 441. Bibcode:1997ApJ...479..441J.doi:10.1086/303853.

[34] Jenniskens, P.; Lyytinen, E. (2005). “Meteor Show-ers from the Debris of Broken Comets: D/1819W1 (Blanpain), 2003 WY25, and the Phoeni-cids”. Astronomical Journal 130 (3): 1286–1290.Bibcode:2005AJ....130.1286J. doi:10.1086/432469.

[35] Brian G. Marsden (1983-10-25). “IAUC 3881: 1983TB AND THE GEMINID METEORS; 1983 SA; KRAur”. International Astronomical Union Circular. Re-trieved 2011-07-05.

[36] Jenniskens, P.; Lyytinen, E.; De Lignie, M.C.; Johannink,C.; Jobse, K.; Schievink, R.; Langbroek, M.; Koop, M.;Gural, P.; Wilson, M.A.; Yrjölä, I.; Suzuki, K.; Ogawa,H.; De Groote, P. (2002). “Dust Trails of 8P/Tuttleand the Unusual Outbursts of the Ursid Shower”.Icarus 159: 197–209. Bibcode:2002Icar..159..197J.doi:10.1006/icar.2002.6855.

[37] A possible meteor shower on the Moon, by D. M. Hunten,R. W. H. Kozlowski and A. L. Sprague, Article first pub-lished online: 7 DEC 2012, DOI: 10.1029/91GL02543

[38] Lunar Impacts

[39] Meteor showers at Mars

[40] Can Meteors Exist at Mars?

[41] Meteor Showers and their Parent Bodies

[42] Rosemary M. Killen; Joseph M. Hahn (December10, 2014). “Impact Vaporization as a PossibleSource of Mercury’s Calcium Exosphere”. Icarus.doi:10.1016/j.icarus.2014.11.035. (subscription required(help)).

[43] The P/Halley Stream: Meteor Showers on Earth, VenusandMars, by Apoistolos A. Christou, Geremie Vaubaillonand Paul Withers, Earth, Moon, and Planets, vol 102, # 1-4, DOI:10.1007/s11038-007-9201-3

[44] Lakdawalla, Emily. “Meteor showers on Titan: an ex-ample of why Twitter is awesome for scientists and thepublic”. Retrieved 3 June 2013.

• note also the Huygens lander was studied for its me-teoric entry and an observation campaign was at-tempted: An Artificial meteor on Titan?, by RalphD. Lorenz, journal??, vol 43, issue 5, October2002, pp. 14–17 and Huygens entry emission: Ob-servation campaign, results, and lessons learned byRalph D. Lorenz, Olivier Witasse, Jean-Pierre Le-breton, Thierry Blancquaert, Imke de Pater, FranckMazoue, Henry Roe, Mark T. Lemmon, Bonnie J.Burratti, Shadrian Holmes and Keith Noll, Journalof Geophysical Research: Planets, 20 JUN 2006,DOI: 10.1029/2005JE002603

[45] Watching meteors on Triton, W. Dean Pesnell, J.M. Gre-bowsky, and Andrew L. Weisman, Icarus, issue 169,(2004) pp. 482-491

30 CHAPTER 3. METEOR SHOWER

[46] IR Flashes induced bymeteoroid impacts onto Pluto’s sur-face, by I.B. Kosarev, I. V. Nemtchinov,Microsymposium,vol. 36, MS 050, 2002

3.10 External links• Worldwide viewing times for 2014 Meteor Showers

• Live Meteor Screen

• Meteor Showers, by Mark Fortune

• Basics of Meteor Observing, by Sky and Telescope

• Infography about Meteor Showers

• North American Meteor Network

• Meteor Shower Photos and Info (AOL Research &Learn)

• Meteor Showers, by Sky and Telescope

• Meteor showers Astronomy Cast episode #8, in-cludes full transcript in PDF-format.

• Meteor Showers Online , by Gary W. Kronk

• Meteor Streams

• National Geographic News – Sky-Watcher Alert:Meteor Show Peaks This Week

• Six Not-So-Famous Summer Meteor Showers JoeRao (SPACE.com)

• The American Meteor Society

• The International Meteor Organisation

• IMO 2008 Meteor Shower Calendar

• The Space Book by Eonitus

• Digital Astrolabe calendar of meteor showers

• A very precise Meteor Shower Map from Sono-taco Network, illustrate meteor shower sources from240,000 records

Chapter 4

Nebula

For other uses, see Nebula (disambiguation).A nebula (Latin for “cloud";[1] pl. nebulae, nebulæ, or

Portion of the Carina Nebula

nebulas) is an interstellar cloud of dust, hydrogen, heliumand other ionized gases. Originally, nebula was a namefor any diffuse astronomical object, including galaxiesbeyond the Milky Way. The Andromeda Galaxy, for in-stance, was referred to as the Andromeda Nebula (andspiral galaxies in general as “spiral nebulae”) before thetrue nature of galaxies was confirmed in the early 20thcentury by Vesto Slipher, Edwin Hubble and others.Most nebulae are of vast size, reaching sizes of even hun-dreds of light years in diameter, yet they appear as smallclouds to the naked eye.[2] Although denser than the spacesurrounding them, most nebulae are far less dense thanany vacuum created in an Earthen environment - a neb-ular cloud the size of the Earth would weigh only a fewkilograms.Nebulae are often star-forming regions, such as in theEagle Nebula. This nebula is depicted in one of NASA'smost famous images, the "Pillars of Creation". In these

The Crab Nebula photographed by Hubble

regions the formations of gas, dust, and other materials“clump” together to form larger masses, which attract fur-ther matter, and eventually will become massive enoughto form stars. The remaining materials are then believedto form planets, and other planetary system objects.

4.1 Observational history

Around 150 AD, Claudius Ptolemaeus (Ptolemy)recorded, in books VII-VIII of his Almagest, five starsthat appeared nebulous. He also noted a region ofnebulosity between the constellations Ursa Major andLeo that was not associated with any star.[4] The first truenebula, as distinct from a star cluster, was mentionedby the Persian/Muslim astronomer, Abd al-Rahmanal-Sufi, in his Book of Fixed Stars (964).[5] He noted “alittle cloud” where the Andromeda Galaxy is located.[6]He also cataloged the Omicron Velorum star clusteras a “nebulous star” and other nebulous objects, suchas Brocchi’s Cluster.[5] The supernova that created theCrab Nebula, the SN 1054, was observed by Arabic andChinese astronomers in 1054.[7][8]

31

32 CHAPTER 4. NEBULA

Tycho Supernova remnant in X-ray light

The "Pillars of Creation" from the Eagle Nebula. Evidence fromthe Spitzer Telescope suggests that the pillars may already havebeen destroyed by a supernova explosion, but the light showing usthe destruction will not reach the Earth for another millennium.[3]

On 26 November 1610, Nicolas-Claude Fabri de Peirescdiscovered the Orion Nebula using a telescope. This neb-ula was also observed by Johann Baptist Cysat in 1618.However, the first detailed study of the Orion Nebulawouldn't be performed until 1659 by Christiaan Huygens,who also believed himself to be the first person to dis-cover this nebulosity.[6]

In 1715, Edmund Halley published a list of six nebulae.[9]This number steadily increased during the century, withJean-Philippe de Cheseaux compiling a list of 20 (includ-ing eight not previously known) in 1746. From 1751–53, Nicolas Louis de Lacaille cataloged 42 nebulae fromthe Cape of Good Hope, with most of them being previ-ously unknown. Charles Messier then compiled a catalogof 103 “nebulae” (now called Messier objects, which in-cluded what are now known to be galaxies) by 1781; hisinterest was detecting comets, and these were objects that

might be mistaken for them, wasting time.[10]

The number of nebulae was then greatly expanded by theefforts of William Herschel and his sister Caroline Her-schel. Their Catalogue of One Thousand New Nebulaeand Clusters of Stars was published in 1786. A secondcatalog of a thousand was published in 1789 and the thirdand final catalog of 510 appeared in 1802. During muchof their work, William Herschel believed that these neb-ulae were merely unresolved clusters of stars. In 1790,however, he discovered a star surrounded by nebulosityand concluded that this was a true nebulosity, rather thana more distant cluster.[10]

Beginning in 1864, William Huggins examined the spec-tra of about 70 nebulae. He found that roughly a third ofthem had the emission spectrum of a gas. The rest showeda continuous spectrum and thus were thought to consist ofa mass of stars.[11][12] A third category was added in 1912when Vesto Slipher showed that the spectrum of the neb-ula that surrounded the star Merope matched the spectraof the Pleiades open cluster. Thus the nebula radiates byreflected star light.[13]

In about 1922, following the Great Debate, it had becomeclear that many “nebulae” were in fact galaxies far fromour own.Slipher and Edwin Hubble continued to collect the spec-tra from many diffuse nebulae, finding 29 that showedemission spectra and 33 had the continuous spectra ofstar light.[12] In 1922, Hubble announced that nearly allnebulae are associated with stars, and their illuminationcomes from star light. He also discovered that the emis-sion spectrum nebulae are nearly always associated withstars having spectral classifications of B1 or hotter (in-cluding all O-type main sequence stars), while nebulaewith continuous spectra appear with cooler stars.[14] BothHubble and Henry Norris Russell concluded that the neb-ulae surrounding the hotter stars are transformed in somemanner.[12]

4.2 Formation

Many nebulae or stars form from the gravitational col-lapse of gas in the interstellar medium or ISM. As thematerial collapses under its own weight, massive starsmay form in the center, and their ultraviolet radiationionizes the surrounding gas, making it visible at opticalwavelengths. Examples of these types of nebulae are theRosette Nebula and the Pelican Nebula. The size of thesenebulae, known as HII regions, varies depending on thesize of the original cloud of gas. New stars are formed inthe nebulae. The formed stars are sometimes known as ayoung, loose cluster.Some nebulae are formed as the result of supernova ex-plosions, the death throes of massive, short-lived stars.The materials thrown off from the supernova explosionare ionized by the energy and the compact object that it

4.3. TYPES OF NEBULAE 33

NGC 2024, The Flame Nebula

The Triangulum Emission Garren Nebula NGC 604

can produce. One of the best examples of this is the CrabNebula, in Taurus. The supernova event was recorded inthe year 1054 and is labelled SN 1054. The compact ob-ject that was created after the explosion lies in the centerof the Crab Nebula and is a neutron star.Other nebulae may form as planetary nebulae. This is thefinal stage of a low-mass star’s life, like Earth's Sun. Starswith a mass up to 8–10 solar masses evolve into red gi-ants and slowly lose their outer layers during pulsations intheir atmospheres. When a star has lost enough material,its temperature increases and the ultraviolet radiation itemits can ionize the surrounding nebula that it has thrownoff. The nebula is almost 97% hydrogen and 3% helium,plus trace amounts of other elements.

4.3 Types of nebulae• Herbig–Haro object HH 161 and HH 164.[1]

• HH 46/47 are small patches of nebulosity associatedwith newly born stars[2]

• The Omega Nebula, an example of an emission neb-ula

• The Horsehead Nebula, an example of a dark neb-ula.

• The Cat’s Eye Nebula, an example of a planetarynebula.

• The Red Rectangle Nebula, an example of aprotoplanetary nebula.

• The delicate shell of SNR B0509-67.5

1. ^ “A stellar sneezing fit”. ESA/Hubble Picture of theWeek. Retrieved 16 December 2013.

2. ^ “ALMA Takes Close Look at Drama of Star-birth”. ESO Press Release. Retrieved 21 August2013.

4.3.1 Classical types

Objects named nebulae belong to four major groups. Be-fore their nature was understood, galaxies (“spiral nebu-lae”) and star clusters too distant to be resolved as starswere also classified as nebulae, but no longer are.

• H II regions, large diffuse nebulae containing ion-ized hydrogen

• Planetary nebulae

• Supernova remnant (e.g., Crab Nebula)

• Dark nebula

Not all cloud-like structures are named nebulae; Herbig–Haro objects are an example.

4.3.2 Diffuse nebulae

Close-up view of the diffuse nebula NGC 6357[15]

34 CHAPTER 4. NEBULA

The Carina Nebula is a diffusion nebula

Most nebulae can be described as diffuse nebulae, whichmeans that they are extended and contain no well-definedboundaries.[16] In visible light these nebulae may be di-vided into emission and reflection nebulae. Emission neb-ulae emit spectral line radiation from ionized gas (mostlyionized hydrogen);[17] they are often called HII regions(the term “HII” is used in professional astronomy to referto ionized hydrogen).Reflection nebulae themselves do not emit significantamounts of visible light, but are near stars and reflect lightfrom them.[17] Similar nebulae not illuminated by starsdo not exhibit visible radiation, but may be detected asopaque clouds blocking light from luminous objects be-hind them; they are called “dark nebulae”.[17]

Although these nebulae have different visibility at opticalwavelengths, they are all bright sources of infrared emis-sion, chiefly from dust within the nebulae.[17]

4.3.3 Planetary nebulae

Main article: Planetary nebula

Planetary nebulae form from the gaseous shells that areejected from low-mass asymptotic giant branch starswhen they transform into white dwarfs.[17] They are emis-sion nebulae with spectra similar to those of emissionnebulae found in star formation regions.[17] Technicallythey are HII regions, because most hydrogen will be ion-ized, but they are denser and more compact than thenebulae in star formation regions.[17] Planetary nebulaewere given their name by the first astronomical observerswho became able to distinguish them from planets, whotended to confuse them with planets, of more interest tothem. Our Sun is expected to spawn a planetary nebulaabout 12 billion years after its formation.[18]

Four different planetary nebulae

Protoplanetary nebula

Main article: Protoplanetary nebula

A protoplanetary nebula (PPN) is an astronomical objectwhich is at the short-lived episode during a star’s rapidstellar evolution between the late asymptotic giant branch(LAGB) phase and the following planetary nebula (PN)phase.[19] During theAGBphase, the star undergoesmassloss, emitting a circumstellar shell of hydrogen gas. Whenthis phase comes to an end, the star enters the PPN phase.The PPN is energized by the central star, causing it toemit strong infrared radiation and become a reflectionnebula. Collaminated stellar winds from the central starshape and shock the shell into an axially symmetric form,while producing a fast moving molecular wind.[20] Theexact point when a PPN becomes a planetary nebula (PN)is defined by the temperature of the central star. The PPNphase continues until the central star reaches a tempera-ture of 30,000 K, after which is it hot enough to ionizethe surrounding gas.[21]

4.3.4 Supernova remnants

A supernova occurs when a high-mass star reaches theend of its life. When nuclear fusion in the core of the starstops, the star collapses. The gas falling inward either re-bounds or gets so strongly heated that it expands outwardsfrom the core, thus causing the star to explode.[17] Theexpanding shell of gas forms a supernova remnant, a spe-cial diffuse nebula.[17] Although much of the optical andX-ray emission from supernova remnants originates fromionized gas, a great amount of the radio emission is a formof non-thermal emission called synchrotron emission.[17]This emission originates from high-velocity electrons os-

4.5. SEE ALSO 35

The Crab Nebula, an example of a supernova remnant

The Crab Nebula video by NASA (2009)

cillating within magnetic fields.

4.4 Notable named nebulae• Ant Nebula

• Barnard’s Loop

• Boomerang Nebula

• Cat’s Eye Nebula

• Crab Nebula

• Eagle Nebula

• Eskimo Nebula

• Eta Carinae Nebula

• Fox Fur Nebula

• Helix Nebula

• Hourglass Nebula

• Horsehead Nebula

• Lagoon Nebula

• Orion Nebula

• Pelican Nebula

• Red Square Nebula

• Ring Nebula

• Rosette Nebula

• Tarantula Nebula

4.4.1 Nebula catalogs

• Gum catalog

• RCW Catalogue

• Sharpless catalog

4.5 See also• H I region

• H II region

• List of diffuse nebulae

• Lists of nebulae

• Molecular cloud

• Magellanic Clouds

• Messier object

• Nebulae in fiction

• Nebular hypothesis

• Orion Molecular Cloud Complex

• Timeline of knowledge about the interstellar and in-tergalactic medium

4.6 References[1] Nebula, Online Etymology Dictionary

[2] 'Formey, Johann Heinrich Samuel. “Nebula.” The Ency-clopedia of Diderot & d'Alembert Collaborative Transla-tion Project. Translated by Amanda Oberski. Ann Ar-bor: Michigan Publishing, University of Michigan Li-brary, 2011. Web. 1 April 2015. <http://hdl.handle.net/2027/spo.did2222.0002.664>. Trans. of “Nébuleux,”Encyclopédie ou Dictionnaire raisonné des sciences, desarts et des métiers, vol. 11. Paris, 1765.'

[3] Famous Space Pillars Feel the Heat of Star’s Explosion –Jet Propulsion Laboratory

36 CHAPTER 4. NEBULA

[4] Kunitzsch, P. (1987), “A Medieval Reference to theAndromeda Nebula”, ESO Messenger 49: 42–43,Bibcode:1987Msngr..49...42K, retrieved 2009-10-31

[5] Kenneth Glyn Jones (1991). Messier’s nebulae and starclusters. Cambridge University Press. p. 1. ISBN 0-521-37079-5.

[6] Harrison, T. G. (March 1984). “The Orion Neb-ula — where in History is it”. Quarterly Journalof the Royal Astronomical Society 25 (1): 70–73.Bibcode:1984QJRAS..25...65H.

[7] Lundmark K. (1921), Suspected New Stars Recorded inthe Old Chronicles and Among Recent Meridian Observa-tions’', Publications of the Astronomical Society of the Pa-cific, v. 33, p.225

[8] Mayall N.U. (1939), The Crab Nebula, a Probable Super-nova, Astronomical Society of the Pacific Leaflets, v. 3,p.145

[9] Halley, E. (1714–16). “An account of several nebulae orlucid spots like clouds, lately discovered among the fixtstars by help of the telescope”. Philosophical Transac-tions. XXXIX: 390–2.

[10] Hoskin, Michael (2005). “Unfinished Business:William Herschel’s Sweeps for Nebulae”. BritishJournal for the History of Science 43: 305–320.Bibcode:2005HisSc..43..305H.

[11] Watts, William Marshall; Huggins, Sir William; LadyHuggins (1904). An introduction to the study of spectrumanalysis. Longmans, Green, and Co. pp. 84–85. Re-trieved 2009-10-31.

[12] Struve, Otto (1937). “Recent Progress in the Studyof Reflection Nebulae”. Popular Astronomy 45: 9–22.Bibcode:1937PA.....45....9S.

[13] Slipher, V. M. (1912). “On the spectrum of the nebulain the Pleiades”. Lowell Observatory Bulletin 1: 26–27.Bibcode:1912LowOB...2...26S.

[14] Hubble, E. P. (December 1922). “The source ofluminosity in galactic nebulae.”. Astrophysical Jour-nal 56: 400–438. Bibcode:1922ApJ....56..400H.doi:10.1086/142713.

[15] “VLT Takes a Close Look at NGC 6357”. ESO Press Re-lease. Retrieved 21 June 2012.

[16] “The Messier Catalog: Diffuse Nebulae”. SEDS.Archived from the original on 1996-12-25. Retrieved2007-06-12.

[17] F. H. Shu (1982). The Physical Universe. Mill Valley,California: University Science Books. ISBN 0-935702-05-9.

[18] E. Chaisson; S. McMillan (1995). Astronomy: a begin-ner’s guide to the universe (2nd ed.). Upper Saddle River,New Jersey: Prentice-Hall. ISBN 0-13-733916-X.

[19] R. Sahai; C. Sánchez Contreras; M. Morris(2005). “A Starfish Preplanetary Nebula: IRAS19024+0044”. Astrophysical Journal 620 (2): 948–960.Bibcode:2005ApJ...620..948S. doi:10.1086/426469.

[20] Davis, C. J.; Smith, M. D.; Gledhill, T. M.; Var-ricatt, W. P. (2005). “Near-infrared echelle spec-troscopy of protoplanetary nebulae: probing the fastwind in H2". Monthly Notices of the Royal Astronomi-cal Society 360 (1): 104–118. arXiv:astro-ph/0503327.Bibcode:2005MNRAS.360..104D. doi:10.1111/j.1365-2966.2005.09018.x.

[21] Volk, Kevin M.; Kwok, Sun (July 1, 1989). “Evo-lution of protoplanetary nebulae”. AstrophysicalJournal, Part 1 (ISSN 0004-637X) 342: 345–363.Bibcode:1989ApJ...342..345V. doi:10.1086/167597.

4.7 External links• Nebulae, SEDS Messier Pages

• Fusedweb.pppl.gov

• Information on star formation, geocities.com

• Clickable table of Messier objects, space-and-telescope.com

Chapter 5

Star

For other uses, see Star (disambiguation).

A star-forming region in the LargeMagellanic Cloud. NASA/ESAimage.

False-color imagery of the Sun, a G-type main-sequence star, theclosest to Earth

A star is a luminous sphere of plasma held together by itsown gravity. The nearest star to Earth is the Sun. Otherstars, mostly in the Milky Way, are visible from Earthduring the night, appearing as a multitude of fixed lu-minous points in the sky due to their immense distancefrom Earth. Historically, the most prominent stars were

grouped into constellations and asterisms, and the bright-est stars gained proper names. Extensive catalogues ofstars have been assembled by astronomers, which providestandardized star designations.For at least a portion of its life, a star shines due tothermonuclear fusion of hydrogen into helium in its core,releasing energy that traverses the star’s interior and thenradiates into outer space. Once the hydrogen in the coreof a star is nearly exhausted, almost all naturally occurringelements heavier than helium are created by stellar nucle-osynthesis during the star’s lifetime and, for some stars,by supernova nucleosynthesis when it explodes. Near theend of its life, a star can also contain degenerate mat-ter. Astronomers can determine themass, age, metallicity(chemical composition), and many other properties of astar by observing its motion through space, luminosity,and spectrum respectively. The total mass of a star isthe principal determinant of its evolution and eventualfate. Other characteristics of a star, including diame-ter and temperature, change over its life, while the star’senvironment affects its rotation and movement. A plotof the temperature of many stars against their luminosi-ties, known as a Hertzsprung–Russell diagram (H–R di-agram), allows the age and evolutionary state of a star tobe determined.A star’s life begins with the gravitational collapse of agaseous nebula of material composed primarily of hy-drogen, along with helium and trace amounts of heavierelements. Once the stellar core is sufficiently dense, hy-drogen becomes steadily converted into helium throughnuclear fusion, releasing energy in the process.[1] The re-mainder of the star’s interior carries energy away from thecore through a combination of radiative and convectiveprocesses. The star’s internal pressure prevents it fromcollapsing further under its own gravity. Once the hy-drogen fuel at the core is exhausted, a star with at least0.4 times the mass of the Sun[2] expands to become a redgiant, in some cases fusing heavier elements at the coreor in shells around the core. The star then evolves intoa degenerate form, recycling a portion of its matter intothe interstellar environment, where it will contribute tothe formation of a new generation of stars with a higherproportion of heavy elements.[3] Meanwhile, the core be-comes a stellar remnant: a white dwarf, a neutron star, or

37

38 CHAPTER 5. STAR

(if it is sufficiently massive) a black hole.Binary andmulti-star systems consist of two or more starsthat are gravitationally bound, and generally move aroundeach other in stable orbits. When two such stars have arelatively close orbit, their gravitational interaction canhave a significant impact on their evolution.[4] Stars canform part of a much larger gravitationally bound struc-ture, such as a star cluster or a galaxy.

5.1 Observation history

People have seen patterns in the stars since ancient times.[5]

This 1690 depiction of the constellation of Leo, the lion, is byJohannes Hevelius.[6]

Historically, stars have been important to civilizationsthroughout the world. They have been part of religiouspractices and used for celestial navigation and orienta-tion. Many ancient astronomers believed that stars werepermanently affixed to a heavenly sphere, and that theywere immutable. By convention, astronomers groupedstars into constellations and used them to track the mo-tions of the planets and the inferred position of the Sun.[5]The motion of the Sun against the background stars (andthe horizon) was used to create calendars, which could beused to regulate agricultural practices.[7] The Gregoriancalendar, currently used nearly everywhere in the world,is a solar calendar based on the angle of the Earth’s rota-tional axis relative to its local star, the Sun.The oldest accurately dated star chart appeared in ancientEgyptian astronomy in 1534 BC.[8] The earliest knownstar catalogues were compiled by the ancient Babylonianastronomers of Mesopotamia in the late 2nd millenniumBC, during the Kassite Period (ca. 1531–1155 BC).[9]

The first star catalogue in Greek astronomy was createdby Aristillus in approximately 300 BC, with the help ofTimocharis.[10] The star catalog of Hipparchus (2nd cen-tury BC) included 1020 stars and was used to assemblePtolemy's star catalogue.[11] Hipparchus is known for thediscovery of the first recorded nova (new star).[12] Manyof the constellations and star names in use today derivefrom Greek astronomy.

The constellation of Leo as it can be seen by the naked eye. Lineshave been added.

In spite of the apparent immutability of the heavens,Chinese astronomers were aware that new stars couldappear.[13] In 185 AD, they were the first to observe andwrite about a supernova, now known as the SN 185.[14]The brightest stellar event in recorded history was the SN1006 supernova, which was observed in 1006 and writ-ten about by the Egyptian astronomer Ali ibn Ridwan andseveral Chinese astronomers.[15] The SN 1054 supernova,which gave birth to the Crab Nebula, was also observedby Chinese and Islamic astronomers.[16][17][18]

Medieval Islamic astronomers gave Arabic names tomany stars that are still used today, and they inventednumerous astronomical instruments that could computethe positions of the stars. They built the first largeobservatory research institutes, mainly for the purposeof producing Zij star catalogues.[19] Among these, theBook of Fixed Stars (964) was written by the Persian as-tronomer Abd al-Rahman al-Sufi, who observed a num-ber of stars, star clusters (including the Omicron Velo-rum and Brocchi’s Clusters) and galaxies (including theAndromeda Galaxy).[20] According to A. Zahoor, in the11th century, the Persian polymath scholar Abu RayhanBiruni described the Milky Way galaxy as a multitudeof fragments having the properties of nebulous stars, andalso gave the latitudes of various stars during a lunareclipse in 1019.[21]

According to Josep Puig, the Andalusian astronomer Ibn

5.1. OBSERVATION HISTORY 39

Bajjah proposed that the Milky Way was made up ofmany stars which almost touched one another and ap-peared to be a continuous image due to the effect ofrefraction from sublunary material, citing his observa-tion of the conjunction of Jupiter and Mars on 500AH (1106/1107 AD) as evidence.[22] Early European as-tronomers such as Tycho Brahe identified new stars inthe night sky (later termed novae), suggesting that theheavens were not immutable. In 1584 Giordano Brunosuggested that the stars were like the Sun, and may haveother planets, possibly even Earth-like, in orbit aroundthem,[23] an idea that had been suggested earlier by the an-cient Greek philosophers, Democritus and Epicurus,[24]and by medieval Islamic cosmologists[25] such as Fakhral-Din al-Razi.[26] By the following century, the idea ofthe stars being the same as the Sun was reaching a consen-sus among astronomers. To explain why these stars ex-erted no net gravitational pull on the Solar System, IsaacNewton suggested that the stars were equally distributedin every direction, an idea prompted by the theologianRichard Bentley.[27]

The Italian astronomer Geminiano Montanari recordedobserving variations in luminosity of the star Algol in1667. Edmond Halley published the first measurementsof the proper motion of a pair of nearby “fixed” stars,demonstrating that they had changed positions from thetime of the ancient Greek astronomers Ptolemy andHipparchus.[23]

William Herschel was the first astronomer to attempt todetermine the distribution of stars in the sky. During the1780s, he performed a series of gauges in 600 directions,and counted the stars observed along each line of sight.From this he deduced that the number of stars steadilyincreased toward one side of the sky, in the direction ofthe Milky Way core. His son John Herschel repeated thisstudy in the southern hemisphere and found a correspond-ing increase in the same direction.[28] In addition to hisother accomplishments, William Herschel is also notedfor his discovery that some stars do not merely lie alongthe same line of sight, but are also physical companionsthat form binary star systems.The science of stellar spectroscopy was pioneered byJoseph von Fraunhofer and Angelo Secchi. By com-paring the spectra of stars such as Sirius to the Sun,they found differences in the strength and number oftheir absorption lines—the dark lines in a stellar spectradue to the absorption of specific frequencies by the at-mosphere. In 1865 Secchi began classifying stars intospectral types.[29] However, the modern version of thestellar classification scheme was developed by Annie J.Cannon during the 1900s.The first direct measurement of the distance to a star (61Cygni at 11.4 light-years) was made in 1838 by FriedrichBessel using the parallax technique. Parallax measure-ments demonstrated the vast separation of the stars in theheavens.[23] Observation of double stars gained increasing

Alpha Centauri A and B over limb of Saturn

importance during the 19th century. In 1834, FriedrichBessel observed changes in the proper motion of the starSirius, and inferred a hidden companion. Edward Pick-ering discovered the first spectroscopic binary in 1899when he observed the periodic splitting of the spectrallines of the star Mizar in a 104-day period. Detailed ob-servations of many binary star systems were collected byastronomers such as William Struve and S. W. Burnham,allowing the masses of stars to be determined from com-putation of the orbital elements. The first solution to theproblem of deriving an orbit of binary stars from tele-scope observations was made by Felix Savary in 1827.[30]The twentieth century saw increasingly rapid advances inthe scientific study of stars. The photograph became avaluable astronomical tool. Karl Schwarzschild discov-ered that the color of a star, and hence its temperature,could be determined by comparing the visual magnitudeagainst the photographic magnitude. The development ofthe photoelectric photometer allowed very precise mea-surements of magnitude at multiple wavelength intervals.In 1921 Albert A. Michelson made the first measure-ments of a stellar diameter using an interferometer on theHooker telescope.[31]

Important theoretical work on the physical structure ofstars occurred during the first decades of the twentiethcentury. In 1913, the Hertzsprung-Russell diagram wasdeveloped, propelling the astrophysical study of stars.Successful models were developed to explain the interiorsof stars and stellar evolution. Cecilia Payne-Gaposchkinfirst proposed that stars were made primarily of hydro-gen and helium in her 1925 PhD thesis.[32] The spec-tra of stars were further understood through advances inquantum physics. This allowed the chemical compositionof the stellar atmosphere to be determined.[33]

With the exception of supernovae, individual starshave primarily been observed in our Local Group ofgalaxies,[34] and especially in the visible part of the MilkyWay (as demonstrated by the detailed star cataloguesavailable for our galaxy).[35] But some stars have beenobserved in the M100 galaxy of the Virgo Cluster, about100 million light years from the Earth.[36] In the LocalSupercluster it is possible to see star clusters, and currenttelescopes could in principle observe faint individual starsin the Local Cluster[37] (see Cepheids). However, out-side the Local Supercluster of galaxies, neither individual

40 CHAPTER 5. STAR

stars nor clusters of stars have been observed. The onlyexception is a faint image of a large star cluster contain-ing hundreds of thousands of stars located at a distanceof one billion light years[38]—ten times further than themost distant star cluster previously observed.

5.2 Designations

Main articles: Star designation, Astronomical namingconventions and Star catalogueThe concept of the constellation was known to exist dur-

This view contains blue stars known as "Blue stragglers", for theirapparent location on the Hertzsprung–Russell diagram

ing the Babylonian period. Ancient sky watchers imag-ined that prominent arrangements of stars formed pat-terns, and they associated these with particular aspectsof nature or their myths. Twelve of these formations layalong the band of the ecliptic and these became the basisof astrology.[39] Many of the more prominent individualstars were also given names, particularly with Arabic orLatin designations.As well as certain constellations and the Sun it-self, individual stars have their own myths.[40] To theAncient Greeks, some “stars”, known as planets (Greekπλανήτης (planētēs), meaning “wanderer”), representedvarious important deities, from which the names of theplanets Mercury, Venus, Mars, Jupiter and Saturn weretaken.[40] (Uranus and Neptune were also Greek andRoman gods, but neither planet was known in Antiquitybecause of their low brightness. Their names were as-signed by later astronomers.)Circa 1600, the names of the constellations were used toname the stars in the corresponding regions of the sky.The German astronomer Johann Bayer created a seriesof star maps and applied Greek letters as designations tothe stars in each constellation. Later a numbering systembased on the star’s right ascension was invented and addedto John Flamsteed's star catalogue in his book “Histo-ria coelestis Britannica” (the 1712 edition), whereby this

numbering system came to be called Flamsteed designa-tion or Flamsteed numbering.[41][42]

The only internationally recognized authority for namingcelestial bodies is the International Astronomical Union(IAU).[43] A number of private companies sell namesof stars, which the British Library calls an unregulatedcommercial enterprise.[44][45] However, the IAU has dis-associated itself from this commercial practice, andthese names are neither recognized by the IAU norused by them.[46] One such star naming company is theInternational Star Registry, which, during the 1980s,was accused of deceptive practice for making it ap-pear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scamand a fraud,[47][48][49][50] and the New York City Depart-ment of Consumer Affairs issued a violation against ISRfor engaging in a deceptive trade practice.[51][52]

5.3 Units of measurement

Although stellar parameters can be expressed in SI unitsor CGS units, it is often most convenient to express mass,luminosity, and radii in solar units, based on the charac-teristics of the Sun:

Large lengths, such as the radius of a giant star orthe semi-major axis of a binary star system, are of-ten expressed in terms of the astronomical unit (AU)—approximately the mean distance between the Earth andthe Sun (150 million km or 93 million miles).

5.4 Formation and evolution

Low-mass stars High-mass stars

Star-formingnebula

Eagle Nebula

ProtostarV1647 Orionis

Mid-sized starThe Sun

Red giantArcturus

Planetarynebula

DumbbellNebula

Reddwarf

ProximaCentauri

Blue dwarf

Whitedwarf

Sirius B Blackdwarf

Massive starSpica

Red supergiantBetelgeuse

SupernovaKepler's Star

(remnant: Crab Nebula)

Neutron starLGM-1 pulsar

Black holeCygnus X-1

Low-mass stars High-mass stars

Star-formingnebula

Eagle Nebula

ProtostarV1647 Orionis

Mid-sized starThe Sun

Red giantArcturus

Planetarynebula

DumbbellNebula

Reddwarf

ProximaCentauri

Blue dwarf

Whitedwarf

Sirius B Blackdwarf

Massive starSpica

Red supergiantBetelgeuse

SupernovaKepler's Star

(remnant: Crab Nebula)

Neutron starLGM-1 pulsar

Black holeCygnus X-1

Stellar evolution of low-mass (left cycle) and high-mass (rightcycle) stars, with examples in italics

Main article: Stellar evolution

Stars form within extended regions of higher density inthe interstellar medium, although the density is still lower

5.4. FORMATION AND EVOLUTION 41

than the inside of a vacuum chamber. These regions -known as molecular clouds - consist mostly of hydrogen,with about 23 to 28 percent helium and a few percentheavier elements. One example of such a star-formingregion is the Orion Nebula.[55] As massive stars formfrom molecular clouds, they powerfully illuminate thoseclouds. They also ionize the hydrogen, creating an H IIregion.All stars spend the majority of their existence as mainsequence stars, fueled primarily by the nuclear fusion ofhydrogen into helium within their cores. However, starsof different masses have markedly different properties atvarious stages of their development. The ultimate fate ofmore massive stars differs from that of less massive stars,as do their luminosity and the impact they have on theirenvironment. Accordingly, astronomers often group starsby their mass:[56]

• Very low mass stars with masses below 0.5 M☉ donot enter the asymptotic giant branch (AGB) butevolve directly into white dwarfs

• Low mass stars (including the Sun) with a massabove about 0.5 and below about 1.8–2.2 M☉ (de-pending on composition) do enter the AGB, wherethey develop a degenerate helium core

• Intermediate-mass stars undergo helium fusion anddevelop a degenerate carbon-oxygen core.

• Massive stars have a minimum mass of 7–10 M☉,but this may be as low as 5–6 M☉. These starsundergo carbon fusion, with their lives ending in acore-collapse supernova explosion.

5.4.1 Protostar formation

Main article: Star formation

The formation of a star begins with gravitational instabil-ity within a molecular cloud, caused by regions of higherdensity - often triggered by shock-waves from nearbysupernovae (massive stellar explosions), the collision ofdifferent molecular clouds, or the collision of galaxies(as in a starburst galaxy). Once a region reaches a suffi-cient density of matter to satisfy the criteria for Jeans in-stability, it begins to collapse under its own gravitationalforce.[57]

As the cloud collapses, individual conglomerations ofdense dust and gas form "Bok globules". As a globule col-lapses and the density increases, the gravitational energyconverts into heat and the temperature rises. When theprotostellar cloud has approximately reached the stablecondition of hydrostatic equilibrium, a protostar forms atthe core.[58] These pre–main sequence stars are often sur-rounded by a protoplanetary disk and powered mainly bythe release of gravitational energy. The period of gravi-tational contraction lasts about 10 to 15 million years.

Artist’s conception of the birth of a star within a dense molecularcloud. NASA image

Early stars of less than 2 M☉ are called T Tauri stars,while those with greater mass are Herbig Ae/Be stars.These newly formed stars emit jets of gas along their axisof rotation, which may reduce the angular momentum ofthe collapsing star and result in small patches of nebu-losity known as Herbig–Haro objects.[59][60] These jets,in combination with radiation from nearby massive stars,may help to drive away the surrounding cloud from whichthe star was formed.[61]

Early in their development, T Tauri stars follow theHayashi track—they contract and decrease in luminositywhile remaining at roughly the same temperature. Lessmassive T Tauri stars follow this track to the main se-quence, while more massive stars turn onto the Henyeytrack.

5.4.2 Main sequence

Main article: Main sequence

Stars spend about 90% of their existence fusing hydrogeninto helium in high-temperature and high-pressure reac-tions near the core. Such stars are said to be on the mainsequence and are called dwarf stars. Starting at zero-agemain sequence, the proportion of helium in a star’s corewill steadily increase, the rate of nuclear fusion at the corewill slowly increase, as will the star’s temperature andluminosity.[62] The Sun, for example, is estimated to haveincreased in luminosity by about 40% since it reached themain sequence 4.6 billion (4.6 × 109) years ago.[63]

Every star generates a stellar wind of particles that causesa continual outflow of gas into space. For most stars,the mass lost is negligible. The Sun loses 10−14 M☉every year,[64] or about 0.01% of its total mass over itsentire lifespan. However, very massive stars can lose10−7 to 10−5 M☉ each year, significantly affecting theirevolution.[65] Stars that begin with more than 50 M☉can lose over half their total mass while on the main

42 CHAPTER 5. STAR

sequence.[66]

An example of a Hertzsprung–Russell diagram for a set of starsthat includes the Sun (center). (See “Classification” below.)

The duration that a star spends on the main sequence de-pends primarily on the amount of fuel it has to fuse andthe rate at which it fuses that fuel, i.e. its initial massand its luminosity. For the Sun, its life is estimated tobe about 10 billion (1010) years. Massive stars consumetheir fuel very rapidly and are short-lived. Lowmass starsconsume their fuel very slowly. Stars less massive than0.25 M☉, called red dwarfs, are able to fuse nearly allof their mass as fuel while stars of about 1 M☉ can onlyuse about 10% of their mass as fuel. The combinationof their slow fuel-consumption and relatively large usablefuel supply allows about 0.25 M☉ stars to last for aboutone trillion (1012) years according to stellar-evolution cal-culations, while the least-massive hydrogen-fusing stars(0.08 M☉) will last for about 12 trillion years.[67] At theend of their lives, red dwarfs simply become dimmer anddimmer.[2] However, since the lifespan of such stars isgreater than the current age of the universe (13.8 billionyears), no stars under about 0.85 M☉[68] are expected tohave moved off the main sequence.Besides mass, the elements heavier than helium can playa significant role in the evolution of stars. Astronomersconsider all elements heavier than helium “metals”, andcall the chemical concentration of these elements themetallicity. The metallicity can influence the durationthat a star will burn its fuel, control the formation ofmagnetic fields[69] and modify the strength of the stel-lar wind.[70] Older, population II stars have substantiallyless metallicity than the younger, population I stars dueto the composition of the molecular clouds from whichthey formed. Over time these clouds become increasinglyenriched in heavier elements as older stars die and shed

portions of their atmospheres.

5.4.3 Post–main sequence

Main article: Red giant

As stars of at least 0.4 M☉[2] exhaust their supply of hy-drogen at their core, their outer layers expand greatly andcool to form a red giant. In about 5 billion years, whenthe Sun enters this phase, it will expand to a maximumradius of roughly 1 astronomical unit (150 million kilo-metres), 250 times its present size. As a giant, the Sunwill lose roughly 30% of its current mass.[63][71]

In a red giant of up to 2.25M☉, hydrogen fusion proceedsin a shell surrounding the core.[72] Eventually the core iscompressed enough to start helium fusion, and the starnow gradually shrinks in radius and its surface tempera-ture increases. For larger stars, the core region transitionsdirectly from fusing hydrogen to fusing helium.[4]

After the star has consumed the helium at the core, fusioncontinues in a shell around a hot core of carbon and oxy-gen. The star then follows an evolutionary path that par-allels the original red giant phase, but at a higher surface-temperature.

Massive stars

Main article: Red supergiant

During their helium-burning phase, very high-mass starswith more than nine solar masses expand to form red su-pergiants. Once this fuel is exhausted at the core, theycontinue to fuse elements heavier than helium.The core contracts until the temperature and pressure suf-fice to fuse carbon (see carbon burning process). Thisprocess continues, with the successive stages being fu-eled by neon (see neon burning process), oxygen (seeoxygen burning process), and silicon (see silicon burn-ing process). Near the end of the star’s life, fusion con-tinues along a series of onion-layer shells within the star.Each shell fuses a different element, with the outermostshell fusing hydrogen; the next shell fusing helium, andso forth.[73]

The final stage occurs when a massive star begins produc-ing iron. Since iron nuclei are more tightly bound thanany heavier nuclei, any fusion beyond iron does not pro-duce a net release of energy—the process would, on thecontrary, consume energy. Likewise, since they are moretightly bound than all lighter nuclei, energy cannot be re-leased by fission.[72] In relatively old, very massive stars,a large core of inert iron will accumulate in the centerof the star. The heavier elements in these stars can worktheir way to the surface, forming evolved objects knownas Wolf-Rayet stars that have a dense stellar wind which

5.5. DISTRIBUTION 43

sheds the outer atmosphere.

Collapse

As a star’s core shrinks, the intensity of radiation fromthat surface increases, creating such radiation pressure onthe outer shell of gas that it will push those layers away,forming a planetary nebula. If what remains after theouter atmosphere has been shed is less than 1.4 M☉, itshrinks to a relatively tiny object about the size of Earth,known as a white dwarf. White dwarfs lack the massfor further gravitational compression to take place.[74]The electron-degenerate matter inside a white dwarf is nolonger a plasma, even though stars are generally referredto as being spheres of plasma. Eventually, white dwarfsfade into black dwarfs over a very long period of time.

The Crab Nebula, remnants of a supernova that was first ob-served around 1050 AD

In larger stars, fusion continues until the iron core hasgrown so large (more than 1.4 M☉) that it can no longersupport its own mass. This core will suddenly collapse asits electrons are driven into its protons, forming neutrons,neutrinos and gamma rays in a burst of electron captureand inverse beta decay. The shockwave formed by thissudden collapse causes the rest of the star to explodein a supernova. Supernovae become so bright that theymay briefly outshine the star’s entire home galaxy. Whenthey occur within the Milky Way, supernovae have his-torically been observed by naked-eye observers as “newstars” where none seemingly existed before.[75]

Supernova explosions blow away most of their stars’ mat-ter (forming nebulae such as the Crab Nebula).[75] Thereremains a neutron star (which sometimes manifests it-self as a pulsar or X-ray burster) or, in the case of thelargest stars (large enough to leave a remnant greater thanroughly 4M☉), a black hole.[76] In a neutron star the mat-ter is in a state known as neutron-degenerate matter, with

a more exotic form of degenerate matter, QCD matter,possibly present in the core. Within a black hole the mat-ter is in a state that is not currently understood.The blown-off outer layers of dying stars include heavyelements, which may be recycled during the formation ofnew stars. These heavy elements allow the formation ofrocky planets. The outflow from supernovae and the stel-lar wind of large stars play an important part in shapingthe interstellar medium.[75]

5.5 Distribution

A white dwarf star in orbit around Sirius (artist’s impression).NASA image

In addition to isolated stars, a multi-star system can con-sist of two or more gravitationally bound stars that orbiteach other. The simplest and most common multi-starsystem is a binary star, but systems of three or more starsare also found. For reasons of orbital stability, suchmulti-star systems are often organized into hierarchical sets ofbinary stars.[77] Larger groups called star clusters also ex-ist. These range from loose stellar associations with onlya few stars, up to enormous globular clusters with hun-dreds of thousands of stars.It has been a long-held assumption that the majority ofstars occur in gravitationally bound, multiple-star sys-tems. This is particularly true for very massive O andB class stars, where 80% of the stars are believed to bepart of multiple-star systems. However the proportion ofsingle star systems increases for smaller stars, so that only25% of red dwarfs are known to have stellar companions.As 85% of all stars are red dwarfs, most stars in theMilkyWay are likely single from birth.[78]

Stars are not spread uniformly across the universe, butare normally grouped into galaxies along with interstel-lar gas and dust. A typical galaxy contains hundredsof billions of stars, and there are more than 100 billion(1011) galaxies in the observable universe.[79] A 2010 starcount estimate was 300 sextillion (3 × 1023) in the ob-servable universe.[80] While it is often believed that stars

44 CHAPTER 5. STAR

only exist within galaxies, intergalactic stars have beendiscovered.[81]

The nearest star to the Earth, apart from the Sun, isProxima Centauri, which is 39.9 trillion kilometres, or4.2 light-years away. Travelling at the orbital speed of theSpace Shuttle (8 kilometres per second—almost 30,000kilometres per hour), it would take about 150,000 years toget there.[82] Distances like this are typical inside galacticdiscs, including in the vicinity of the solar system.[83]Stars can be much closer to each other in the centres ofgalaxies and in globular clusters, or much farther apart ingalactic halos.Due to the relatively vast distances between stars outsidethe galactic nucleus, collisions between stars are thoughtto be rare. In denser regions such as the core of globu-lar clusters or the galactic center, collisions can be morecommon.[84] Such collisions can produce what are knownas blue stragglers. These abnormal stars have a higher sur-face temperature than the other main sequence stars withthe same luminosity in the cluster.[85]

5.6 Characteristics

The Sun is the nearest star to Earth.

Almost everything about a star is determined by its initialmass, including essential characteristics such as luminos-ity and size, as well as its evolution, lifespan, and eventualfate.

5.6.1 Age

Most stars are between 1 billion and 10 billion years old.Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet dis-covered, HD 140283, nicknamed Methuselah star, is anestimated 14.46 ± 0.8 billion years old.[86] (Due to the un-certainty in the value, this age for the star does not conflictwith the age of the Universe, determined by the Plancksatellite as 13.798 ± 0.037.[86])

The more massive the star, the shorter its lifespan, pri-marily because massive stars have greater pressure ontheir cores, causing them to burn hydrogen more rapidly.The most massive stars last an average of a few millionyears, while stars of minimum mass (red dwarfs) burntheir fuel very slowly and can last tens to hundreds of bil-lions of years.[87][88]

5.6.2 Chemical composition

See also: Metallicity and Molecules in stars

“From a chemist’s point of view, the sur-face or interior of a star…is boring—there areno molecules there.”--Roald Hoffmann[89]

When stars form in the presentMilkyWay galaxy they arecomposed of about 71% hydrogen and 27% helium,[90]as measured by mass, with a small fraction of heavier el-ements. Typically the portion of heavy elements is mea-sured in terms of the iron content of the stellar atmo-sphere, as iron is a common element and its absorptionlines are relatively easy to measure. The portion of heav-ier elements may be an indicator of the likelihood that thestar has a planetary system.[91]

The star with the lowest iron content ever measured is thedwarf HE1327-2326, with only 1/200,000th the iron con-tent of the Sun.[92] By contrast, the super-metal-rich starμ Leonis has nearly double the abundance of iron as theSun, while the planet-bearing star 14 Herculis has nearlytriple the iron.[93] There also exist chemically peculiarstars that show unusual abundances of certain elementsin their spectrum; especially chromium and rare earth el-ements.[94]

5.6.3 Diameter

Stars vary widely in size. In each image in the sequence, the right-most object appears as the left-most object in the next panel. TheEarth appears at right in panel 1 and the Sun is second from theright in panel 3. The rightmost star at panel 6 is UY Scuti, thelargest known star.

5.6. CHARACTERISTICS 45

Due to their great distance from the Earth, all stars exceptthe Sun appear to the unaided eye as shining points in thenight sky that twinkle because of the effect of the Earth’satmosphere. The Sun is also a star, but it is close enoughto the Earth to appear as a disk instead, and to providedaylight. Other than the Sun, the star with the largestapparent size is R Doradus, with an angular diameter ofonly 0.057 arcseconds.[95]

The disks of most stars are much too small in angular sizeto be observed with current ground-based optical tele-scopes, and so interferometer telescopes are required toproduce images of these objects. Another technique formeasuring the angular size of stars is through occultation.By precisely measuring the drop in brightness of a staras it is occulted by the Moon (or the rise in brightnesswhen it reappears), the star’s angular diameter can becomputed.[96]

Stars range in size from neutron stars, which vary any-where from 20 to 40 km (25 mi) in diameter, tosupergiants like Betelgeuse in the Orion constellation,which has a diameter approximately 1,070 times thatof the Sun—about 1,490,171,880 km (925,949,878 mi).Betelgeuse, however, has a much lower density than theSun.[97]

5.6.4 Kinematics

Main article: Stellar kinematicsThemotion of a star relative to the Sun can provide useful

The Pleiades, an open cluster of stars in the constellation ofTaurus. These stars share a common motion through space.[98]

NASA photo

information about the origin and age of a star, as wellas the structure and evolution of the surrounding galaxy.The components of motion of a star consist of the radialvelocity toward or away from the Sun, and the traverseangular movement, which is called its proper motion.Radial velocity is measured by the doppler shift of thestar’s spectral lines, and is given in units of km/s. Theproper motion of a star is determined by precise astro-metric measurements in units of milli-arc seconds (mas)

per year. By determining the parallax of a star, the propermotion can then be converted into units of velocity. Starswith high rates of proper motion are likely to be relativelyclose to the Sun, making them good candidates for paral-lax measurements.[99]

Once both rates of movement are known, the space ve-locity of the star relative to the Sun or the galaxy can becomputed. Among nearby stars, it has been found thatyounger population I stars have generally lower veloci-ties than older, population II stars. The latter have ellipti-cal orbits that are inclined to the plane of the galaxy.[100]A comparison of the kinematics of nearby stars has alsoled to the identification of stellar associations. These aremost likely groups of stars that share a common point oforigin in giant molecular clouds.[101]

5.6.5 Magnetic field

Main article: Stellar magnetic fieldThe magnetic field of a star is generated within regions

Surface magnetic field of SU Aur (a young star of T Tauri type),reconstructed by means of Zeeman-Doppler imaging

of the interior where convective circulation occurs. Thismovement of conductive plasma functions like a dynamo,generatingmagnetic fields that extend throughout the star.The strength of the magnetic field varies with the massand composition of the star, and the amount of magneticsurface activity depends upon the star’s rate of rotation.This surface activity produces starspots, which are re-gions of strong magnetic fields and lower than normal sur-face temperatures. Coronal loops are arching magneticfields that reach out into the corona from active regions.Stellar flares are bursts of high-energy particles that areemitted due to the same magnetic activity.[102]

Young, rapidly rotating stars tend to have high levels ofsurface activity because of their magnetic field. The mag-

46 CHAPTER 5. STAR

netic field can act upon a star’s stellar wind, functioningas a brake to gradually slow the rate of rotation with time.Thus, older stars such as the Sun have a much slowerrate of rotation and a lower level of surface activity. Theactivity levels of slowly rotating stars tend to vary in acyclical manner and can shut down altogether for periodsof time.[103] During the Maunder minimum, for exam-ple, the Sun underwent a 70-year period with almost nosunspot activity.

5.6.6 Mass

Main article: Stellar mass

One of the most massive stars known is Eta Carinae,[104]which, with 100–150 times as much mass as the Sun, willhave a lifespan of only several million years. A study ofthe Arches cluster suggests that 150M☉ is the upper limitfor stars in the current era of the universe.[105] The rea-son for this limit is not precisely known, but it is partiallydue to the Eddington luminosity which defines the maxi-mum amount of luminosity that can pass through the at-mosphere of a star without ejecting the gases into space.However, a star named R136a1 in the Large MagellanicCloud, RMC 136a star cluster has been measured at 256M☉, which puts this limit into question.[106] A study de-termined that stars larger than 150M☉ in R136 were cre-ated through the collision and merger of massive stars inclose binary systems, providing a way to sidestep the 150M☉ limit.[107]

The reflection nebula NGC 1999 is brilliantly illuminated byV380 Orionis (center), a variable star with about 3.5 times themass of the Sun. The black patch of sky is a vast hole of emptyspace and not a dark nebula as previously thought. NASA image

The first stars to form after the Big Bang may have beenlarger, up to 300 M☉ or more,[108] due to the completeabsence of elements heavier than lithium in their com-

position. This generation of supermassive, population IIIstars is long extinct, however, and currently only theoret-ical.With a mass only 80 times that of Jupiter (MJ), 2MASSJ0523-1403 is the smallest known star undergoing nuclearfusion in its core.[109] For stars with similar metallicity tothe Sun, the theoretical minimum mass the star can have,and still undergo fusion at the core, is estimated to beabout 75 MJ.[110][111] When the metallicity is very low,however, a recent study of the faintest stars found that theminimum star size seems to be about 8.3% of the solarmass, or about 87 MJ.[111][112] Smaller bodies are calledbrown dwarfs, which occupy a poorly defined grey areabetween stars and gas giants.The combination of the radius and the mass of a star de-termines the surface gravity. Giant stars have a muchlower surface gravity than main sequence stars, while theopposite is the case for degenerate, compact stars such aswhite dwarfs. The surface gravity can influence the ap-pearance of a star’s spectrum, with higher gravity causinga broadening of the absorption lines.[33]

5.6.7 Rotation

Main article: Stellar rotation

The rotation rate of stars can be determined throughspectroscopic measurement, or more exactly determinedby tracking the rotation rate of starspots. Young starscan have a rapid rate of rotation greater than 100 km/sat the equator. The B-class star Achernar, for example,has an equatorial rotation velocity of about 225 km/s orgreater, causing its equator to be slung outward and giv-ing it an equatorial diameter that is more than 50% largerthan the distance between the poles. This rate of rota-tion is just below the critical velocity of 300 km/s wherethe star would break apart.[113] By contrast, the Sun onlyrotates once every 25 – 35 days, with an equatorial ve-locity of 1.994 km/s. The star’s magnetic field and thestellar wind serve to slow a main sequence star’s rate ofrotation by a significant amount as it evolves on the mainsequence.[114]

Degenerate stars have contracted into a compact mass,resulting in a rapid rate of rotation. However they haverelatively low rates of rotation compared to what wouldbe expected by conservation of angular momentum—thetendency of a rotating body to compensate for a contrac-tion in size by increasing its rate of spin. A large portionof the star’s angular momentum is dissipated as a result ofmass loss through the stellar wind.[115] In spite of this, therate of rotation for a pulsar can be very rapid. The pulsarat the heart of the Crab nebula, for example, rotates 30times per second.[116] The rotation rate of the pulsar willgradually slow due to the emission of radiation.

5.7. RADIATION 47

5.6.8 Temperature

The surface temperature of a main sequence star is de-termined by the rate of energy production at the coreand by its radius, and is often estimated from the star’scolor index.[117] The temperature is normally given as theeffective temperature, which is the temperature of an ide-alized black body that radiates its energy at the same lu-minosity per surface area as the star. Note that the effec-tive temperature is only a representative value, as the tem-perature increases toward the core.[118] The temperaturein the core region of a star is several million kelvins.[119]

The stellar temperature will determine the rate of ioniza-tion of various elements, resulting in characteristic ab-sorption lines in the spectrum. The surface temperatureof a star, along with its visual absolute magnitude andabsorption features, is used to classify a star (see classifi-cation below).[33]

Massive main sequence stars can have surface tempera-tures of 50,000 K. Smaller stars such as the Sun have sur-face temperatures of a few thousand K. Red giants haverelatively low surface temperatures of about 3,600 K; butthey also have a high luminosity due to their large exteriorsurface area.[120]

5.7 Radiation

The energy produced by stars, as a product of nuclearfusion, radiates into space as both electromagnetic radi-ation and particle radiation. The particle radiation emit-ted by a star is manifested as the stellar wind,[121] whichstreams from the outer layers as free protons, and electri-cally charged alpha, and beta particles. Although almostmassless there also exists a steady stream of neutrinos em-anating from the star’s core.The production of energy at the core is the reason starsshine so brightly: every time two or more atomic nu-clei fuse together to form a single atomic nucleus of anew heavier element, gamma ray photons are releasedfrom the nuclear fusion product. This energy is convertedto other forms of electromagnetic energy of lower fre-quency, such as visible light, by the time it reaches thestar’s outer layers.The color of a star, as determined by the most intensefrequency of the visible light, depends on the temperatureof the star’s outer layers, including its photosphere.[122]Besides visible light, stars also emit forms of electro-magnetic radiation that are invisible to the human eye.In fact, stellar electromagnetic radiation spans the entireelectromagnetic spectrum, from the longest wavelengthsof radio waves through infrared, visible light, ultraviolet,to the shortest of X-rays, and gamma rays. From thestandpoint of total energy emitted by a star, not all com-ponents of stellar electromagnetic radiation are signifi-cant, but all frequencies provide insight into the star’s

physics.Using the stellar spectrum, astronomers can also deter-mine the surface temperature, surface gravity, metallicityand rotational velocity of a star. If the distance of thestar is known, such as by measuring the parallax, then theluminosity of the star can be derived. The mass, radius,surface gravity, and rotation period can then be estimatedbased on stellar models. (Mass can be calculated for starsin binary systems bymeasuring their orbital velocities anddistances. Gravitational microlensing has been used tomeasure the mass of a single star.[123]) With these pa-rameters, astronomers can also estimate the age of thestar.[124]

5.7.1 Luminosity

The luminosity of a star is the amount of light and otherforms of radiant energy it radiates per unit of time. It hasunits of power. The luminosity of a star is determined bythe radius and the surface temperature. However, manystars do not radiate a uniform flux (the amount of energyradiated per unit area) across their entire surface. Therapidly rotating star Vega, for example, has a higher en-ergy flux at its poles than along its equator.[125]

Surface patches with a lower temperature and luminos-ity than average are known as starspots. Small, dwarfstars such as our Sun generally have essentially featurelessdisks with only small starspots. Larger, giant stars havemuch larger, more obvious starspots,[126] and they alsoexhibit strong stellar limb darkening. That is, the bright-ness decreases towards the edge of the stellar disk.[127]Red dwarf flare stars such as UV Ceti may also possessprominent starspot features.[128]

5.7.2 Magnitude

Main articles: Apparent magnitude and Absolute magni-tude

The apparent brightness of a star is expressed in termsof its apparent magnitude, which is the brightness of astar and is a function of the star’s luminosity, distancefrom Earth, and the altering of the star’s light as it passesthrough Earth’s atmosphere. Intrinsic or absolute magni-tude is directly related to a star’s luminosity and is whatthe apparent magnitude a star would be if the distance be-tween the Earth and the star were 10 parsecs (32.6 light-years).Both the apparent and absolute magnitude scales arelogarithmic units: one whole number difference in mag-nitude is equal to a brightness variation of about 2.5times[130] (the 5th root of 100 or approximately 2.512).This means that a first magnitude star (+1.00) is about 2.5times brighter than a second magnitude (+2.00) star, andapproximately 100 times brighter than a sixth magnitude

48 CHAPTER 5. STAR

star (+6.00). The faintest stars visible to the naked eyeunder good seeing conditions are about magnitude +6.On both apparent and absolute magnitude scales, thesmaller the magnitude number, the brighter the star; thelarger the magnitude number, the fainter. The bright-est stars, on either scale, have negative magnitude num-bers. The variation in brightness (ΔL) between two starsis calculated by subtracting the magnitude number ofthe brighter star (m ) from the magnitude number of thefainter star (m ), then using the difference as an exponentfor the base number 2.512; that is to say:

∆m = mf −mb

2.512∆m = ∆L

Relative to both luminosity and distance from Earth, astar’s absolute magnitude (M) and apparent magnitude(m) are not equivalent;[130] for example, the bright starSirius has an apparent magnitude of −1.44, but it has anabsolute magnitude of +1.41.The Sun has an apparent magnitude of −26.7, but itsabsolute magnitude is only +4.83. Sirius, the brighteststar in the night sky as seen from Earth, is approximately23 times more luminous than the Sun, while Canopus,the second brightest star in the night sky with an abso-lute magnitude of −5.53, is approximately 14,000 timesmore luminous than the Sun. Despite Canopus beingvastly more luminous than Sirius, however, Sirius appearsbrighter than Canopus. This is because Sirius is merely8.6 light-years from the Earth, while Canopus is muchfarther away at a distance of 310 light-years.As of 2006, the star with the highest known absolutemagnitude is LBV 1806-20, with a magnitude of −14.2.This star is at least 5,000,000 times more luminous thanthe Sun.[131] The least luminous stars that are currentlyknown are located in the NGC 6397 cluster. The faintestred dwarfs in the cluster were magnitude 26, while a 28thmagnitude white dwarf was also discovered. These faintstars are so dim that their light is as bright as a birthdaycandle on the Moon when viewed from the Earth.[132]

5.8 Classification

Main article: Stellar classification

The current stellar classification system originated in theearly 20th century, when stars were classified from A toQ based on the strength of the hydrogen line.[134] It wasnot known at the time that the major influence on theline strength was temperature; the hydrogen line strengthreaches a peak at over 9000 K, and is weaker at bothhotter and cooler temperatures. When the classificationswere reordered by temperature, it more closely resembledthe modern scheme.[135]

Stars are given a single-letter classification according totheir spectra, ranging from type O, which are very hot, toM, which are so cool that molecules may form in their at-mospheres. The main classifications in order of decreas-ing surface temperature are: O, B, A, F, G, K, and M. Avariety of rare spectral types have special classifications.The most common of these are types L and T, which clas-sify the coldest low-mass stars and brown dwarfs. Eachletter has 10 sub-divisions, numbered from 0 to 9, in orderof decreasing temperature. However, this system breaksdown at extreme high temperatures: classO0 andO1 starsmay not exist.[136]

In addition, stars may be classified by the luminosity ef-fects found in their spectral lines, which correspond totheir spatial size and is determined by the surface gravity.These range from 0 (hypergiants) through III (giants) toV (main sequence dwarfs); some authors add VII (whitedwarfs). Most stars belong to the main sequence, whichconsists of ordinary hydrogen-burning stars. These fallalong a narrow, diagonal band when graphed according totheir absolute magnitude and spectral type.[136] The Sunis a main sequence G2V yellow dwarf of intermediatetemperature and ordinary size.Additional nomenclature, in the form of lower-case let-ters, can follow the spectral type to indicate peculiar fea-tures of the spectrum. For example, an "e" can indicatethe presence of emission lines; "m" represents unusuallystrong levels of metals, and "var" can mean variations inthe spectral type.[136]

White dwarf stars have their own class that begins withthe letter D. This is further sub-divided into the classesDA, DB, DC, DO, DZ, and DQ, depending on the typesof prominent lines found in the spectrum. This is fol-lowed by a numerical value that indicates the temperatureindex.[137]

5.9 Variable stars

Main article: Variable starVariable stars have periodic or random changes in lu-minosity because of intrinsic or extrinsic properties. Ofthe intrinsically variable stars, the primary types can besubdivided into three principal groups.During their stellar evolution, some stars pass throughphases where they can become pulsating variables. Pul-sating variable stars vary in radius and luminosity overtime, expanding and contracting with periods rangingfrom minutes to years, depending on the size of the star.This category includes Cepheid and cepheid-like stars,and long-period variables such as Mira.[138]

Eruptive variables are stars that experience sudden in-creases in luminosity because of flares or mass ejectionevents.[138] This group includes protostars, Wolf-Rayetstars, and Flare stars, as well as giant and supergiant stars.

5.10. STRUCTURE 49

The asymmetrical appearance of Mira, an oscillating variablestar. NASA HST image

Cataclysmic or explosive variable stars are those that un-dergo a dramatic change in their properties. This groupincludes novae and supernovae. A binary star systemthat includes a nearby white dwarf can produce certaintypes of these spectacular stellar explosions, including thenova and a Type 1a supernova.[4] The explosion is createdwhen the white dwarf accretes hydrogen from the com-panion star, building up mass until the hydrogen under-goes fusion.[139] Some novae are also recurrent, havingperiodic outbursts of moderate amplitude.[138]

Stars can also vary in luminosity because of extrinsic fac-tors, such as eclipsing binaries, as well as rotating starsthat produce extreme starspots.[138] A notable exampleof an eclipsing binary is Algol, which regularly varies inmagnitude from 2.3 to 3.5 over a period of 2.87 days.

5.10 Structure

Main article: Stellar structureThe interior of a stable star is in a state of hydrostaticequilibrium: the forces on any small volume almost ex-actly counterbalance each other. The balanced forces areinward gravitational force and an outward force due to thepressure gradient within the star. The pressure gradientis established by the temperature gradient of the plasma;the outer part of the star is cooler than the core. The tem-perature at the core of a main sequence or giant star is atleast on the order of 107 K. The resulting temperature andpressure at the hydrogen-burning core of a main sequencestar are sufficient for nuclear fusion to occur and for suf-ficient energy to be produced to prevent further collapseof the star.[140][141]

As atomic nuclei are fused in the core, they emit energyin the form of gamma rays. These photons interact with

Internal structures of main sequence stars, convection zones witharrowed cycles and radiative zones with red flashes. To the lefta low-mass red dwarf, in the center a mid-sized yellow dwarfand at the right a massive blue-white main sequence star.

the surrounding plasma, adding to the thermal energy atthe core. Stars on the main sequence convert hydrogeninto helium, creating a slowly but steadily increasing pro-portion of helium in the core. Eventually the helium con-tent becomes predominant and energy production ceasesat the core. Instead, for stars of more than 0.4M☉, fusionoccurs in a slowly expanding shell around the degeneratehelium core.[142]

In addition to hydrostatic equilibrium, the interior ofa stable star will also maintain an energy balance ofthermal equilibrium. There is a radial temperature gradi-ent throughout the interior that results in a flux of energyflowing toward the exterior. The outgoing flux of energyleaving any layer within the star will exactly match theincoming flux from below.The radiation zone is the region within the stellar interiorwhere radiative transfer is sufficiently efficient to maintainthe flux of energy. In this region the plasma will not beperturbed and any mass motions will die out. If this is notthe case, however, then the plasma becomes unstable andconvection will occur, forming a convection zone. Thiscan occur, for example, in regions where very high energyfluxes occur, such as near the core or in areas with highopacity as in the outer envelope.[141]

The occurrence of convection in the outer envelope of amain sequence star depends on the mass. Stars with sev-eral times the mass of the Sun have a convection zonedeep within the interior and a radiative zone in the outerlayers. Smaller stars such as the Sun are just the opposite,with the convective zone located in the outer layers.[143]Red dwarf stars with less than 0.4 M☉ are convectivethroughout, which prevents the accumulation of a heliumcore.[2] For most stars the convective zones will also varyover time as the star ages and the constitution of the inte-rior is modified.[141]

The portion of a star that is visible to an observer is calledthe photosphere. This is the layer at which the plasma ofthe star becomes transparent to photons of light. Fromhere, the energy generated at the core becomes free topropagate out into space. It is within the photosphere thatsun spots, or regions of lower than average temperature,

50 CHAPTER 5. STAR

This diagram shows a cross-section of the Sun. NASA image

appear.Above the level of the photosphere is the stellar atmo-sphere. In a main sequence star such as the Sun, the low-est level of the atmosphere is the thin chromosphere re-gion, where spicules appear and stellar flares begin. Thisis surrounded by a transition region, where the tempera-ture rapidly increases within a distance of only 100 km(62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several millionkilometres.[144] The existence of a corona appears to bedependent on a convective zone in the outer layers of thestar.[143] Despite its high temperature, the corona emitsvery little light. The corona region of the Sun is normallyonly visible during a solar eclipse.From the corona, a stellar wind of plasma particles ex-pands outward from the star, propagating until it interactswith the interstellar medium. For the Sun, the influenceof its solar wind extends throughout the bubble-shapedregion of the heliosphere.[145]

5.11 Nuclear fusion reaction path-ways

Main article: Stellar nucleosynthesis1H 1H 1H 1H

2H 1H 1H 2H

3He 3He

1H 1H

4Heνγ

ν

Gamma Ray

Neutrino

Proton

Neutron

Positron

ν

γγ

Overview of the proton-proton chain

12C13N

13C14N

15O

15N

1H1H

1H1H

4He

Proton

Neutron

Positron

Gamma Ray

Neutrino

The carbon-nitrogen-oxygen cycle

A variety of different nuclear fusion reactions take placeinside the cores of stars, depending upon their mass andcomposition, as part of stellar nucleosynthesis. The netmass of the fused atomic nuclei is smaller than the sumof the constituents. This lost mass is released as electro-magnetic energy, according to the mass-energy equiva-lence relationship E = mc2.[1]

The hydrogen fusion process is temperature-sensitive, soa moderate increase in the core temperature will resultin a significant increase in the fusion rate. As a resultthe core temperature of main sequence stars only variesfrom 4million kelvin for a smallM-class star to 40millionkelvin for a massive O-class star.[119]

In the Sun, with a 10-million-kelvin core, hydrogen fusesto form helium in the proton-proton chain reaction:[146]

41H→ 22H + 2e+ + 2νₑ (4.0 MeV + 1.0 MeV)21H + 22H → 23He + 2γ (5.5 MeV)

5.13. REFERENCES 51

23He → 4He + 21H (12.9 MeV)

These reactions result in the overall reaction:

41H → 4He + 2e+ + 2γ + 2νₑ (26.7 MeV)

where e+ is a positron, γ is a gamma ray photon, νₑ isa neutrino, and H and He are isotopes of hydrogen andhelium, respectively. The energy released by this reactionis in millions of electron volts, which is actually only atiny amount of energy. However enormous numbers ofthese reactions occur constantly, producing all the energynecessary to sustain the star’s radiation output.In more massive stars, helium is produced in a cycleof reactions catalyzed by carbon—the carbon-nitrogen-oxygen cycle.[146]

In evolved stars with cores at 100 million kelvin andmasses between 0.5 and 10 M☉, helium can be trans-formed into carbon in the triple-alpha process that usesthe intermediate element beryllium:[146]

4He + 4He + 92 keV → 8*Be4He + 8*Be + 67 keV → 12*C12*C → 12C + γ + 7.4 MeV

For an overall reaction of:

34He → 12C + γ + 7.2 MeV

In massive stars, heavier elements can also be burned ina contracting core through the neon burning process andoxygen burning process. The final stage in the stellar nu-cleosynthesis process is the silicon burning process thatresults in the production of the stable isotope iron-56.Fusion can not proceed any further except through anendothermic process, and so further energy can only beproduced through gravitational collapse.[146]

The example below shows the amount of time requiredfor a star of 20M☉ to consume all of its nuclear fuel. Asan O-class main sequence star, it would be 8 times thesolar radius and 62,000 times the Sun’s luminosity.[148]

5.12 See also• Exoplanet host stars

• Lists of stars

• List of largest known stars

• Outline of astronomy

• Sidereal time

• Star clocks

• Star count

• Stars and planetary systems in fiction

• Stellar astronomy

• Stellar dynamics

• Twinkle Twinkle Little Star (children’s nurseryrhyme)

5.13 References[1] Bahcall, John N. (June 29, 2000). “How the Sun Shines”.

Nobel Foundation. Retrieved 2006-08-30.

[2] Richmond, Michael. “Late stages of evolution for low-mass stars”. Rochester Institute of Technology. Retrieved2006-08-04.

[3] “Stellar Evolution & Death”. NASA Observatorium.Archived from the original on 2008-02-10. Retrieved2006-06-08.

[4] Iben, Icko, Jr. (1991). “Single and binary star evolu-tion”. Astrophysical Journal Supplement Series 76: 55–114. Bibcode:1991ApJS...76...55I. doi:10.1086/191565.

[5] Forbes, George (1909). History of Astronomy. London:Watts & Co. ISBN 1-153-62774-4.

[6] Hevelius, Johannis (1690). Firmamentum Sobiescianum,sive Uranographia. Gdansk.

[7] Tøndering, Claus. “Other ancient calendars”. WebEx-hibits. Retrieved 2006-12-10.

[8] von Spaeth, Ove (2000). “Dating the Oldest EgyptianStar Map”. Centaurus International Magazine of the His-tory of Mathematics, Science and Technology 42 (3): 159–179. Bibcode:2000Cent...42..159V. doi:10.1034/j.1600-0498.2000.420301.x. Retrieved 2007-10-21.

[9] North, John (1995). The Norton History of Astronomyand Cosmology. New York and London: W.W. Norton& Company. pp. 30–31. ISBN 0-393-03656-1.

[10] Murdin, P. (November 2000). “Aristillus (c.200 BC)". Encyclopedia of Astronomy andAstrophysics. Bibcode:2000eaa..bookE3440.doi:10.1888/0333750888/3440. ISBN 0-333-75088-8.

[11] Grasshoff, Gerd (1990). The history of Ptolemy’s star cat-alogue. Springer. pp. 1–5. ISBN 0-387-97181-5.

[12] Pinotsis, Antonios D. “Astronomy in Ancient Rhodes”.Section of Astrophysics, Astronomy and Mechanics, De-partment of Physics, University of Athens. Retrieved2009-06-02.

[13] Clark, D. H.; Stephenson, F. R. (June 29, 1981). “TheHistorical Supernovae”. Supernovae: A survey of cur-rent research; Proceedings of the Advanced Study Insti-tute. Cambridge, England: Dordrecht, D. Reidel Publish-ing Co. pp. 355–370. Bibcode:1982sscr.conf..355C.

52 CHAPTER 5. STAR

[14] Zhao, Fu-Yuan; Strom, R. G.; Jiang, Shi-Yang (2006).“The Guest Star of AD185 Must Have Been a Su-pernova”. Chinese Journal of Astronomy and Astro-physics 6 (5): 635–640. Bibcode:2006ChJAA...6..635Z.doi:10.1088/1009-9271/6/5/17.

[15] “Astronomers Peg Brightness of History’s Brightest Star”.NAOA News. March 5, 2003. Retrieved 2006-06-08.

[16] Frommert, Hartmut; Kronberg, Christine (August 30,2006). “Supernova 1054 – Creation of the Crab Nebula”.SEDS. University of Arizona.

[17] Duyvendak, J. J. L. (April 1942). “Further Data Bearingon the Identification of the Crab Nebula with the Super-nova of 1054 A.D. Part I. The Ancient Oriental Chroni-cles”. Publications of the Astronomical Society of the Pa-cific 54 (318): 91–94. Bibcode:1942PASP...54...91D.doi:10.1086/125409.Mayall, N. U.; Oort, Jan Hendrik (April 1942). “Fur-ther Data Bearing on the Identification of the CrabNebula with the Supernova of 1054 A.D. Part II.The Astronomical Aspects”. Publications of the As-tronomical Society of the Pacific 54 (318): 95–104.Bibcode:1942PASP...54...95M. doi:10.1086/125410.

[18] Brecher, K. et al. (1983). “Ancient records and theCrabNebula supernova”. The Observatory 103: 106–113.Bibcode:1983Obs...103..106B.

[19] Kennedy, Edward S. (1962). “Review: The Observa-tory in Islam and Its Place in the General History of theObservatory by Aydin Sayili”. Isis 53 (2): 237–239.doi:10.1086/349558.

[20] Jones, Kenneth Glyn (1991). Messier’s nebulae and starclusters. Cambridge University Press. p. 1. ISBN 0-521-37079-5.

[21] Zahoor, A. (1997). “Al-Biruni”. Hasanuddin University.Archived from the original on 2008-06-26. Retrieved2007-10-21.

[22] Montada, Josep Puig (September 28, 2007). “Ibn Bajja”.Stanford Encyclopedia of Philosophy. Retrieved 2008-07-11.

[23] Drake, Stephen A. (August 17, 2006). “A Brief His-tory of High-Energy (X-ray &Gamma-Ray) Astronomy”.NASA HEASARC. Retrieved 2006-08-24.

[24] Greskovic, Peter; Rudy, Peter (July 24, 2006).“Exoplanets”. ESO. Retrieved 2012-06-15.

[25] Ahmad, I. A. (1995). “The impact of the Qur'anicconception of astronomical phenomena on Islamic civ-ilization”. Vistas in Astronomy 39 (4): 395–403[402]. Bibcode:1995VA.....39..395A. doi:10.1016/0083-6656(95)00033-X.

[26] Setia, Adi (2004). “Fakhr Al-Din Al-Razi on Physics andthe Nature of the Physical World: A Preliminary Survey”.Islam & Science 2. Retrieved 2010-03-02.

[27] Hoskin, Michael (1998). “The Value of Archives in Writ-ing the History of Astronomy”. Space Telescope ScienceInstitute. Retrieved 2006-08-24.

[28] Proctor, Richard A. (1870). “Are any of thenebulæ star-systems?". Nature 1 (13): 331–333.Bibcode:1870Natur...1..331P. doi:10.1038/001331a0.

[29] MacDonnell, Joseph. “Angelo Secchi, S.J. (1818–1878) the Father of Astrophysics”. Fairfield University.Archived from the original on 2011-07-21. Retrieved2006-10-02.

[30] Aitken, Robert G. (1964). The Binary Stars. New York:Dover Publications Inc. p. 66. ISBN 0-486-61102-7.

[31] Michelson, A. A.; Pease, F. G. (1921). “Measure-ment of the diameter of Alpha Orionis with the in-terferometer”. Astrophysical Journal 53: 249–259.Bibcode:1921ApJ....53..249M. doi:10.1086/142603.

[32] "" Payne-Gaposchkin, Cecilia Helena.” CWP”.University of California. Retrieved 2013-02-21.

[33] Unsöld, Albrecht (2001). The New Cosmos (5th ed.).New York: Springer. pp. 180–185, 215–216. ISBN 3-540-67877-8.

[34] e. g. Battinelli, Paolo; Demers, Serge; Letarte, Bruno(2003). “Carbon Star Survey in the Local Group. V.The Outer Disk of M31”. The Astronomical Journal125 (3): 1298–1308. Bibcode:2003AJ....125.1298B.doi:10.1086/346274.

[35] “Millennium Star Atlas marks the completion of ESA’sHipparcos Mission”. ESA. December 8, 1997. Retrieved2007-08-05.

[36] Villard, Ray; Freedman, Wendy L. (October 26, 1994).“Hubble Space Telescope Measures Precise Distance tothe Most Remote Galaxy Yet”. Hubble Site. Retrieved2007-08-05.

[37] “Hubble Completes Eight-Year Effort to Measure Ex-panding Universe”. Hubble Site. May 25, 1999. Re-trieved 2007-08-02.

[38] “UBC Prof., alumnus discover most distant star clusters:a billion light-years away.”. UBC Public Affairs. January8, 2007. Retrieved 2007-08-02.

[39] Koch-Westenholz, Ulla; Koch, Ulla Susanne (1995).Mesopotamian astrology: an introduction to Babylonianand Assyrian celestial divination. Carsten Niebuhr Insti-tute Publications 19. Museum Tusculanum Press. p. 163.ISBN 87-7289-287-0.

[40] Coleman, Leslie S. “Myths, Legends and Lore”. FrostyDrew Observatory. Retrieved 2012-06-15.

[41] “Naming Astronomical Objects”. International Astro-nomical Union (IAU). Retrieved 2009-01-30.

[42] “Naming Stars”. Students for the Exploration and Devel-opment of Space (SEDS). Retrieved 2009-01-30.

[43] Lyall, Francis; Larsen, Paul B. (2009). “Chapter 7: TheMoon and Other Celestial Bodies”. Space Law: A Trea-tise. Ashgate Publishing, Ltd. p. 176. ISBN 0-7546-4390-5.

[44] “Star naming”. Scientia Astrophysical Organization.2005. Retrieved 2010-06-29.

5.13. REFERENCES 53

[45] “Disclaimer: Name a star, name a rose and other, sim-ilar enterprises”. British Library. The British LibraryBoard. Archived from the original on 2010-01-19. Re-trieved 2010-06-29.

[46] Andersen, Johannes. “Buying Stars and Star Names”. In-ternational Astronomical Union. Retrieved 2010-06-24.

[47] Pliat, Phil (September–October 2006). “Name Dropping:Want to Be a Star?". Skeptical Inquirer 30.5. Retrieved2010-06-29.

[48] Adams, Cecil (April 1, 1998). “Can you pay $35 to geta star named after you?". The Straight Dope. Retrieved2006-08-13.

[49] Golden, Frederick; Faflick, Philip (January 11, 1982).“Science: Stellar Idea or Cosmic Scam?". Times Maga-zine (Time Inc.). Retrieved 2010-06-24.

[50] Di Justo, Patrick (December 26, 2001). “Buy a Star, ButIt’s Not Yours”. Wired (Condé Nast Digital). Retrieved2010-06-29.

[51] Plait, Philip C. (2002). Bad astronomy: misconceptionsand misuses revealed, from astrology to the moon landing“hoax”. John Wiley and Sons. pp. 237–240. ISBN 0-471-40976-6.

[52] Sclafani, Tom (May 8, 1998). “Consumer Affairs Com-missioner Polonetsky Warns Consumers: “Buying A StarWon't Make You One"". National Astronomy and Iono-sphere Center, Aricebo Observatory. Retrieved 2010-06-24.

[53] Sackmann, I.-J.; Boothroyd, A. I. (2003). “OurSun. V. A Bright Young Sun Consistent withHelioseismology and Warm Temperatures on An-cient Earth and Mars”. The Astrophysical Journal583 (2): 1024–1039. arXiv:astro-ph/0210128.Bibcode:2003ApJ...583.1024S. doi:10.1086/345408.

[54] Tripathy, S. C.; Antia, H.M. (1999). “Influence of surfacelayers on the seismic estimate of the solar radius”. SolarPhysics 186 (1/2): 1–11. Bibcode:1999SoPh..186....1T.doi:10.1023/A:1005116830445.

[55] Woodward, P. R. (1978). “Theoretical models of star for-mation”. Annual review of astronomy and astrophysics16 (1): 555–584. Bibcode:1978ARA&A..16..555W.doi:10.1146/annurev.aa.16.090178.003011.

[56] Kwok, Sun (2000). The origin and evolution of planetarynebulae. Cambridge astrophysics series 33. CambridgeUniversity Press. pp. 103–104. ISBN 0-521-62313-8.

[57] Smith, Michael David (2004). The Origin of Stars. Impe-rial College Press. pp. 57–68. ISBN 1-86094-501-5.

[58] Seligman, Courtney. “Slow Contraction of ProtostellarCloud”. Self-published. Archived from the original on2008-06-23. Retrieved 2006-09-05.

[59] Bally, J.; Morse, J.; Reipurth, B. (1996). “The Birth ofStars: Herbig-Haro Jets, Accretion and Proto-PlanetaryDisks”. In Benvenuti, Piero; Macchetto, F. D.; Schreier,Ethan J. Science with the Hubble Space Telescope – II. Pro-ceedings of a workshop held in Paris, France, December

4–8, 1995. Space Telescope Science Institute. p. 491.Bibcode:1996swhs.conf..491B.

[60] Smith, Michael David (2004). The origin of stars. Impe-rial College Press. p. 176. ISBN 1-86094-501-5.

[61] Megeath, Tom (May 11, 2010). “Herschel finds a hole inspace”. ESA. Retrieved 2010-05-17.

[62] Mengel, J. G. et al. (1979). “Stellar evolu-tion from the zero-age main sequence”. Astro-physical Journal Supplement Series 40: 733–791.Bibcode:1979ApJS...40..733M. doi:10.1086/190603.

[63] Sackmann, I. J.; Boothroyd, A. I.; Kraemer, K. E.(1993). “Our Sun. III. Present and Future”. Astrophys-ical Journal 418: 457. Bibcode:1993ApJ...418..457S.doi:10.1086/173407.

[64] Wood, B. E. et al. (2002). “Measured Mass-Loss Ratesof Solar-like Stars as a Function of Age and Activity”.The Astrophysical Journal 574 (1): 412–425. arXiv:astro-ph/0203437. Bibcode:2002ApJ...574..412W.doi:10.1086/340797.

[65] de Loore, C.; de Greve, J. P.; Lamers, H. J. G. L. M.(1977). “Evolution of massive stars with mass loss by stel-lar wind”. Astronomy and Astrophysics 61 (2): 251–259.Bibcode:1977A&A....61..251D.

[66] “The evolution of stars between 50 and 100 times themassof the Sun”. Royal Greenwich Observatory. Retrieved2006-09-07.

[67] Adams, Fred C.; Laughlin, Gregory; Graves, GenevieveJ. M. “Red Dwarfs and the End of the Main Sequence”.Gravitational Collapse: FromMassive Stars to Planets. Re-vista Mexicana de Astronomía y Astrofísica. pp. 46–49.Retrieved 2008-06-24.

[68] “Main Sequence Lifetime”. Swinburne Astronomy On-line Encyclopedia of Astronomy. Swinburne University ofTechnology.

[69] Pizzolato, N. et al. (2001). “Subphotospheric con-vection and magnetic activity dependence on metallicityand age: Models and tests”. Astronomy & Astrophysics373 (2): 597–607. Bibcode:2001A&A...373..597P.doi:10.1051/0004-6361:20010626.

[70] “Mass loss and Evolution”. UCL Astrophysics Group.June 18, 2004. Archived from the original on 2004-11-22. Retrieved 2006-08-26.

[71] Schröder, K.-P.; Smith, Robert Connon (2008). “Dis-tant future of the Sun and Earth revisited”. MonthlyNotices of the Royal Astronomical Society 386 (1): 155.arXiv:0801.4031. Bibcode:2008MNRAS.386..155S.doi:10.1111/j.1365-2966.2008.13022.x. See alsoPalmer, Jason (February 22, 2008). “Hope dims thatEarth will survive Sun’s death”. NewScientist.com newsservice. Retrieved 2008-03-24.

[72] Hinshaw, Gary (August 23, 2006). “The Life and Deathof Stars”. NASA WMAP Mission. Retrieved 2006-09-01.

54 CHAPTER 5. STAR

[73] “What is a star?". Royal Greenwich Observatory. Re-trieved 2006-09-07.

[74] Liebert, J. (1980). “White dwarf stars”. An-nual review of astronomy and astrophysics 18(2): 363–398. Bibcode:1980ARA&A..18..363L.doi:10.1146/annurev.aa.18.090180.002051.

[75] “Introduction to Supernova Remnants”. Goddard SpaceFlight Center. April 6, 2006. Retrieved 2006-07-16.

[76] Fryer, C. L. (2003). “Black-hole formation from stellarcollapse”. Classical and Quantum Gravity 20 (10): S73–S80. Bibcode:2003CQGra..20S..73F. doi:10.1088/0264-9381/20/10/309.

[77] Szebehely, Victor G.; Curran, Richard B. (1985). Stabilityof the Solar System and Its Minor Natural and ArtificialBodies. Springer. ISBN 90-277-2046-0.

[78] “Most Milky Way Stars Are Single” (Press release).Harvard-Smithsonian Center for Astrophysics. January30, 2006. Retrieved 2006-07-16.

[79] “What is a galaxy? How many stars in a galaxy / the Uni-verse?". Royal Greenwich Observatory. Retrieved 2006-07-18.

[80] Borenstein, Seth (December 1, 2010). “Universe’s StarCount Could Triple”. CBS News. Retrieved 2011-07-14.

[81] “Hubble Finds Intergalactic Stars”. Hubble News Desk.January 14, 1997. Retrieved 2006-11-06.

[82] 3.99 × 1013 km / (3 × 104 km/h × 24 × 365.25) = 1.5 ×105 years.

[83] Holmberg, J.; Flynn, C. (2000). “The local density ofmat-ter mapped by Hipparcos”. Monthly Notices of the RoyalAstronomical Society 313 (2): 209–216. arXiv:astro-ph/9812404. Bibcode:2000MNRAS.313..209H.doi:10.1046/j.1365-8711.2000.02905.x.

[84] “Astronomers: Star collisions are rampant, catastrophic”.CNN News. June 2, 2000. Retrieved 2014-01-21.

[85] Lombardi, Jr., J. C. et al. (2002). “Stellar Collisions andthe Interior Structure of Blue Stragglers”. The Astrophys-ical Journal 568 (2): 939–953. arXiv:astro-ph/0107388.Bibcode:2002ApJ...568..939L. doi:10.1086/339060.

[86] H. E. Bond; E. P. Nelan; D. A. VandenBerg; G. H.Schaefer; D. Harmer (2013). “HD 140283: A Star inthe Solar Neighborhood that Formed Shortly After theBig Bang”. The Astrophysical Journal Letters 765 (1):L12. arXiv:1302.3180. Bibcode:2013ApJ...765L..12B.doi:10.1088/2041-8205/765/1/L12.

[87] Naftilan, S. A.; Stetson, P. B. (July 13, 2006). “How doscientists determine the ages of stars? Is the techniquereally accurate enough to use it to verify the age of theuniverse?". Scientific American. Retrieved 2007-05-11.

[88] Laughlin, G.; Bodenheimer, P.; Adams, F. C. (1997).“The End of theMain Sequence”. The Astrophysical Jour-nal 482 (1): 420–432. Bibcode:1997ApJ...482..420L.doi:10.1086/304125.

[89] Stellar Molecules » American Scientist

[90] Irwin, Judith A. (2007). Astrophysics: Decoding the Cos-mos. John Wiley and Sons. p. 78. ISBN 0-470-01306-0.

[91] Fischer, D. A.; Valenti, J. (2005). “The Planet-Metallicity Correlation”. The Astrophysical Journal622 (2): 1102–1117. Bibcode:2005ApJ...622.1102F.doi:10.1086/428383.

[92] “Signatures Of The First Stars”. ScienceDaily. April 17,2005. Retrieved 2006-10-10.

[93] Feltzing, S.; Gonzalez, G. (2000). “The nature of super-metal-rich stars: Detailed abundance analysis of 8 super-metal-rich star candidates”. Astronomy & Astrophysics367 (1): 253–265. Bibcode:2001A&A...367..253F.doi:10.1051/0004-6361:20000477.

[94] Gray, David F. (1992). The Observation and Analysis ofStellar Photospheres. Cambridge University Press. pp.413–414. ISBN 0-521-40868-7.

[95] “The Biggest Star in the Sky”. ESO. March 11, 1997.Retrieved 2006-07-10.

[96] Ragland, S.; Chandrasekhar, T.; Ashok, N. M. (1995).“Angular Diameter of Carbon Star Tx-Piscium fromLunar Occultation Observations in the Near Infrared”.Journal of Astrophysics and Astronomy 16: 332.Bibcode:1995JApAS..16..332R.

[97] Davis, Kate (December 1, 2000). “Variable Star of theMonth—December, 2000: Alpha Orionis”. AAVSO.Archived from the original on 2006-07-12. Retrieved2006-08-13.

[98] Loktin, A. V. (September 2006). “Kinematics ofstars in the Pleiades open cluster”. Astronomy Re-ports 50 (9): 714–721. Bibcode:2006ARep...50..714L.doi:10.1134/S1063772906090058.

[99] “Hipparcos: High ProperMotion Stars”. ESA. September10, 1999. Retrieved 2006-10-10.

[100] Johnson, Hugh M. (1957). “The Kinematics and Evo-lution of Population I Stars”. Publications of theAstronomical Society of the Pacific 69 (406): 54.Bibcode:1957PASP...69...54J. doi:10.1086/127012.

[101] Elmegreen, B.; Efremov, Y. N. (1999). “TheFormation of Star Clusters”. American Scien-tist 86 (3): 264. Bibcode:1998AmSci..86..264E.doi:10.1511/1998.3.264. Archived from the original onMarch 23, 2005. Retrieved 2006-08-23.

[102] Brainerd, Jerome James (July 6, 2005). “X-rays fromStellar Coronas”. The Astrophysics Spectator. Retrieved2007-06-21.

[103] Berdyugina, Svetlana V. (2005). “Starspots: A Key to theStellar Dynamo”. Living Reviews. Retrieved 2007-06-21.

[104] Smith, Nathan (1998). “The Behemoth Eta Carinae: ARepeat Offender”. Mercury Magazine (Astronomical So-ciety of the Pacific) 27: 20. Retrieved 2006-08-13.

5.13. REFERENCES 55

[105] “NASA’s Hubble Weighs in on the Heaviest Stars in theGalaxy”. NASA News. March 3, 2005. Retrieved 2006-08-04.

[106] Hainich, R.; Rühling, U.; Todt, H.; Oskinova, L. M.;Liermann, A.; Gräfener, G.; Foellmi, C.; Schnurr,O.; Hamann, W. -R. (2014). “The Wolf-Rayet starsin the Large Magellanic Cloud”. Astronomy & Astro-physics 565: A27. arXiv:1401.5474. doi:10.1051/0004-6361/201322696.

[107] Wolchover, Natalie (August 7, 2012). “Mystery of the'Monster Stars’ Solved: It Was a Monster Mash”. Live-Science.com.

[108] “Ferreting Out The First Stars”. Harvard-SmithsonianCenter for Astrophysics. September 22, 2005. Retrieved2006-09-05.

[109] “2MASS J05233822-1403022”. SIMBAD - Centre deDonnées astronomiques de Strasbourg. Retrieved 14 De-cember 2013.

[110] Boss, Alan (April 3, 2001). “Are They Planets orWhat?".Carnegie Institution of Washington. Archived from theoriginal on 2006-09-28. Retrieved 2006-06-08.

[111] Shiga, David (August 17, 2006). “Mass cut-off be-tween stars and brown dwarfs revealed”. New Scientist.Archived from the original on 2006-11-14. Retrieved2006-08-23.

[112] Leadbeater, Elli (August 18, 2006). “Hubble glimpsesfaintest stars”. BBC. Retrieved 2006-08-22.

[113] “Flattest Star Ever Seen”. ESO. June 11, 2003. Retrieved2006-10-03.

[114] Fitzpatrick, Richard (February 13, 2006). “Introductionto Plasma Physics: A graduate course”. The University ofTexas at Austin. Archived from the original on 2010-01-04. Retrieved 2006-10-04.

[115] Villata, Massimo (1992). “Angular momentum loss bya stellar wind and rotational velocities of white dwarfs”.Monthly Notices of the Royal Astronomical Society 257 (3):450–454. Bibcode:1992MNRAS.257..450V.

[116] “A History of the Crab Nebula”. ESO. May 30, 1996.Retrieved 2006-10-03.

[117] Strobel, Nick (August 20, 2007). “Properties ofStars: Color and Temperature”. Astronomy Notes.Primis/McGraw-Hill, Inc. Archived from the original on2007-06-26. Retrieved 2007-10-09.

[118] Seligman, Courtney. “Review of Heat Flow Inside Stars”.Self-published. Retrieved 2007-07-05.

[119] “Main Sequence Stars”. The Astrophysics Spectator.February 16, 2005. Retrieved 2006-10-10.

[120] Zeilik, Michael A.; Gregory, Stephan A. (1998). Intro-ductory Astronomy & Astrophysics (4th ed.). SaundersCollege Publishing. p. 321. ISBN 0-03-006228-4.

[121] Koppes, Steve (June 20, 2003). “University of Chicagophysicist receives Kyoto Prize for lifetime achievementsin science”. The University of Chicago News Office. Re-trieved 2012-06-15.

[122] “TheColour of Stars”. Australian TelescopeOutreach andEducation. Retrieved 2006-08-13.

[123] “AstronomersMeasureMass of a Single Star—First Sincethe Sun”. Hubble News Desk. July 15, 2004. Retrieved2006-05-24.

[124] Garnett, D. R.; Kobulnicky, H. A. (2000). “Dis-tance Dependence in the Solar NeighborhoodAge-Metallicity Relation”. The Astrophysical Jour-nal 532 (2): 1192–1196. arXiv:astro-ph/9912031.Bibcode:2000ApJ...532.1192G. doi:10.1086/308617.

[125] Staff (January 10, 2006). “Rapidly Spinning Star Vega hasCool Dark Equator”. National Optical Astronomy Obser-vatory. Retrieved 2007-11-18.

[126] Michelson, A. A.; Pease, F. G. (2005). “Starspots: A Keyto the Stellar Dynamo”. Living Reviews in Solar Physics(Max Planck Society).

[127] Manduca, A.; Bell, R. A.; Gustafsson, B. (1977). “Limbdarkening coefficients for late-type giant model atmo-spheres”. Astronomy and Astrophysics 61 (6): 809–813.Bibcode:1977A&A....61..809M.

[128] Chugainov, P. F. (1971). “On the Cause of Pe-riodic Light Variations of Some Red Dwarf Stars”.Information Bulletin on Variable Stars 520: 1–3.Bibcode:1971IBVS..520....1C.

[129] “Magnitude”. National Solar Observatory—SacramentoPeak. Archived from the original on 2008-02-06. Re-trieved 2006-08-23.

[130] “Luminosity of Stars”. Australian Telescope Outreachand Education. Retrieved 2006-08-13.

[131] Hoover, Aaron (January 15, 2004). “Star may be biggest,brightest yet observed”. HubbleSite. Archived from theoriginal on 2007-08-07. Retrieved 2006-06-08.

[132] “Faintest Stars in Globular Cluster NGC 6397”. Hubble-Site. August 17, 2006. Retrieved 2006-06-08.

[133] Smith, Gene (April 16, 1999). “Stellar Spectra”. Univer-sity of California, San Diego. Retrieved 2006-10-12.

[134] Fowler, A. (February 1891). “The Draper Cat-alogue of Stellar Spectra”. Nature 45: 427–8.Bibcode:1892Natur..45..427F. doi:10.1038/045427a0.

[135] Jaschek, Carlos; Jaschek, Mercedes (1990). The Classifi-cation of Stars. Cambridge University Press. pp. 31–48.ISBN 0-521-38996-8.

[136] MacRobert, Alan M. “The Spectral Types of Stars”. Skyand Telescope. Retrieved 2006-07-19.

[137] “White Dwarf (wd) Stars”. White Dwarf Research Cor-poration. Archived from the original on 2009-10-08. Re-trieved 2006-07-19.

56 CHAPTER 5. STAR

[138] “Types of Variable”. AAVSO. May 11, 2010. Retrieved2010-08-20.

[139] “Cataclysmic Variables”. NASA Goddard Space FlightCenter. 2004-11-01. Retrieved 2006-06-08.

[140] Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia(2004). Stellar Interiors. Springer. pp. 32–33. ISBN0-387-20089-4.

[141] Schwarzschild, Martin (1958). Structure and Evolution ofthe Stars. PrincetonUniversity Press. ISBN 0-691-08044-5.

[142] “Formation of the High Mass Elements”. Smoot Group.Retrieved 2006-07-11.

[143] “What is a Star?". NASA. 2006-09-01. Retrieved 2006-07-11.

[144] “The Glory of a Nearby Star: Optical Light from a HotStellar Corona Detected with the VLT” (Press release).ESO. August 1, 2001. Retrieved 2006-07-10.

[145] Burlaga, L. F. et al. (2005). “Crossing the TerminationShock into the Heliosheath: Magnetic Fields”. Science309 (5743): 2027–2029. Bibcode:2005Sci...309.2027B.doi:10.1126/science.1117542. PMID 16179471.

[146] Wallerstein, G. et al. (1999). “Synthesis ofthe elements in stars: forty years of progress”(PDF). Reviews of Modern Physics 69 (4):995–1084. Bibcode:1997RvMP...69..995W.doi:10.1103/RevModPhys.69.995. Retrieved 2006-08-04.

[147] Girardi, L.; Bressan, A.; Bertelli, G.; Chiosi, C.(2000). “Evolutionary tracks and isochrones for low-and intermediate-mass stars: From 0.15 to 7 M ᵤ ,and from Z=0.0004 to 0.03”. Astronomy and Astro-physics Supplement 141 (3): 371–383. arXiv:astro-ph/9910164. Bibcode:2000A&AS..141..371G.doi:10.1051/aas:2000126.

[148] Woosley, S. E.; Heger, A.; Weaver, T. A.(2002). “The evolution and explosion of mas-sive stars”. Reviews of Modern Physics 74 (4):1015–1071. Bibcode:2002RvMP...74.1015W.doi:10.1103/RevModPhys.74.1015.

[149] 11.5 days is 0.0315 years.

5.14 Further reading

• Pickover, Cliff (2001). The Stars of Heaven. OxfordUniversity Press. ISBN 0-19-514874-6.

• Gribbin, John; Gribbin, Mary (2001). Stardust: Su-pernovae and Life—The Cosmic Connection. YaleUniversity Press. ISBN 0-300-09097-8.

• Hawking, Stephen (1988). A Brief History of Time.Bantam Books. ISBN 0-553-17521-1.

5.15 External links• Kaler, James. “Portraits of Stars and their Constel-lations”. University of Illinois. Retrieved 2010-08-20.

• “Query star by identifier, coordinates or referencecode”. SIMBAD. Centre de Données astronomiquesde Strasbourg. Retrieved 2010-08-20.

• “How To Decipher Classification Codes”. Astro-nomical Society of South Australia. Retrieved2010-08-20.

• “Live Star Chart”. Dobsonian Telescope Commu-nity. Retrieved 2010-08-20. View the stars aboveyour location

• Prialnick, Dina et al. (2001). “Stars: Stellar Atmo-spheres, Structure, & Evolution”. University of St.Andrews. Retrieved 2010-08-20.

5.16. TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES 57

5.16 Text and image sources, contributors, and licenses

5.16.1 Text• Constellation Source: http://en.wikipedia.org/wiki/Constellation?oldid=656293863 Contributors: Kpjas, Bryan Derksen, Zundark, As-

troNomer, Andre Engels, XJaM, Roadrunner, Heron, B4hand, Montrealais, Bignose, Olivier, Lorenzarius, Nealmcb, Zocky, Martin-Harper, Ixfd64, Minesweeper, Looxix, Ahoerstemeier, Caid Raspa, Brettz9, Александър, Andres, Tristanb, Jiang, Jeandré du Toit, JohnK, Ruhrjung, Smack, Timwi, Radiojon, Ishu, Furrykef, Omegatron, Xevi, JonathanDP81, Bloodshedder, Rossumcapek, Jni, ChuunenBaka, Donarreiskoffer, Robbot, Moriori, Kizor, Zandperl, Mirv, Rursus, Bkell, Conrad Leviston, Borislav, Benc, Xanzzibar, Vacuum,Giftlite, Dbenbenn, DavidCary, Jyril, Amorim Parga, Harp, Tom harrison, Zigger, Herbee, Piquan, Moyogo, Curps, Michael Devore,Wikibob, Dmmaus, Jason Quinn, Python eggs, Meddlin' Pedant, Antandrus, HorsePunchKid, Maximaximax, Tomruen, Icairns, Tail, SamHocevar, Adashiel, Spiffy sperry, CALR, Moverton, Diagonalfish, Discospinster, Rich Farmbrough, Guanabot, Spundun, Eitheladar, Ja-madagni, Dbachmann, SpookyMulder, Janderk, Eric Forste, RJHall, El C, Kwamikagami, Hayabusa future, Shanes, Juppiter, Art LaPella,Pablo X, Bobo192, NetBot, Smalljim, Neotuli, Man vyi, Eritain, LostLeviathan, Caeruleancentaur, Pearle, Alansohn, Eric Kvaalen, An-drew Gray, Lord Pistachio, Riana, Goldom, Wdfarmer, DreamGuy, Snowolf, HenkvD, Amorymeltzer, Inge-Lyubov, Pi lambda, Zoohouse,Alai, Mosesofmason, Feezo, WilliamKF, Woohookitty, TigerShark, Ukulele, Rocastelo, Sburke, StradivariusTV, Davidkazuhiro, Falconer,Kristaga, -Ril-, Trapolator, Sengkang, Andromeda321, Graham87, JIP, Mendaliv, Angusmclellan, JakeWartenberg, Sdornan, Mike s, Veg-aswikian, Matt Deres, DuKot, Vuong Ngan Ha, SchuminWeb, Merv, Nihiltres, Nivix, Hottentot, RexNL, Gurch, Irregulargalaxies, King ofHearts, CiaPan, Chobot, Samwaltz, The Rambling Man, Spventi, Deeptrivia, Jimp, Quince, Brandmeister (old), Petiatil, Arado, Red Slash,GLaDOS, SpuriousQ, RadioFan2 (usurped), Stephenb, CambridgeBayWeather, Wimt, Shanel, NawlinWiki, Wiki alf, Buster79, Grafen,RyanGardner, Anomity, Nick, E rulez, Aldux, Moe Epsilon, Kortoso, DeadEyeArrow, Nlu, Ms2ger, Light current, Closedmouth, Pb30,Livitup, TBadger, Airodyssey, Archer7, Argo Navis, Allens, Junglecat, Moomoomoo, Cookiedog, Cmglee, SmackBot, Stellar McArt-ney, Prodego, Vald, Hu Gadarn, Delldot, Canthusus, BiT, Gilliam, Ohnoitsjamie, Skizzik, GwydionM, Andy M. Wang, Frédérick Lacasse,Saros136, Chris the speller, Bluebot, Keegan, MK8, B00P, SchfiftyThree, Raistuumum, Colonies Chris, Jdthood, Can't sleep, clown will eatme, Shalom Yechiel, Moonsword, Frap, Argyriou, LiamH, Rrburke, Celarnor, AgentFade2Black, Aldaron, Rajwoodson, Zrulli, Makemi,Bowlhover, Metebelis, Dreadstar, DMacks, Kalathalan, Bejnar, Kukini, Thor Dockweiler, TenPoundHammer, Thejerm, EmirA, Lambiam,Swatjester, J 1982, Gobonobo, Ckatz, RandomCritic, Booksworm, Avs5221, Unistardust, BranStark, Fredil Yupigo, Dead3y3, Iridescent,Dekaels, Kostya30, Joseph Solis in Australia, Wjejskenewr, PrettyMuchBryce, Marysunshine, Courcelles, Tawkerbot2, Anthony Arrigo,Benabik, CmdrObot, JohnCD, Verdi1, Gogo Dodo, Dougweller, Fcn, FastLizard4, Quadrius, Thijs!bot, Epbr123, Mercury, N5iln, Berria,Headbomb, Marek69, The Proffesor, Dawnseeker2000, Escarbot, Thadius856, Tham153, AntiVandalBot, Majorly, Luna Santin, Summer-PhD, CZmarlin, AstroLynx, DarkAudit, Rabbi-m, Dr. Submillimeter, Gh5046, Golgofrinchian, JAnDbot, Dan D. Ric, MER-C, Dsp13,Kerotan, Bongwarrior, VoABot II, Theranos, Froid, KConWiki, Catgut, Seberle, Shijualex, Glen, DerHexer, JaGa, Edward321, Jorgy,Kheider, Bieb, MartinBot, TinaD, Twtunes, Ultraviolet scissor flame, AlexiusHoratius, Ash, J.delanoy, Trusilver, Skeptic2, Pharos04, Bo-gey97, 12dstring, Richontaban, McSly, Kishor narayan, Canadian Scouter, Coin945, Hut 6.5, NewEnglandYankee, Antony-22, Fountainsof BrynMawr, SmilesALot, Zerokitsune, Bobianite, Donmarkdixon, KylieTastic, Juliancolton, CF90, Vanished user 39948282, Apd02468,Ketchuphed, Pdcook, Ja 62, Meatwad761, Izno, Xiahou, CardinalDan, Redtigerxyz, VolkovBot, Philip Trueman, TXiKiBoT, ElinorD, Pa-pajfd, Qxz, Someguy1221, Martin451, JhsBot, Psyche825, Delbert Grady, Inductiveload, Lala botpot nef, Dirkbb, Turgan, AlleborgoBot,Noncompliant one, EmxBot, EJF, Thomas94, SieBot, Mikemoral, Tresiden, PlanetStar, Malcolmxl5, BotMultichill, Calabraxthis, Srushe,Keilana, PookeyMaster, Bentogoa, Happysailor, RadicalOne, Flyer22, Hiddenfromview, Redmarkviolinist, Oxymoron83, Leonsito, OK-Bot, Spitfire19, Dear Reader, Mr. Stradivarius, Felizdenovo, Denisarona, Escape Orbit, Vonones, REDYVA, Netopyr-e, Stranded888,Twinsday, Martarius, ClueBot, PipepBot, Fyyer, The Thing That Should Not Be, IceUnshattered, EoGuy, Jan1nad, Gaia Octavia Agrippa,J.H.McDonnell, Uncle Milty, DrFO.Jr.Tn, Blanchardb, Big-dynamo, ChandlerMapBot, Wasangtse, EcoJack, Excirial, Jusdafax, Wikitum-nus, Dancechica, Jerry Zhang, Jotterbot, Gmaliha, Razorflame, SockPuppetForTomruen, JasonAQuest, Aitias, Versus22, PCHS-NJROTC,SoxBot III, DumZiBoT, Minacee1996, XLinkBot, Gnowor, Dark Mage, Feinoha, Mifter, Heeryung97, The Rationalist, RyanCross, That-guyflint, CalumH93, Addbot, Proofreader77, Jojhutton, Atethnekos, Fort miner, Dgroseth, CanadianLinuxUser, Download, LaaknorBot,Glane23, Chzz, Petifile, West.andrew.g, 5 albert square, Numbo3-bot, Theking17825, Issyl0, Tide rolls, Lightbot, OlEnglish, Jhayeur,Hihi77, Luckas-bot, Yobot, AnakngAraw, AnomieBOT, HairyPerry, JackieBot, Archie carnforth, AdjustShift, Apau98, Ulric1313, Mannjess, Materialscientist, Limideen, RadioBroadcast, ArdWar, Citation bot, Nikolay94, Maxis ftw, Iloveham, Xqbot, Random astronomer,Capricorn42, Khajidha, Renaissancee, Blue4511, Arsia Mons, Vagabond writer, GrouchoBot, Wizardist, G2345, Brandon5485, Matho-nius, AirFury, Trafford09, Moxy, Shadowjams, Ehird, Erik9, GHJmover, Dougofborg, BoomerAB, Tangent747, Younggi96, Sirtywell,Fooguu, Jc3s5h, VS6507, Prisonermonkeys, Musicluver123, Wireless Keyboard, AstaBOTh15, Pinethicket, I dream of horses, Osan-tos2009, Tom.Reding, Rushbugled13, Lithium cyanide, RedBot, Mono, Vrenator, Einstein Li 37, Reaper Eternal, CharlesJoshua, Jeffrd10,Schmei, Coco950, Reach Out to the Truth, RobertMfromLI, Whisky drinker, Mean as custard, TheArguer, Bhawani Gautam, Tesseract2,DASHBot, Spectral Diagram, Zaqq, EmausBot, Orphan Wiki, WikitanvirBot, Heracles31, Kellylynnluv, Racerx11, GoingBatty, RA0808,Till Credner, RenamedUser01302013, Tommy2010, Wikipelli, P. S. F. Freitas, Lamb99, Italia2006, ZéroBot, Daonguyen95, ClaudioM Souza, StringTheory11, NicatronTg, Dffgd, Azuris, Hobick54, Cline92, Wayne Slam, Openstrings, Мл.научный сотрудник, Don-ner60, Moocow121, ChuispastonBot, ديرانية ,عباد Sven Manguard, Alihubbard, Nirakka, ClueBot NG, Gareth Griffith-Jones, Manubot,MelbourneStar, रामा, Mouse20080706, Coleandzoey, KasandraMartinez, Theopolisme, Periferomenos, Oddbodz, Helpful Pixie Bot, Cal-abe1992, DBigXray, Vagobot, Ninney, Michael Cockrell, Colinmartin74, Tung889, ולדמן שמחה ,יהודה Adamstraw99, The Illusive Man,Metalello, Tahc, Harsh 2580, Dexbot, Webclient101, Dubcityrockin, S000001.5, Hrithik5, Saleem100, Reatlas, Rfassbind, Sjbrae0859,Rahite6202, MarchOrDie, Varun papola, Kenneth deleon, SamX, DavidLeighEllis, Navzvanz, HalfGig, Noyster, Charlotte Charity,Wiki.authoring, Vieque, BrightonC, LOLLABABA, Amortias, Axel Azzopardi, Gephyra, Catobonus, Tetra quark, KILLtoWIN326 andAnonymous: 915

• Galaxy Source: http://en.wikipedia.org/wiki/Galaxy?oldid=655020980 Contributors: AxelBoldt, MagnusManske, The Epopt, Derek Ross,Mav, AstroNomer, -- April, XJaM, Chrislintott, Arvindn, DavidLevinson, Ben-Zin, Imran, B4hand, Olivier, Spiff, Frecklefoot, Infrogma-tion, Liftarn, Menchi, Ixfd64, Cyde, Arpingstone, Minesweeper, Alfio, Kosebamse, Looxix, Ellywa, Ahoerstemeier, Docu, Julesd, Jll,Glenn, Marteau, AugPi, Nikai, Andres, TonyClarke, Lee M, Conti, Seth ze, Schneelocke, Timwi, Wikiborg, Birkett, DW40, Traroth,AlexPlank, Nyh, Romanm, Smallweed, Mirv, Rursus, Diderot, Borislav, Profoss, Seth Ilys, Clementi, Centrx, Giftlite, Graeme Bartlett,DocWatson42, Jyril, Cobaltbluetony, Holofect, Xerxes314, Curps, Alison, JamesHoadley, Gareth Wyn, Siroxo, Jackol, Mateuszica, JosephDwayne, Wolfgang1018, Chowbok, Cbraga, Antandrus, Beland, MisfitToys, Piotrus, Jossi, Karol Langner, The Land, Kesac, Latitude0116,Mike Storm, Satori, Icairns, Iantresman, Trevor MacInnis, SYSS Mouse, Mike Rosoft, D6, Jiy, Discospinster, Rich Farmbrough, DIU-ZOMA, Vsmith, Jpk, Florian Blaschke, Fleung, Mani1, SpookyMulder, Bender235, Neurophyre, Neko-chan, RJHall, Livajo, El C, Edwin-

58 CHAPTER 5. STAR

stearns, Lycurgus, DamianFinol, Bletch, Shanes, Art LaPella, Triona, Bobo192, Smalljim, John Vandenberg, I9Q79oL78KiL0QTFHgyc,Joe Jarvis, Irrawaddy, Jojit fb, Nk, PeterisP, Ardric47, Obradovic Goran, MPerel, Mpulier, HasharBot, Ranveig, Quaoar, Alansohn,DimaY2K, Keenan Pepper, Sade, Mailer diablo, Mysdaao, Avenue, Snowolf, H2g2bob, Bsadowski1, Kusma, Gene Nygaard, HenryLi,Kitch, Bruce89, Tariqabjotu, Stephen, Hojimachong, WilliamKF, Zanaq, FrancisTyers, Woohookitty, Sburke, Plek, Oliphaunt, Jeff3000,Schzmo, Sengkang, GregorB, Cedrus-Libani, Palica, Dysepsion, SqueakBox, A Train, Kbdank71, Zoz, Drbogdan, Rjwilmsi, Zbxgscqf,Vary, Marasama, Hjb26, JHMM13, Captmondo, Mike s, Ligulem, Jehochman, ScottJ, Brighterorange, Krash, The wub, Watcharakorn,SchuminWeb, RobertG, Salva31, Matthewhayes, RexNL, Gurch, DannyZ, Jesse0986, MoRsE, King of Hearts, CJLL Wright, Chobot,DVdm, Korg, Cactus.man, Wjfox2005, Scaryrobot, YurikBot, Wavelength, TexasAndroid, Spacepotato, Reverendgraham, Sceptre, HuwPowell, Phantomsteve, DanMS, Lucinos, Stephenb, Polluxian, Hogghogg, Rsrikanth05, Greenyoda, Wimt, Anomalocaris, NawlinWiki,Wiki alf, Grafen, Tfine80, Johann Wolfgang, Dbmag9, Banes, Peter Delmonte, Matticus78, Dputig07, Bobak, Raven4x4x, Number 57,RL0919, Nick C, Semperf, Syrthiss, Gadget850, DeadEyeArrow, Bota47, Everyguy, Essexmutant, Martinwilke1980, Dna-webmaster,User27091, Wknight94, Ageekgal, Closedmouth, Pb30, JoanneB, Flehmen, HereToHelp, Garion96, Argo Navis, Katieh5584, Junglecat,GrinBot, Serendipodous, DVD R W, AndrewWTaylor, Mhardcastle, Tyler Oderkirk, SmackBot, Afita, Unschool, Thierry Caro, Ashill,Abhimat.gautam, Prodego, KnowledgeOfSelf, MHD, Felix Dance, Jagged 85, Dims, Took, Edgar181, SmartGuy Old, Gaff, Cool3, Pe-ter Isotalo, Gilliam, Betacommand, Skizzik, Rst20xx, DetlevSchm, Green meklar, MalafayaBot, SchfiftyThree, DHN-bot, E946, Tsca.bot,Can't sleep, clownwill eat me, OrphanBot, NLUT,Xiner, Rrburke, Tyche151, Japeo, Addshore, Mr.Z-man, Deathlie, SundarBot, Phaedriel,Robma, Ne0Freedom, John D. Croft, Monotonehell, DMPalmer, DMacks, Jeremyb, Henning Makholm, Salamurai, LeoNomis, Evlekis,Vina-iwbot, Kukini, Ceoil, SashatoBot, ArglebargleIV, Rory096, Petr Kopač, Sophia, Kuru, Microchip08, Bilboon, Scientizzle, So9q,Drkencarter, Linnell, JorisvS, Tim Q. Wells, Mr. Lefty, IronGargoyle, Serpentinite, Rock4arolla, Optakeover, Johnchiu, Ryulong, MTS-bot, Jose77, Colin Johnston, Hu12, Marshall Stax, BranStark, Iridescent, JMK, Halfblue, CapitalR, Courcelles, Tawkerbot2, Dlohcierekim,JForget, Stifynsemons, Kvantti, Irwangatot, Scohoust, Aherunar, Eric, Melicans, AshLin, BigMar992, Lazulilasher, Logical2u, Gran2,Ispy1981, Funnyfarmofdoom, Dominicanpapi82, Gogo Dodo, Bellerophon5685, Pedia wiki, Not me, EricandHolli, Michael C Price,Tawkerbot4, DumbBOT, Kozuch, Omicronpersei8, JodyB, UberScienceNerd, Crum375, Casliber, Thijs!bot, Epbr123, Lynndunn, KingBee, Notjake13, Sry85, Memty Bot, Ginosal, Headbomb, Tonyle, Marek69, James086, Lars Lindberg Christensen, Ideogram, Nick Num-ber, Big Bird, Sbandrews, Lfastrup, Spud Gun, AntiVandalBot, Yonatan, Luna Santin, Seaphoto, Sean K, DarkAudit, Lyricmac, Ste4k, Dr.Submillimeter, Trippcook, Jhsounds, Gdo01, Spencer, Ioeth, Barek, MER-C, Janejellyroll, Hut 8.5, Rothorpe, Chad Hennings, Bencher-lite, WolfmanSF, PacificBoy, Murgh, Bongwarrior, VoABot II, Khan singh, JNW, Mbc362, Faizhaider, CTF83!, Homie07, Aka042,Midgrid, Theroadislong, Panser Born, KirinX, Animum, MojoTas, 28421u2232nfenfcenc, Allstarecho, Spssbkp, Shijualex, Glen, ChrisG, DerHexer, Kestasjk, Iazz, IvoShandor, Danielratiu, Cocytus, NatureA16, Hdt83, MartinBot, Mschel, CommonsDelinker, AlexiusHor-atius, Player 03, Fellwalker57, Tgeairn, Artaxiad, Watch37264, J.delanoy, DrKiernan, Euku, Rrostrom, Bogey97, UBeR, Uncle Dick,Ashcraft, Foober, Supergra, Jack of ages, Katalaveno, Rocket71048576, GhostPirate, TomasBat, Rominandreu, Ohms law, Touch OfLight, Workofthedevil, Stuckinmyhead, TottyBot, 2help, Петър Петров, Cometstyles, WJBscribe, Necromancer44, DorganBot, Natl1,Asdfdsafg, Gfes, Pdcook, Ja 62, Jarry1250, Useight, BernardZ, Vndragon4, Rubbrchikin, Squids and Chips, ThePointblank, CardinalDan,Robprain, Idioma-bot, TNTfan101, ACSE, Chinneeb, Deor, VolkovBot, Morenooso, Tolone, ABF, DSRH, AlnoktaBOT, VasilievVV,Ryan032, Philip Trueman, Boo2u89, TXiKiBoT, Kaffi, Zidonuke, Adamwang, Blah master man, Hqb, Sean D Martin, Qxz, MrJuan-cho03, Clarince63, JhsBot, CanOfWorms, Fbs. 13, LeaveSleaves, Amog, Psyche825, Bentley4, Jordanhorn, Thompson2266, Induc-tiveload, Madhero88, Drtgjhjiddf, Cwilliamsdog, Cocomeco, Chiggen, Brynpttrsn, Billinghurst, Blurpeace, SwordSmurf, Brainmuncher,FC190, Latinquasar, Su37amelia, James McBride, Falcon8765, Heroesrule17, Turgan, Smartie960, Sylent, Timmytootoo, Kaci8567, Igat-sios, Farquharsons, Malachi007, Spinningspark, Placeneck, Wellsy1992, Insanity Incarnate, Jsponge96, Dmcq, Kurtroscillo, Dooflotchie,Logan, NHRHS2010, EmxBot, Gagundathar, SieBot, Magickmonkey54, Watercleanerperson, Euryalus, Winchelsea, Mbz1, Dawn Bard,Caltas, Cwkmail, RJaguar3, Calabraxthis, Moey1, Dombom, Keilana, Bentogoa, Flyer22, Tiptoety, Tjbvista, Hiddenfromview, Aruton,Antonio Lopez, Lightmouse, W3rH3re, BenoniBot, Coolerguy101, Macy, Harry the Dirty Dog, Arthana, Mr.wang, LonelyMarble, AndrijKursetsky, Torchwoodwho, Fasttiger100, Cosmo0, Scottyoak2, Mygerardromance, WikiLaurent, Dabomb87, Neo., Pinkadelica, Denis-arona, Escape Orbit, Naturespace, Athenean, Sfan00 IMG, Elassint, ClueBot, Fribbler, Fyyer, Foxj, The Thing That Should Not Be,Supersonicstars, Wwheaton, Vasyatka1, Husky2002, CounterVandalismBot, Gordo1717, Niceguyedc, Auquacutie, Dylan620, Leadwind,Trueheartless, Nicksallama, DragonBot, Excirial, Ritchiemate, Vivio Testarossa, Tuckerson1, NuclearWarfare, Jotterbot, PhySusie, Njar-darlogar, DeltaQuad, Pie053, There are no names, Panos84, Thingg, Astrotwitch, Aitias, Horselover Frost, Versus22, CameronsAshley,SoxBot III, Wnt, Bearsona, Tarheel95, Spitfire, Gonzonoir, Patilsagar09, Jovianeye, Duncan, Purnajitphukon, Little Mountain 5, Avoided,Crimson chin7, ZooFari, Uni223344, Lemchesvej, CalumH93, Avprslayer, Addbot, Willking1979, Some jerk on the Internet, DOI bot, Jo-jhutton, Tcncv, Alienware9955, Friginator, Binary TSO, Dimosvki, Chinese baybay, Hattar393, MartinezMD, Njaelkies Lea, Ashton1983,Richmond96, Cst17, Cypkerth, Download, Protonk, CarsracBot, Gifðas, AndersBot, LemmeyBOT, LinkFA-Bot, Andy17061993, RL579,Slayer094, Xario, Tide rolls, Lightbot, OlEnglish, Gail, Greatorix, Hartz, Legobot, Math Champion, Luckas-bot, Yobot, 2D, Fraggle81,Julia W, Aldebaran66, Crispmuncher, Qlwinsor, THENWHOWAS PHONE?, Nallimbot, Aldren kenji, Tempodivalse, AnomieBOT, An-drewrp, Metalhead94, Marauder40, Master of Pies, Jim1138, Harloshaply, AdjustShift, Jim Birkenshaw, Kingpin13, Ulric1313, Crystalwhacker, Caseyisgay, Bluerasberry, Materialscientist, Hbkrishnan, The High Fin Sperm Whale, Citation bot, Hypocrite9901, Nikolay94,Elm-39, Maxis ftw, Odd126, Obersachsebot, Xqbot, Zad68, TinucherianBot II, Tasudrty, Random astronomer, SamForestell, Capricorn42,TechBot, Hanberke, Waachiperchow, Grim23, Anna Frodesiak, Mlpearc, UlmPhysiker, GrouchoBot, Jhbdel, Jacob Hand, Abce2, Ribot-BOT, Amaury, Cburb13, GhalyBot, Moxy, Shadowjams, Keenanmeboy, 05jdunn, Fotaun, A.amitkumar, Lkatkinsmith, BoomerAB, Fres-coBot, Jakethakid, Tobby72, Kiwipeel, Kittins floating in the sky yay, Recognizance, Concorde4950, Footyfanatic3000, Sae1962, Adrianis-good, Tetraedycal, Louperibot, Citation bot 1, Pinethicket, HRoestBot, Tom.Reding, Rgbower, Lmiller777, Fumitol, Dude1818, Kashiera,Reconsider the static, Irbisgreif, FoxBot, TobeBot, Marvinandmilo, Niobrara, TBloemink, Bobsteel09, Reaper Eternal, Zachareth, Di-annaa, RepodudexDXDxD, Mean as custard, RjwilmsiBot, Bhawani Gautam, Billypancho, DASHBot, Jeffdunhamfan123, EmausBot,WikitanvirBot, Immunize, Franz123, Akhil.aggarwal2, Hula Hup, Racerx11, GoingBatty, Gebinsk, NotAnonymous0, Solarra, Jmenci-som, Tommy2010, Crimsonvalor, Wikipelli, K6ka, P. S. F. Freitas, Uncle Dick2, Italia2006, Chelseamarie322, Claudio M Souza, Fæ,Josve05a,Wackywace, Doomedtx, Ialsofedthisup, H3llBot, Iyragaura, Gz33,Wayne Slam, Kelvinmoulden, OnePt618, Tmobileloverdeluxe,Tolly4bolly, PhantomPlugger, Pyxelator, Andattaca2010, Aidarzver, Karthikndr, L Kensington, Donner60, Ronkilburn, Carmichael, Billwilliam compton, Clementina, Homer.Hapmouche, Herk1955, Mjbmrbot, ClueBot NG, Terrorking101, CaitlinStewart, KagakuKyouju,Dr. Persi, Jan5899, Satellizer, Catinthehat93, Chester Markel, Jj1236, Samalambam1, Corusant, Braincricket, 65rytg, Widr, GlassLa-dyBug, Macinew101, Danim, Michael5046, North Atlanticist Usonian, Helpful Pixie Bot, Titodutta, Bibcode Bot, Furkhaocean, Bstbll,Fsuinbed, AvocatoBot, Rm1271, Cadiomals, Drift chambers, Joydeep, Snow Blizzard, MrBill3, 23haveblue, Zedshort, Eguinto, MorningSunshine, Klilidiplomus, SoylentPurple, Jason from nyc, Sudhar1996, Jonadin93, Pendragon5, Meltingbutterflies14, NGC 2736, Dnlbring,SuperAwesomeMegaChipmunk, ChrisGualtieri, Khazar2, Lolgag98, JYBot, Tow, Azwan.adza, EagerToddler39, Dexbot, JOSHUAHHEBY, Reatlas, Ross Hill, Jomatt456, Yopogi74, Greengreengreenred, Jgritton, Chavre18, Metadox, G3a3master, Ugog Nizdast, Lah-

5.16. TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES 59

macunKebab, The Herald, Imanawesomedude, Johndric Valdez, Damián A. Fernández Beanato, Kind Tennis Fan, Rohit123P, Wackyike,Kaitlyncranston, Seabuckthorn, Thewordsmith9, JaconaFrere, Skr15081997, AT-Walkerman, Yeyo345436675, Bumliker, Monkbot, Liv-inusFranciscus, Filedelinkerbot, Hhiklmao, Darkeyes5, Evillegend, ConnorJones7699, Kinslayer1997, BethNaught, Klaogert, Appleshift,Carl Marxs, Kriswuswagswag, Leomayalu, Kris wuwu, Justanothergirl01, Laynajacks, Totomato, Exotic 7, Omo mahkaylahh, Squirtyu-nicorn, Lactonotgalaxias, Kyungsoofully, Krisismystyle, Lamekibum, Beatrizluvkpop, Spideratseds, Narky Blert, Stefania.deluca, SomeGadget Geek, Tetra quark, Isambard Kingdom, KCCkamaljitchakraborty, ExperiencedArticleFixer and Anonymous: 1084

• Meteor shower Source: http://en.wikipedia.org/wiki/Meteor%20shower?oldid=650753749 Contributors: Tim Chambers, Vignaux,Looxix, Ahoerstemeier, Julesd, Hike395, RickK, Jusjih, Shantavira, Contrazz, Robbot, Chopchopwhitey, Postdlf, Rursus, Hadal, Modeha,Robinh, Lupo, Rsduhamel, Jyril, Bkonrad, Curps, Jrdioko, Neilc, Gadfium, DavidBrooks, Antandrus, Haah, Icairns, Slidewinder, Zon-dor, Mike Rosoft, Discospinster, Vsmith, Eitheladar, Roo72, Dbachmann, RJHall, Walden, Worldtraveller, Dystopos, John Vandenberg,Walkiped, Diceman, Kjkolb, MPerel, Enirac Sum, Geke, Snowolf, GrouchyDan, 25or6to4, ^demon, WadeSimMiser, Andromeda321,Eras-mus, Palica, Bebenko, Rjwilmsi, Dracontes, FlaBot, RobertG, Chobot, DVdm, YurikBot, Wavelength, Brandmeister (old), RadioFan,KevinCuddeback, Gaius Cornelius, Bota47, Smkolins, Chesnok, Ahmad510, BorgQueen, Katieh5584, Serendipodous, Nekura, Vanka5,SmackBot, Melchoir, WilyD, Nickst, Delldot, Kintetsubuffalo, Rmosler2100, Bluebot, B00P, WikiFlier, DHN-bot, Atropos, Rrburke,Bowlhover, Earthsky, Alexander110, Kalathalan, Jtm71, RandomCritic, Ghelae, Waggers, Joseph Solis in Australia, Courcelles, J Milburn,JForget, Van helsing, Rigel1, Mystylplx, Ruslik0, Bill.albing, Myasuda, Int3gr4te, Thijs!bot, D4g0thur, Mojo Hand, Headbomb, Amity150,Leon7, Nick Number, AntiVandalBot, Mikenorton, JAnDbot, Harryzilber, Ericoides, MegX, Burga, Bongwarrior, VoABot II, Rain-dreamer, Bleh999, Cyktsui, Kheider, BunsenH, Yobol, MartinBot, Keith D, CommonsDelinker, M1ooo, J.delanoy, Pharaoh of the Wiz-ards, Ryan Postlethwaite, SriMesh, DorganBot, Pparazorback, Philip Trueman, Zamphuor, UnitedStatesian, Wingedsubmariner, 1981willy,Thereginator28, Norhelt, Gkronk, Tresiden, Gerakibot, Jsc83, Wilson44691, Sillyfattie12345, Moongurl101, Makeignafunni, Hamilton-daniel, Martarius, ClueBot, Iandiver, LisaSmegal, DragonBot, Iohannes Animosus, 7&6=thirteen, Muro Bot, Aitias, Gikü, SwampFox2,SilvonenBot, Addbot, Willking1979, Some jerk on the Internet, Jojhutton, CL, Glane23, Obsidianspider, 84user, Williamhortner, Tiderolls, Luckas-bot, TaBOT-zerem, Plasticbot, Killiondude, Galoubet, Oofis, Masursky, Materialscientist, Stefansquintet, Citation bot, Dirl-Bot, Xqbot, RibotBOT, Rainald62, Fotaun, Sock, Paine Ellsworth, D'ohBot, Alfredoconconi, Citation bot 1, Tom.Reding, Full-date unlink-ing bot, Tim1357, Vrenator, MrX, Makki98, Wikiborg4711, SMSshark, TjBot, EmausBot, Racerx11, GoingBatty, Tommy2010, ZéroBot,A930913, Bulwersator, 28bot, Washington Irving Esquire, Fjörgynn, ClueBot NG, CRJ200flyer, CocuBot, MelbourneStar, Crazymon-key1123, Helpful Pixie Bot, KLBot2, Bibcode Bot, ElphiBot, Sumanchakra143, Meteormark, Pocketthis, BattyBot, NLO-STARGAZER,Dexbot, Br'er Rabbit, YasBot, Michael Vaillant, Tnorbs, Kogge, Monkbot and Anonymous: 218

• Nebula Source: http://en.wikipedia.org/wiki/Nebula?oldid=654512793 Contributors: Bryan Derksen, Berek, Andre Engels, Jkominek,XJaM, William Avery, 0, Heron, Fxmastermind, Edward, Michael Hardy, Ixfd64, Arpingstone, Alfio, Looxix, Ahoerstemeier, Angela,Aarchiba, Milkfish, Glenn, Marteau, Pizza Puzzle, Shizhao, Bloodshedder, Jni, Robbot, Kizor, Netizen, Sam Spade, Babbage, Sverdrup,Ojigiri, Wikibot, Nerval, Ancheta Wis, Peruvianllama, Wwoods, Curps, NeoJustin, JamesHoadley, Waltpohl, Rick Block, Malyctenar,Guanaco, SWAdair, Joseph Dwayne, DorotaP, Slowking Man, Antandrus, Beland, Mike Storm, Borameer, Icairns, Fratley, Iantresman,Janneok, Mike Rosoft, AAAAA, DanielCD, Discospinster, Vsmith, Spundun, Calair, RJHall, El C, Edwinstearns, Worldtraveller, Ed-ward Z. Yang, Noren, Bobo192, Korivak, Nk, John Nowak, Storm Rider, Danski14, Alansohn, Free Bear, Keenan Pepper, Supine,Snowolf, Wtmitchell, Amorymeltzer, Deathphoenix, DV8 2XL, HenryLi, Kazvorpal, RyanGerbil10, Bobrayner, Bacteria, Rorschach,Jacen Aratan, Ptomato, Mu301, Jersyko, Pol098, CiTrusD, Damicatz, Isnow, Kralizec!, Fxer, Dysepsion, Magister Mathematicae, JIP,Rjwilmsi, Marasama, Amire80, Rillian, Bmenrigh, Yamamoto Ichiro, RobertG, Latka, Pumeleon, Gurch, Ayla, Pevernagie, Chippym22,Silivrenion, Chobot, Krishnavedala, DVdm, Antiuser, E Pluribus Anthony, RattusMaximus, Hairy Dude, John Quincy Adding Machine,Lofty, Farside6, SpuriousQ, Fabricationary, Stephenb, Zhatt, Deskana, Feross, Anetode, PhilipO, CptnMisc, JHCaufield, Dissolve, Dead-EyeArrow, Ageekgal, Flehmen, Gesslein, Argo Navis, NeilN, Ben mcwilliam, Mejor Los Indios, AndrewWTaylor, Luk, SmackBot, Afita,KnowledgeOfSelf, NaiPiak, Pgk, Vald, Jacek Kendysz, Jagged 85, Edgar181, HalfShadow, SmartGuy Old, Peter Isotalo, Gilliam, Tmpst,ERcheck, Bluebot, Jprg1966, B00P, MalafayaBot, Whispering, Ctbolt, Colonies Chris, Hotwiki, CX23882-19, Cantalamessa, Rrburke,Underorbit, Addshore, Khoikhoi, Dreadstar, Dross82, AndyCMyers, Sturm, Doug Bell, Xerocs, Dbtfz, Soap, AmiDaniel, J 1982, Disa-vian, Bjankuloski06en, Scetoaux, IronGargoyle, Ckatz, RandomCritic, JHunterJ, Slakr, Salur, Jose77, The-Pope, BranStark, Iridescent,TwistOfCain, Shoeofdeath, Jynus, Newone, Twas Now, Octane, Color probe, George100, JForget, BeenAroundAWhile, StarScream1007,JohnCD, Ruslik0, Micah hainline, Geekfox, Myasuda, Icarus of old, Treybien, Mato, Gogo Dodo, Pascal.Tesson, DumbBOT, Dinner-bone, Kozuch, Zalgo, Thijs!bot, Epbr123, Andyjsmith, Berria, Headbomb, Tellyaddict, Dainis, Thadius856, Somnabot, AntiVandalBot,GTD Aquitaine, Cchhrriiss, Vic226, Dagibit, Dr. Submillimeter, Dylan Lake, Mighty Ne, Salgueiro, Figma, JAnDbot, MER-C, Dd 8630,IanOsgood, Xeno, J-stan, Vanished user s4irtj34tivkj12erhskj46thgdg, MegX, Ryan4314, Kirrages, Rothorpe, Beaumont, JonnyDomestik,Acroterion, Magioladitis, Bongwarrior, VoABot II, Soulbot, Indon, KirinX, Allstarecho, Ahmad87, DerHexer, KhalidMahmood, Strider01,Funkfrost, NatureA16, Geboy, Stephenchou0722, Cecilkorik, MartinBot, Vanessaezekowitz, Rettetast, Ravichandar84, Matt0527, R'n'B,CommonsDelinker, Tgeairn, J.delanoy, Violask81976, Catmoongirl, Uncle Dick, Gzkn, Q21op, Nealthcounts, Gerard armando, Penissucker 2000, Blahblahblahman, Carlosforonda, Narcberry, DadaNeem, Touch Of Light, $dollarz$, DorganBot, Treisijs, KiraFreedom,Shiznick, TheFilth, Idioma-bot, Azuriteking, Signalhead, Deor, Jonwilliamsl, CWii, Jakeizzle, VasilievVV, TheOtherJesse, TylerJarHead,Philip Trueman, DoorsAjar, TXiKiBoT, Pulsar.co.nr, Alexeivna, Technopat, Qxz, Martin451, Leafyplant, LeaveSleaves, Papercut 2008,1981willy, Drtgjhjiddf, James McBride, Falcon8765, Enviroboy, King editer, AgentCDE, Paston-cooper, Realtyworks, Bobbyjackfan95,Tvinh, NHRHS2010, EmxBot, Kperry5000, EJF, SieBot, K. Annoyomous, Tiddly Tom, Euryalus, Archangelseal, Bearsfan2323, DawnBard, Caltas, Jacotto, Nwright6302, Keilana, Radon210, Green6592, Liz9744, Jc-S0CO, PhilGaskill, Jorge Iani, Techman224, Efe, Tvdm,Troy 07, WikipedianMarlith, Martarius, ClueBot, GorillaWarfare, Tzler, The Thing That Should Not Be, Jdgilbey, Polyamorph, Ob9889,Dylan620, Alig112, Puchiko, Excirial, Ragnarock441, Vivio Testarossa, Jotterbot, Njardarlogar, 7&6=thirteen, Thehelpfulone, Mpozzan,Thingg, Aitias, Versus22, Burner0718, Roberto Mura, DumZiBoT, BarretB, Pedestrian65, Arianewiki1, XLinkBot, Arthur chos, Wa-chowicz, Avoided, Rreagan007, WikHead, NellieBly, RP459, Vianello, ZooFari, RyanCross, Addbot, Cxz111, Willking1979, Some jerkon the Internet, DOI bot, NewAtThis, 15lsoucy, Mr. Wheely Guy, Ka Faraq Gatri, Download, LaaknorBot, CarsracBot, Zemel1993,AndersBot, Hooverhigh, Favonian, AgadaUrbanit, Numbo3-bot, Tide rolls, Cookiecrumbs1530, Luckas-bot, II MusLiM HyBRiD II,Nallimbot, AnomieBOT, Rubinbot, Shamrockman455, Jim1138, Galoubet, Piano non troppo, AdjustShift, Kingpin13, Materialscien-tist, Hunnjazal, The High Fin Sperm Whale, Citation bot, Bob Burkhardt, GB fan, Zad68, Acebulf, 703steve703, GrouchoBot, Abce2,Frankie0607, SassoBot, Amaury, Randomguy182, Shadowjams, Astatine-210, SchnitzelMannGreek, Fotaun, A.amitkumar, Thehelpful-bot, Cekli829, DeNoel, FrescoBot, Chubbychuck2, Paine Ellsworth, Tranletuhan, Nathan2137, Billding6969, Rafudu, Citation bot 1,Awsomezacste, Intelligentsium, Biker Biker, Pekayer11, Pinethicket, I dream of horses, Tom.Reding, Hamtechperson, A8UDI, Jschnur,RedBot, SpaceFlight89, Meaghan, Donutmanz, Merlion444, SkyMachine, IVAN3MAN, Lotje, Vrenator, Extra999, Hilaryrosepiavictoria,Zidanie5, Gwyneth99, Tbhotch, Sideways713, Keegscee, Updatehelper, Nct28th, NerdyScienceDude, EmausBot, Samarhabib80, Immu-nize, Moof3h, Sadalsuud, Corgiman24, Jsilverman001, Racerx11, RA0808, Yowife, Jeremiahdomon, Jmencisom, Tommy2010, Storm-

60 CHAPTER 5. STAR

chaser89, Wikipelli, Italia2006, John Cline, Fæ, Shuipzv3, Monterey Bay, Wayne Slam, Thine Antique Pen, ClamDip, Czeror, ForeverDusk, Llightex, SeanPoston, AMD, Socialservice, ClueBot NG, Mechanical digger, Legend1234567890, TheRogueMan007, Ilistfakestuff,Dekethegeek, Movses-bot, Baseball Watcher, OperaJoeGreen, Lord Roem, Julesmazur, 123Hedgehog456, Cottoncandydogs, MerlIwBot,Helpful Pixie Bot, Unkpunk, Bibcode Bot, Lowercase sigmabot, Claytonscrib, Prcc27, Vagobot, DUCKISJAMMMY, Todobo, Irocklol-skis, Northamerica1000, Absconded Northerner, DaKittehLuvah, Droohr, Hurricanefan25, MusikAnimal, Darkness Shines, Mark Arsten,Cadiomals, Altaïr, Akid21, Cp9543, Achowat, M1234d, Anbu121, AshleyJO000, BattyBot, Nathanmh123, Ninjago10, ChrisGualtieri,Gdfusion, RatWeazle, Elbrado94, G.Kiruthikan, Reatlas, Rfassbind, Donovanzo, Snjón, Jewels Vern, Cmckain14, Ikarus 18, JamesMoose,Impwarhamer, Sakuya Hiyama, StarStorm482, Manju22an, The Herald, Ginsuloft, Kogge, ARichey, Anrnusna, BDwinds, RedCherry720,St170e, Monkbot, Spideratseds, Prayogrd99, James cain 101, Tetra quark, Sagefire925, ShreyaB94, Sabucci and Anonymous: 668

• Star Source: http://en.wikipedia.org/wiki/Star?oldid=654950940 Contributors: Mav, Uriyan, Bryan Derksen, Zundark, The Anome, As-troNomer, Malcolm Farmer, Wayne Hardman, Andre Engels, Danny, XJaM, Toby Bartels, PierreAbbat, Roadrunner, DavidLevinson,B4hand, Lir, Patrick, Brainsik, Michael Hardy, Alan Peakall, Cprompt, Trevor H., MartinHarper, Valery Beaud, Ixfd64, Cyde, Seav,Karada, Dori, Minesweeper, Peter Whysall, Kosebamse, Tregoweth, Card, Looxix, ArnoLagrange, Ahoerstemeier, Stan Shebs, J'raxis,Nanobug, Julesd, Glenn, Rossami, Andres, Cimon Avaro, EdH, Lancevortex, Smack, Pizza Puzzle, Schneelocke, Hike395, Adam Bishop,Ike9898, Terse, Zoicon5, Pedant17, Kaare, Maximus Rex, Nickshanks, Joy, Wetman, Pumpie, Jni, Donarreiskoffer, Robbot, Vardion,Justo, Tlogmer, Kowey, Lowellian, Lsy098, Sverdrup, Kneiphof, Rursus, Ojigiri, Sunray, Bkell, Jeroen, Borislav, Jheise, Mandel, Alanyst,Cordell, Dina, Nephelin, Alexwcovington, Giftlite, Graeme Bartlett, Marnanel, DavidCary, Harp, Inter, Tom harrison, Zigger, Obli,Xerxes314, Everyking, Curps, Michael Devore, Niteowlneils, Karl Naylor, Jackol, Mckaysalisbury, Bobblewik, Joseph Dwayne, Con-radPino, Antandrus, HorsePunchKid, Beland, Phe, Karol Langner, Phil Sandifer, Kesac, Thincat, Kevin B12, Satori, Phil1988, Icairns,Gscshoyru, Iantresman, Urhixidur, Ukexpat, Trilobite, Adashiel, Trevor MacInnis, Randwicked, Flex, Mike Rosoft, Geof, AliveFree-Happy, Jiy, Erc, Moverton, Discospinster, Rich Farmbrough, Vsmith, Smyth, SpookyMulder, Bender235, ESkog, AdamSolomon, Hapsi-ainen, Brian0918, Dpotter, Pmetzger, RJHall, Sfahey, El C, Edwinstearns, Zenohockey, Lankiveil, Bletch, Kwamikagami, Hayabusa fu-ture, Worldtraveller, Shanes, Tom, Art LaPella, Dbalsdon, Femto, Noren, Bobo192, Harley peters, Longhair, Duk, Viriditas, Elipongo,DaveGorman, Oop, I9Q79oL78KiL0QTFHgyc, Man vyi, La goutte de pluie, TheProject, Shereth, Ardric47, BW52, Krellis, Pearle,HasharBot, Jumbuck, Stephen G. Brown, Alansohn, Gary, Enirac Sum, Vitaly, CountdownCrispy, Jeltz, AzaToth, Viridian, Garfield226,Mysdaao, Spangineer, Malo, Dmismir, Uther Dhoul, Bart133, Snowolf, Hapless Hero, Atomicthumbs, TaintedMustard, Rick Sidwell,RainbowOfLight, Bsadowski1, Skatebiker, Itsmine, Gene Nygaard, Agquarx, Macinapp, Nick Mks, Dan100, Natalya, Feezo, Gmaxwell,WilliamKF, Weyes, FrancisTyers, Firsfron, Kenesis, TigerShark, Camw, Aza, BillC, Kokoriko, Robert K S, CaptainTickles, MrDarcy,Lawe, Sengkang, Prashanthns, Gimboid13, Wisq, Dysepsion, Mandarax, SqueakBox, Rnt20, Graham87, Marskell, Noit, Magister Math-ematicae, Zeroparallax, Chun-hian, FreplySpang, Miq, Jclemens, Lord.lucan, Canderson7, Sjakkalle, Rjwilmsi, Mayumashu, Angusm-clellan, Koavf, Саша Стефановић, Vary, Strait, Linuxbeak, MZMcBride, HandyAndy, Mike s, Mike Peel, Daano15, Brighterorange,Krash, Hsriniva, Reinis, MLRoach, Maurog, GregAsche, AySz88, Sango123, Yamamoto Ichiro, FayssalF, FlaBot, Kiba, Patrick1982,SchuminWeb, RobertG, Old Moonraker, Nihiltres, Crazycomputers, RexNL, Gurch, DannyZ, Krun, Alphachimp, Malhonen, Snail-walker, Imnotminkus, King of Hearts, Ourboldhero, Chobot, 334a, Cactus.man, Bomb319, Gwernol, MamboJambo, Satanael, Wave-length, TexasAndroid, Huw Powell, Jimp, RussBot, Splash, RJC, Fabricationary, AlfredoM, Stephenb, Gaius Cornelius, NawlinWiki,Wiki alf, Pagrashtak, Grafen, Jaxl, Lil crazy thing, Thiseye, Irishguy, Retired username, Bobbo, Nucleusboy, Dppowell, Trollderella,E rulez, Ankologist, Moe Epsilon, RL0919, Off!, Nut-meg, Tony1, Freshgavin, Dbfirs, T, BOT-Superzerocool, Jeremy Visser, Dna-webmaster, Wknight94, Ageekgal, Theda, Closedmouth, Xaxafrad, JPK, GraemeL, Vicarious, Jukemaia, CWenger, Kier07, Caco devidro, Katieh5584, Banus, Paul Erik, Zvika, Cmglee, Mejor Los Indios, DVD R W, Eenu, KnightRider, A bit iffy, Andreaskeller, TheDark, Ashill, Moeron, Mangoe, Hux, Tom Lougheed, Prodego, KnowledgeOfSelf, Royalguard11, FloNight, Pgk, Elminster Aumar, Bo-mac, WilyD, Jagged 85, CMD Beaker, Jrockley, Richard B, Frymaster, ImaginaryFriend, Dhochron, Aksi great, Gilliam, Chaojoker,OldsVistaCruiser, Andy M. Wang, Saros136, Master Jay, Zouf, Keegan, Rkitko, Quinsareth, Persian Poet Gal, Ian13, MK8, Tree BitingConspiracy, Anchoress, MalafayaBot, SchfiftyThree, Oni Ookami Alfador, Kourd, Whispering, DHN-bot, Methnor, Darth Panda, Verrai,Tewfik, Diyako, Golradir, Can't sleep, clown will eat me, Jahiegel, AltGrendel, Vanished User 0001, Wikipedia brown, Mystman666,Matthew, Xiner, Rrburke, Rsm99833, Whpq, Percommode, Aldaron, PrometheusX303, Decltype, Bowlhover, Savidan, Insineratehymn,Basileus Basileon Basileuon Basileuoton, Doodle77, Mion, MOO, Pilotguy, Kukini, CIS, Mithadon, Harryboyles, Soap, Kuru, J 1982,AnonEMouse, Sir Nicholas de Mimsy-Porpington, Linnell, JorisvS, Robert Stevens, Todd661, IronGargoyle, Ckatz, RandomCritic, Bis-cuit Vader, Ryulong, Novangelis, Avant Guard, PostAbandoned, Darry2385, Autonova, Mantrogo, Xajel, Lucid, Levineps, Dan Gluck,Iridescent, K, Spebudmak, Dekaels, Missionary, JoeBot, J Di, R, Freelance Intellectual, Marysunshine, Civil Engineer III, Az1568, Rhetth,JustSayin, Tawkerbot2, AbsolutDan, Dc3, Ioannes Pragensis, The Haunted Angel, JForget, Kiiron, Irwangatot, Shyland, Insanephantom,1.618033989, Van helsing, Crescentnebula, Nevermorestr, Runningonbrains, CWY2190, Ruslik0, Tjkiesel, KnightLago, MrFizyx, Dgw,NickW557, Juhachi, MarsRover, Gran2, MrFish, Rudjek, Yaris678, Tringard, Kanags, Steel, SyntaxError55, Gogo Dodo, JFreeman,Tdvance, Tawkerbot4, Clovis Sangrail, DumbBOT, Jay32183, Duccio, ErrantX, RED13, Superbeatles, Bensmith53, Daniel Olsen, Mon-gonikol, Gimmetrow, General Veers, Fomz, FrancoGG, BetacommandBot, Clain, Thijs!bot, Epbr123, Bot-maru, LeeG, Ultimus, Four-nax, Andyjsmith, Sunjae429, Mojo Hand, Headbomb, Xiao191, Wanderinglopez, Marek69, John254, Brichcja, Swin, Dgies, MichaelA. White, Srose, Northumbrian, Mentifisto, Porqin, MarshBot, JDawg923, AntiVandalBot, Roflbater, Majorly, Luna Santin, Targetter,Orionus, Prince Godfather, K.O.T., Jj137, TimVickers, Scepia, Darklilac, Chill doubt, Gdo01, Glennwells, Alphachimpbot, Spartaz,Storkk, Myanw, Res2216firestar, JAnDbot, Richardw, Sabrem, Moonhawk, MER-C, Antony the genius, Hello32020, Andonic, SmithJones, Snowolfd4, Christopher Cooper, Hardee67, Bearly541, Alastair Haines, FaerieInGrey, Gumby600, Penubag, WolfmanSF, Pedro,Murgh, Bongwarrior, VoABot II, Kuyabribri, Ff1959, Farquaadhnchmn, Khalidkhoso, Redaktor, Bigdan201, Pixel ;-), Amitabho, Nyt-tend, SparrowsWing, Avicennasis, KConWiki, Indon, Animum, Captin Shmit, Cyktsui, Allstarecho, P.B. Pilhet, SpaceGuide, Just James,DerHexer, Snowflake215, Khalid Mahmood, JdeJ, Iazz, TheRanger, AOEU, Danielratiu, Gwern, Kornfan71, Pauly04, Hdt83, Martin-Bot, Prgrmr@wrk, AussieBoy, BetBot, Foncea, Rettetast, DerRichter, CommonsDelinker, GarrisonGreen, Nono64, Hairchrm, Sirstubby,Slash, Huzzlet the bot, J.delanoy, Thadriel, DrKiernan, Danimoth, Kulshrax, Hans Dunkelberg, Phewkin, 12dstring, Bot-Schafter, Shawnin Montreal, Pyrospirit, AntiSpamBot, Carlosforonda, Stambouliote, Half-Blood Auror, Aquaepulse, Idjles, SJP, Bobianite, RPIfire-man, Sunderland06, Unclefist, Ukt-zero, Wikihacker331, Smfairlie, Chanel256, Mmmbananas, Misterflower, Ilikepie3823, Diglleball,Potato12345, Noobert123, Moverington, Wikidogia, SBKT, Websta1232000, Elenseel, Inwind, Dorftrottel, Izno, Jefferson Anderson,Idioma-bot, Vranak, Dogggss, VolkovBot, CWii, ABF, Mrh30, Fundamental metric tensor, JohnBlackburne, Ebustad, AlnoktaBOT,WarddrBOT, Smartnoggin, TXiKiBoT, A.Alapini, Cheeweeo, Napoleon Cartilagaparte, Hqb, Walor, Pooleyusk, George Moromisato,Kdriver06, Anonymous Dissident, Wandachka, Mawkernewek, Qxz, I'm the editor, Himynameisbob2, Henrykus, JhsBot, Abdullais4u,LeaveSleaves, UnitedStatesian, Sheridan Zhoy, Majorevil1, Katimawan2005, 1981willy, Kelly Scragg, Madwhizzer, Ossittts01, Silver-man8585, Synthebot, James McBride, Miggyabilay, Purgatory Fubar, Sylent, Stivic, Wiki.free.encyclo, Urannoying, Ceranthor, Allebor-goBot, StevenJohnston, Aishe zq, Vsst, EmxBot, Peter.thelander, Demmy100, SieBot, Coffee, Timb66, PlanetStar, Gprince007, Moon-

5.16. TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES 61

riddengirl, Scarian, Oldag07, Fabullus, Caltas, Jacotto, Joe the coe, GlassCobra, Purbo T, Keilana, Alboking, RadicalOne, Tiptoety, OdaMari, Arbor to SJ, Kosack, Lightmouse, SH84, Techman224, Macy, Harry the Dirty Dog, Theobald2526, Hamiltondaniel, Susan118,WikiLaurent, Dabomb87, Wjmummert, Freewayguy, Athenean, SallyForth123, Martarius, MBK004, ClueBot, PipepBot, The Thing ThatShould Not Be, Supersonicstars, IceUnshattered, Rjd0060, Meluvmusic, Wwheaton, Herakles01, Arakunem, Bpendo, 123 babe8, Counter-VandalismBot, ChandlerMapBot, DragonBot, SteveRamone, Tim-larry, Excirial, Alexbot, Julian47, Darcy7, Hobo626, Krazybunnyschool,NuclearWarfare, Zeno Boy, Jotterbot, PhySusie, Iohannes Animosus, Ryanandtony, J.R.Claypool, Jameshowe2k7, Felix182, Kakofonous,Dghhjdh, Rocketfire22, There are no names, Pascalou petit, Ankurtg, Thingg, Gabehi, Hellokitty899, Aitias, Arf325, Rawrs, Allsvartr, Ver-sus22, Dana boomer, Gaoqingy07, SoxBot III, Ethanisgood, Dohgon Immortal, Vanished User 1004, Scottiedidthis, Shaunl6, XLinkBot,Jjoohhnnoo, Emma mcfly, Skarebo, Noctibus, JinJian, ZooFari, Addbot, 11341134a, Ocrasaroon, Tighe, DOI bot, Wade haffner, NjaelkiesLea, Fieldday-sunday, Mintos.arzai, CanadianLinuxUser, Brittslayer12, OliverTwisted, Cst17, Download, PranksterTurtle, Kravdraa Ulb,Jonnygrifff, Doniago, LinkFA-Bot, Allyjudd01, Heatwave37, Numbo3-bot, Tide rolls, Lightbot, Ct josh no1, Quantumobserver, Frehley,Everyme, Luckas-bot, Yobot, TaBOT-zerem, Aldebaran66, ArchonMagnus, THEN WHO WAS PHONE?, Azcolvin429, Eric-Wester,MacTire02, AnomieBOT, Quangbao, Rubinbot, Piano non troppo, Fahadsadah, Kingpin13, Materialscientist, TrantaLockedOn, The HighFin SpermWhale, Citation bot, Msandhu0504,Maxis ftw, Hi IMBi, ArthurBot, Chell and the cake, Marshallsumter, Xqbot, TinucherianBotII, Timir2, Sionus, LittleMissKool, TechBot, Tobyhinder, Magicknight94, Mlpearc, Abce2, Ataleh, Frosted14, ProtectionTaggingBot, El-bigger1, Doulos Christos, Moxy, Shadowjams, Chaheel Riens, Pi=3.14159265473, Fotaun, Onomatopoeia500, YusrSehl, Prari, FrescoBot,Lolzct, Originalwana, Tobby72, Pepper, Mr.Freeeeze, Carlitosway1111, Dottyteddy, Bionic rabi, Mattwic, KuroiShiroi, TBRays46, Tran-dall15, Xyzprodigy, Citation bot 1, Zed8055, Cowman783, Toughman999, GaussianCopula, DrilBot, Sw33tly shy, Placko123, Killface55,Pinethicket, I dream of horses, Edderso, Tom.Reding, Lars Washington, ScottHW, Jandalhandler, FoxBot, Throwaway85, Cocu, Lotje,Extra999, Begoon, Bobofari, Jhenderson777, Innotata, Tbhotch, Gabeln2, Wikiborg4711, The Utahraptor, RjwilmsiBot, TjBot, Mchcopl,DASHBot, EmausBot, Jw depew,WikitanvirBot, Sadalsuud, Primefac, Navidsincere, Till Credner, Sdicastro, Jmencisom, Djembayz, Weri-eth, ZéroBot, Cogiati, Josve05a, A2soup, Chasrob, AvicAWB, Yiosie2356, H3llBot, Glennconti, Brandmeister, Smexy speaker, Lewis274,Ani.g12, Moocow121, FDLeyda, CocuBot, Sleddog116, Roommate1989, Jack Melnick, Moneya, CasualVisitor, Flendersnod, Diyar se,Helpful Pixie Bot, CPGirlAJ, Gob Lofa, Bibcode Bot, Pine, Dodshe, Cadiomals, Pocketthis, Zedshort, Eguinto, Shawn Worthington LaserPlasma, Wer900, BattyBot, Soulbust, Khazar2, Dexbot, Stas1995, Reatlas, Rushstreetprince, Praemonitus, The Herald, Zinganthropus,Astredita, Elenceq, Monkbot, Filedelinkerbot, BethNaught, SkyFlubbler, I am. furhan., Isambard Kingdom and Anonymous: 1017

5.16.2 Images• File:3d_glasses_red_cyan.svg Source: http://upload.wikimedia.org/wikipedia/commons/a/a0/3d_glasses_red_cyan.svg License: CCBY-

SA 2.5 Contributors: Own work Original artist: Image created using Inkscape by Daniel Schwen.• File:AGOModra_Leonids98.jpg Source: http://upload.wikimedia.org/wikipedia/commons/7/7e/AGOModra_Leonids98.jpg License:

CC BY-SA 3.0 Contributors: Astronomical and geophysical observatory, Comenius University Original artist: Juraj Tóth• File:Alpha_Centauri_AB_over_limb_of_Saturn_PIA10406.jpg Source: http://upload.wikimedia.org/wikipedia/commons/a/ac/

Alpha_Centauri_AB_over_limb_of_Saturn_PIA10406.jpg License: Public domain Contributors: http://photojournal.jpl.nasa.gov/catalog/PIA10406 Original artist:

• Original uploader was Kwamikagami at en.wikipedia• File:Andromeda_galaxy.jpg Source: http://upload.wikimedia.org/wikipedia/commons/f/ff/Andromeda_galaxy.jpg License: Public do-

main Contributors: This image or video was catalogued by Jet Propulsion Laboratory of the United States National Aeronautics and SpaceAdministration (NASA) under Photo ID: PIA04921.Original artist: NASA/JPL/California Institute of Technology

• File:Antennae_galaxies_xl.jpg Source: http://upload.wikimedia.org/wikipedia/commons/f/f6/Antennae_galaxies_xl.jpg License: Public domain Contribu-tors:

• http://www.spacetelescope.org/images/html/heic0615a.html Original artist: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Col-laboration

• File:Artist’{}s_impression_of_a_protocluster_forming_in_the_early_Universe.jpg Source: http://upload.wikimedia.org/wikipedia/commons/2/2e/Artist%27s_impression_of_a_protocluster_forming_in_the_early_Universe.jpg License: CC BY 4.0 Contributors: http://www.eso.org/public/images/eso1431a/ Original artist: ESO/M. Kornmesser

• File:CNO_Cycle.svg Source: http://upload.wikimedia.org/wikipedia/commons/2/21/CNO_Cycle.svg License: CC BY-SA 3.0 Contributors: ? Original artist:?

• File:Carina_Nebula_by_ESO.jpg Source: http://upload.wikimedia.org/wikipedia/commons/2/20/Carina_Nebula_by_ESO.jpg License: CC BY 4.0 Con-tributors: http://www.eso.org/public/images/eso0905a/ Original artist: ESO

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• File:Celestial_map,_signs_of_the_Zodiac_and_lunar_mansions..JPG Source: http://upload.wikimedia.org/wikipedia/commons/8/88/Celestial_map%2C_signs_of_the_Zodiac_and_lunar_mansions..JPG License: Public domain Contributors:

• http://www.ee.bilkent.edu.tr/~{}history/Ext/Zubdat.html Original artist: Painters of Sultan Murad III

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• File:Commons-logo.svg Source: http://upload.wikimedia.org/wikipedia/en/4/4a/Commons-logo.svg License: ? Contributors: ? Original artist: ?

• File:Comparison_of_planets_and_stars_(sheet_by_sheet)_(Jan_2015_update).png Source: http://upload.wikimedia.org/wikipedia/commons/e/ed/Comparison_of_planets_and_stars_%28sheet_by_sheet%29_%28Jan_2015_update%29.png License: CC BY-SA 4.0 Contributors: Own work Original artist:IStoleThePies

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• File:Constellation_Fornax,_EXtreme_Deep_Field.jpg Source: http://upload.wikimedia.org/wikipedia/commons/e/ed/Constellation_Fornax%2C_EXtreme_Deep_Field.jpg License: Public domain Contributors: http://hubblesite.org/newscenter/archive/releases/2012/37/image/a/warn/,http://www.nasa.gov/images/content/690958main_p1237a1.jpg Original artist: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University ofCalifornia, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team

• File:Constellations_ecliptic_equirectangular_plot.svg Source: http://upload.wikimedia.org/wikipedia/commons/d/d4/Constellations_ecliptic_equirectangular_plot.svg License: Public domain Contributors: Own work, http://svs.gsfc.nasa.gov/vis/a000000/a003500/a003572 Original artist: Cmglee,Timwi, NASA

• File:Crab_Nebula.jpg Source: http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg License: Public domain Contributors: HubbleSite:gallery, release. Original artist: NASA, ESA, J. Hester and A. Loll (Arizona State University)

• File:Dibuix_de_Leo.png Source: http://upload.wikimedia.org/wikipedia/commons/0/0f/Dibuix_de_Leo.png License: Public domain Contributors: ? Origi-nal artist: ?

• File:Eagle_nebula_pillars.jpg Source: http://upload.wikimedia.org/wikipedia/commons/b/b2/Eagle_nebula_pillars.jpg License: Public domain Contribu-tors: http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/34/image/a Original artist: Credit: NASA, Jeff Hester, and Paul Scowen (Arizona StateUniversity)

• File:Earth-moon.jpg Source: http://upload.wikimedia.org/wikipedia/commons/5/5c/Earth-moon.jpg License: Public domain Contributors: NASA [1] Orig-inal artist: Apollo 8 crewmember Bill Anders

• File:Earth_Sol63A_UFO-A067R1.jpg Source: http://upload.wikimedia.org/wikipedia/commons/0/00/Earth_Sol63A_UFO-A067R1.jpg License: Publicdomain Contributors: http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20040311a/Earth_Sol63A_UFO-A067R1.jpg http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20040311a.html Original artist: NASA/JPL/Cornell

• File:File-Ngc5866_hst_big.png Source: http://upload.wikimedia.org/wikipedia/commons/5/5c/File-Ngc5866_hst_big.png License: Public domain Contrib-utors: http://www.spacetelescope.org/images/html/opo0624a.html (direct link)Original artist: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

• File:Folder_Hexagonal_Icon.svg Source: http://upload.wikimedia.org/wikipedia/en/4/48/Folder_Hexagonal_Icon.svg License: Cc-by-sa-3.0 Contributors:? Original artist: ?

• File:FusionintheSun.svg Source: http://upload.wikimedia.org/wikipedia/commons/7/78/FusionintheSun.svg License: CC BY-SA 3.0 Contributors: Ownwork Original artist: Borb

• File:GalacticRotation2.svg Source: http://upload.wikimedia.org/wikipedia/commons/b/b9/GalacticRotation2.svg License: CC-BY-SA-3.0 Contributors:Own work in Inkscape 0.42 Original artist: PhilHibbs

• File:H-R_diagram_-edited-3.gif Source: http://upload.wikimedia.org/wikipedia/commons/7/78/H-R_diagram_-edited-3.gif License: CC BY-SA 2.5Contributors: The Hertzsprung Russell Diagram Original artist: Richard Powell

• File:He1523a.jpg Source: http://upload.wikimedia.org/wikipedia/commons/5/5f/He1523a.jpg License: CC BY 4.0 Contributors: http://www.solstation.com/x-objects/he1523.htm Original artist: ESO, European Southern Observatory

• File:Herschel-Galaxy.png Source: http://upload.wikimedia.org/wikipedia/commons/b/ba/Herschel-Galaxy.png License: Public domain Contributors: ?Original artist: Caroline Herschel

• File:Hoag’{}s_object.jpg Source: http://upload.wikimedia.org/wikipedia/commons/d/da/Hoag%27s_object.jpg License: Public domain Contributors:

• http://hubblesite.org/gallery/album/galaxy/spiral/pr2002021a/ Original artist: NASA

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• http://hubblesite.org/newscenter/archive/releases/2005/01/image/a Original artist: NASA, ESA, and The Hubble Heritage Team STScI/AURA)

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• File:Leonid_meteor_shower_as_seen_from_space_(1997).jpg Source: http://upload.wikimedia.org/wikipedia/commons/5/50/Leonid_meteor_shower_as_seen_from_space_%281997%29.jpg License: Public domain Contributors: ? Original artist: ?

• File:M101_hires_STScI-PRC2006-10a.jpg Source: http://upload.wikimedia.org/wikipedia/commons/c/c5/M101_hires_STScI-PRC2006-10a.jpg Li-cense: CC BY 3.0 Contributors: http://www.spacetelescope.org/news/html/heic0602.html (direct link) Original artist:Credit:Image: European Space Agency & NASAAcknowledgements:Project Investigators for the original Hubble data: K.D. Kuntz (GSFC), F. Bresolin (University of Hawaii), J. Trauger (JPL), J. Mould (NOAO), and Y.-H. Chu(University of Illinois, Urbana)Image processing: Davide De Martin (ESA/Hubble)CFHT image: Canada-France-Hawaii Telescope/J.-C. Cuillandre/CoelumNOAO image: George Jacoby, Bruce Bohannan, Mark Hanna/NOAO/AURA/NSF

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• File:M82_HST_ACS_2006-14-a-large_web.jpg Source: http://upload.wikimedia.org/wikipedia/commons/c/ce/M82_HST_ACS_2006-14-a-large_web.jpg License: Public domain Contributors: http://www.spacetelescope.org/images/html/heic0604a.html (direct link)Original artist: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

• File:M87_jet.jpg Source: http://upload.wikimedia.org/wikipedia/commons/3/39/M87_jet.jpg License: Public domain Contributors: HubbleSite: gallery,release. Original artist: NASA and The Hubble Heritage Team (STScI/AURA)

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• File:Milky_Way_Arch.jpg Source: http://upload.wikimedia.org/wikipedia/commons/9/9e/Milky_Way_Arch.jpg License: CC BY 4.0 Contributors: http://www.eso.org/public/images/milkyway/ Original artist: Bruno Gilli/ESO

• File:Mira_1997.jpg Source: http://upload.wikimedia.org/wikipedia/commons/e/e8/Mira_1997.jpg License: Public domain Contributors: HubbleSite STScI-1997-26 Original artist: Margarita Karovska (Harvard-Smithsonian Center for Astrophysics) and NASA

• File:NGC_4414_(NASA-med).jpg Source: http://upload.wikimedia.org/wikipedia/commons/c/c3/NGC_4414_%28NASA-med%29.jpg License: Publicdomain Contributors: http://nix.larc.nasa.gov/info;jsessionid=1sl2so6lc9mab?id=GPN-2000-000933&orgid=12 Original artist: NASA Headquarters - Great-est Images of NASA (NASA-HQ-GRIN)

• File:Ngc1999.jpg Source: http://upload.wikimedia.org/wikipedia/commons/7/7f/Ngc1999.jpg License: Public domain Contributors: ? Original artist: ?

• File:Ngc2024_med.jpg Source: http://upload.wikimedia.org/wikipedia/commons/7/73/Ngc2024_med.jpg License: Public domain Contributors: Transferredfrom en.wikipedia; transferred to Commons by User:Khayman using CommonsHelper. Original artist: Original uploader was Clh288 at en.wikipedia

• File:Ngc6397_hst_blue_straggler.jpg Source: http://upload.wikimedia.org/wikipedia/commons/4/44/Ngc6397_hst_blue_straggler.jpg License: ? Contrib-utors: http://antwrp.gsfc.nasa.gov/apod/ap020220.html and http://www.esa.int/esa-mmg/mmg.pl?b=b&keyword=6397&single=y&start=2&size=b Originalartist: Francesco Ferraro (Bologna Observatory), ESA, NASA

• File:Nursery_of_New_Stars_-_GPN-2000-000972.jpg Source: http://upload.wikimedia.org/wikipedia/commons/7/7a/Nursery_of_New_Stars_-_GPN-2000-000972.jpg License: Public domain Contributors: Great Images in NASA Description Original artist: NASA, Hui Yang University of IllinoisODNursery of New Stars

• File:Office-book.svg Source: http://upload.wikimedia.org/wikipedia/commons/a/a8/Office-book.svg License: Public domain Contributors: This and myself.Original artist: Chris Down/Tango project

• File:OrionCC.jpg Source: http://upload.wikimedia.org/wikipedia/commons/f/f5/OrionCC.jpg License: CC BY-SA 3.0 Contributors: Own work:AlltheSky.com Original artist: Till Credner

• File:Orion_3D_red-green.png Source: http://upload.wikimedia.org/wikipedia/commons/d/db/Orion_3D_red-green.png License: CC BY-SA 3.0 Contribu-tors: Own work Original artist: Periferomenos

• File:Orion_IAU.svg Source: http://upload.wikimedia.org/wikipedia/commons/f/ff/Orion_IAU.svg License: CC BY 3.0 Contributors: [1] Original artist: IAUand Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

• File:PSM_V01_D405_August_meteor_shower_orbit.jpg Source: http://upload.wikimedia.org/wikipedia/commons/e/e9/PSM_V01_D405_August_meteor_shower_orbit.jpg License: Public domain Contributors: Popular Science Monthly Volume 1 Original artist: Unknown

• File:PSM_V18_D201_Shower_of_perseids_sept_6_and_7.jpg Source: http://upload.wikimedia.org/wikipedia/commons/b/be/PSM_V18_D201_Shower_of_perseids_sept_6_and_7.jpg License: Public domain Contributors: Popular Science Monthly Volume 18 Original artist: Unknown

• File:Pic_iroberts1.jpg Source: http://upload.wikimedia.org/wikipedia/commons/5/51/Pic_iroberts1.jpg License: Public domain Contributors: A Selection ofPhotographs of Stars, Star-clusters and Nebulae, Volume II, The Universal Press, London, 1899. Original artist: Isaac Roberts (d. 1904)

• File:Pleiades_large.jpg Source: http://upload.wikimedia.org/wikipedia/commons/4/4e/Pleiades_large.jpg License: Public domain Contributors: http://hubblesite.org/newscenter/archive/releases/2004/20/image/a/ Original artist: NASA, ESA, AURA/Caltech, Palomar ObservatoryT ₑ ᵢₑ ₑ ₑₐ ₒ ᵢ ₒ : D. Sₒ ₑᵣ ₒ ₐ E. Nₑ ₐ ₍STS I₎, F. Bₑ ₑ ᵢ ₐ B. Aᵣ ᵤᵣ ₍U. Tₑₓₐ ₎, ₐ B. Jₒ ₑ ₍Lᵢ O .₎

• File:Portal-puzzle.svg Source: http://upload.wikimedia.org/wikipedia/en/f/fd/Portal-puzzle.svg License: Public domain Contributors: ? Original artist: ?

• File:Senenmut.jpg Source: http://upload.wikimedia.org/wikipedia/commons/a/a4/Senenmut.jpg License: Public domain Contributors: Own work Originalartist: SenemmTSR

• File:Seyfert_Sextet_full.jpg Source: http://upload.wikimedia.org/wikipedia/commons/2/20/Seyfert_Sextet_full.jpg License: Public domain Contributors:http://www.hubblesite.org/newscenter/archive/2002/22/image/a Original artist: NASA

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• File:Sirius_A_and_B_artwork.jpg Source: http://upload.wikimedia.org/wikipedia/commons/c/c9/Sirius_A_and_B_artwork.jpg License: Public domainContributors: http://www.spacetelescope.org/images/html/heic0516b.html Original artist: NASA, ESACredit: G. Bacon (STScI)

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• File:Suaur.jpg Source: http://upload.wikimedia.org/wikipedia/commons/0/00/Suaur.jpg License: GFDL Contributors: Own work Original artist: Pascaloupetit

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