CLASSIFICATION OFSTARS

• PREPARED BY: AVIRAL BAJPAI B.TECH C.S. (2ND YEAR) 1210111017 FACULTY’S NAME: DR. B.M. DIXIT SIR

In astronomy stellar classification is the classification of stars based on their spectral characteristics. Light from the star is analyzed by splitting it with a prismor diffraction grating into a spectrum exhibiting the rainbow of colours interspersed with absorption lines. Each line indicates an ion of a certain chemical elementwith the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere. The spectral class of a star is a short code summarising the ionization state, giving an objective measure of the photosphere's temperature and density.

Most stars are currently classified under the Morgan–Keenan (MKK) system using the letters O, B, A, F, G, K, and M, a sequence from hottest (O) to coolest (M). Useful mnemonics for remembering the spectral type letters are "Oh, Be A Fine Guy/Girl, Kiss Me" or "Oh Boy, An F Grade Kills Me".

To also include the colder spectral classes L, T and Y, the first mnemonic can be extended to "Oh, Be A Fine Guy/Girl, Kiss Me Later Today, Yolo". Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, F1 form a sequence from hotter to cooler).In the MKK system a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum which vary with the density of the atmosphere and so distinguish giant stars from dwarfs.

STELLAR CLASSIFICATION

Luminosity class I stars are supergiants, class III regular giants, and class V dwarfs or main-sequence stars, with II for bright giants, IV for sub-giants, and VI for sub-dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800K.

During the 1860s and 1870s, pioneering stellar spectroscopist Father Angelo Secchi created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra

•Class I: white and blue stars with broad heavy hydrogen line, such as Vega and Altair. This includes the modern class A and early class F.

Class I, Orion subtype: a subtype of class I with narrow lines in place of wide bands, such as Rigel and Bellatrix. In modern terms, this corresponds to early B-type stars

•Class II: yellow stars—hydrogen less strong, but evident metallic lines, such as the Sun, Arcturus, and Capella . This includes the modern classes G and K as well as late class F.

•Class III: orange to red stars with complex band spectra, such as Betelgeuse and Antares. This corresponds to the modern class M. In 1868, he discovered carbon stars, which he put into a distinct groupIn 1877, he added a fifth class.

Class IV: red stars with significant carbon bands and lines (carbon stars.)In 1877, he added a fifth class.Class V: emission-linestars, such as γ Cassiopeiae and Sheliak.In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.

Spectral Class Intrinsic Color Temperature (K) Prominent Absorption Lines

O Blue 41,000 He+, O++, N++, Si++, He, H

B Blue 31,000 He, H, O+, C+, N+, Si+

A Blue-white 9,500 H(strongest), Ca+, Mg+, Fe+

F White 7,240 H(weaker), Ca+, ionized metals

G Yellow-white 5,920H(weaker), Ca+,

ionized & neutral metal

K Orange 5,300Ca+(strongest),

neutral metals strong, H(weak)

M Red 3,850 Strong neutral atoms, TiO

Absorption Spectra From StarsThe light that moves outward through the sun is what astronomers call a continuous spectrum since the interior regions of the sun have high density. However, when the light reaches the low density region of the solar atmosphere called the chromosphere, some colors of light are absorbed. This occurs because the chromosphere is cool enough for electrons to be bound to nuclei there. Thus, the colors of light whose energy corresponds to the energy difference between permitted electron energy levels are absorbed (and later reemitted in random directions). Thus, when astronomers take spectra of the sun and other stars they see an absorption spectrum due to the absorption of the chromosphere.

OBAFGKMAstronomers have devised a classification scheme which describes the absorption lines of a spectrum. They have seven categories (OBAFGKM) each of which is subdivided into 10 subclasses. Thus, the spectral sequence includes B8, B9, A0, A1, etc. A traditional mnemonic for the sequence is Oh, Be, A Fine Girl/Guy, Kiss Me!Although based on the absorption lines, spectral type tells you about the surface temperature of the star. One can see that there are few spectral lines in the early spectral types O and B. This reflects the simplicity of atomic structure associated with high temperature. While the later spectral types K and M have a large number of lines indicating the larger number of atomic structures possible at lower temperatures.

Three blackbody curves at different temperatures.

Standard Stellar Types (O, B, A, F, G, K, and M)

While the differences in spectra might seem to indicate different chemical compositions, in almost all instances, it actually reflects different surface temperatures. With some exceptions (e.g. the R, N, and S stellar types discussed below), material on the surface of stars is "primitive": there is no significant chemical or nuclear processing of the gaseous outer envelope of a star once it has formed. Fusion at the core of the star results in fundamental compositional changes, but material does not generally mix between the visible surface of the star and its core.Ordered from highest temperature to lowest, the seven main stellar types are O, B, A, F, G, K, and M. Astronomers use one of several mnemonics to remember the order of the classification scheme. O, B, and A type stars are often referred to as early spectral types, while cool stars (G, K, and M) are known as late type stars. The nomenclature is rooted in long-obsolete ideas about stellar evolution, but the terminology remains. The spectral characteristics of these types are summarized below:

type color Approximate Surface Temperature Main Characteristics Examples

O Blue > 25,000 KSingly ionized helium lines either in emission or absorption. Strong ultraviolet continuum.

10 Lacertra

B Blue 11,000 - 25,000 Neutral helium lines in absorption.

RigelSpica

A Blue 7,500 - 11,000Hydrogen lines at maximum strength for A0 stars, decreasing thereafter.

SiriusVega

F Blue to White 6,000 - 7,500 Metallic lines become noticeable.

CanopusProcyon

G White to Yellow 5,000 - 6,000

Solar-type spectra. Absorption lines of neutral metallic atoms and ions (e.g. once-ionized calcium) grow in strength.

SunCapella

K Orange to Red 3,500 - 5,000 Metallic lines dominate. Weak blue continuum.

ArcturusAldebaran

M Red < 3,500Molecular bands of titanium oxide noticeable.

BetelgeuseAntares

Harvard spectral classificationThe Harvard classification system is a one-dimensional classification scheme. Stars vary in surface temperature from about 2,000 to 40,000 kelvin. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest, as is done in the following table:Note: The conventional color description takes into account only the peak of the stellar spectrum. However, in actuality stars radiate in all parts of the spectrum, and since all spectral colors combined appear white, the actual apparent colors the human eye would observe are lighter than the conventional color descriptions.

Yerkes spectral classification

The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, PhilP C. Keenan, and Edith Kellman from Yerkes Observatory.This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity which is related to luminosity (whilst the Harvard classification is based on surface temperature only). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named MK (by William Wilson Morgan and Phillip C. Keenan initials).Since the radius of a giant star is much greater than a dwarf starwhile their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf. These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured. Denser stars with higher surface gravity will exhibit greater pressure broadening of spectral lines.

A number of different luminosity classes are distinguished:0 hypergiantsI supergiants

Ia-0 (hypergiants or extremely luminous supergiants (later addition)), Example: Eta Carinae(spectrum-peculiar)Ia (luminous supergiants), Example: Deneb (spectrum is A2Ia)Iab (intermediate luminous supergiants) Example: Betelgeuse (spectrum is M2Iab)Ib (less luminous supergiants)

II bright giants IIa, Example: β Scuti (HD 173764) (spectrum is G4 IIa)IIab Example: HR 8752 (spectrum is G0Iab:)IIb, Example: HR 6902 (spectrum is G9 IIb)

• III normal giants IIIa, Example: ρ Persei (spectrum is M4 IIIa)IIIab Example: δ Reticuli (spectrum is M2 IIIab)IIIb, Example: Pollux (spectrum is K2 IIIb)

VI subdwarfs. Subdwarfs are generally represented with a prescript of sd or esd (extreme subdwarf) in front of the spectra.

sd, Example: SSSPM J1930-4311 (spectrum sdM7)esd, Example: APMPM J0559-2903 (spectrum esdM7)

VII (uncommon) white dwarfs. White dwarfs are represented with a prescript wD or WD.Marginal cases are allowed; for instance a star classified as Ia-0 would be a very luminous supergiant, verging on hypergiant. Examples are below. The spectral type of the star is not a factor.

IV subgiantsIVa, Example: ε Reticuli (spectrum is K1–2 IVa-III)IVabIVb, Example: HR 672 A (spectrum is G0.5 IVb)

V main-sequence stars (dwarfs) Va, Example: AD Leonis (spectrum M4Vae)VabVb, Example: 85 Pegasi A (spectrum G5 Vb)"Vz", Example: LH10 : 3102 (spectrum O7 Vz), located in the Large Magellanic Cloud.

Spectral types

Class O

O-type main-sequence starClass O stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are class O stars. Some of the most massive stars lie within this spectral class. Class O stars frequently have complicated surroundings which make measurement of their Spectra difficult.Spectrum of an O5 V starO stars have dominant lines of absorption and sometimes emission for He II lines, prominent ionized (Si IV, O III, N III, and C III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines, although not as strong as in later types. Because they are so massive, class O stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the main sequence.When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5 The MKK scheme was extended to O9.7 in 1971and O4 in 1978 and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5.

CLASS BClass B stars are very luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include Mg II and Si II. As O and B stars are so powerful, they only live for a relatively short time, and thus they do not stray far from the area in which they were formed.These stars tend to be found in their originating OB associations which are associated with giant molecular clouds. The Orion OB1 association occupies a large portion of a spiral arm of our galaxy and contains many of the brighter stars of the constellation Orion. About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are class B stars.

Class A

Class A stars are among the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point. About 1 in 160 (0.625%) of the main-sequence stars in the solar neighborhood are class A stars.

Two class F stars: Supergiant Polaris A and its distant companion Polaris B .

Examples: Sirius, Deneb, Altair, Vega,, Fomalhaut.

CLASS F

Class F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, Cr I) beginning to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 (3.03%) of the main-sequence stars in the solar neighborhood are class F stars.

Class G

Class G stars are probably the best known, if only for the reason that the Sun is of this class. They make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood.Most notable are the H and K lines of Ca II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the yellow supergiantG classification as this is an extremely unstable place for a supergiant to be.Examples: The Sun, Alpha Centauri A, Capella, Tau Ceti, Kepler-22

CLASS K

Class K stars are orangish stars that are slightly cooler than the Sun. They make up about 12%, nearly one in eight, of the main-sequence stars in the solar neighborhood. Some K stars are giants and supergiants, such as Arcturus, while orange dwarfs, like Alpha Centauri B, are main-sequence stars.They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. There is a suggestion that K Spectrum stars may potentially increase the chances of life developing on orbiting planets that are within the habitable zone.[42]

Examples: Alpha Centauri B, Epsilon Eridani, Arcturus, Aldebaran, Algol B.

CLASS M:Class M stars are by far the most common. About 76% of the main-sequence stars in the Solar neighborhood are class M stars. However, because main-sequence stars of spectral class M have such low luminosities, none are bright enough to be visible to see with the unaided eye. The brightest known M-class main-sequence star is M0V Lacaille 8760 at magnitude 6.6 (the fractionally brighter Groombridge 1618 was once considered to be class M0 but is now considered to be as K5) and it is extremely unlikely that any brighter examples will be found.Although most class M stars are red dwarfs, the class also hosts most giants and some supergiants such as VY Canis Majoris, Antares and Betelgeuse, as well as Mira variables. Furthermore, the late-M group holds hotter brown dwarfs that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of a class M star shows lines belonging to oxide molecules, TiO in particular, in the visible and all neutral metals, but absorption lines of hydrogen are usually absent. TiO bands can be strong in class M stars, usually dominating their visible spectrum. VY Canis Majoris is a class M hypergiant. This has at times been reported as the largest known star, but its precise size is debated due to uncertainties over its distance, luminosity, and temperature. Artist's impression.

Examples: NML Cygni, WOH G64, VY Canis Majoris.Examples: Betelgeuse, Antares (supergiants)Examples: Rasalgethi, Beta Pegasi (giants)

Classification on LuminosityHertzsprung–Russell (H-R) Diagram:

H-R diagram is a scatter graphof stars showing the relationship between the stars' absolute magnitudes or luminosity versus their spectral types or classifications and effective temperatures.Rather, it is a plot of each star on a graph measuring the star's absolute magnitude or brightness against its temperature and colour.

Forms of diagram Several forms of the Hertzsprung–Russell diagram. The original diagram displayed the spectral type of stars on the horizontal axis and the absolute magnitude on the vertical axis. The first quantity (i.e. spectral type) is difficult to plot as it is not a numerical quantity and in modern versions of the chart it is replaced by the B-V colour index of the stars.

Although the two types of diagrams are similar but distinction is that the exact transformation from one to the other is not trivial, and depends on the stellar-atmosphere model being used and its parameters (like composition and pressure, apart from temperature and luminosity). The H-R diagram can be used to define different types of stars and to match theoretical predictions of stellar evolution using computer models with observations of actual stars. It is then necessary to convert either the calculated quantities to observables, or the other way around, thus introducing an extra uncertainty.

NOTE:

'Y Dwarf' Chillin' in SpaceThis artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. NASA's Wide-field Infrared Survey Explorer uncovered these elusive objects for the first time, using its heat-sensing, infrared vision. The telescope found six Y dwarfs, ranging in atmospheric temperatures from 350 degrees Fahrenheit (175 degrees Celsius) to less than about 80 degrees Fahrenheit (25 degrees Celsius).

Y dwarfs belong to a larger family of objects called brown dwarfs. Brown dwarfs begin their lives like stars but they never accumulate enough mass to fuse atoms steadily at their cores and shine with starlight -- as our sun does so well. Instead, they fade and cool with time, giving off most of their light in infrared wavelengths.

WISE was able to pick up this faint glow for six Y dwarfs, which are the coldest class of brown dwarfs and the latest letter in the stellar classification scheme. This scheme describes stars of all temperatures, beginning with the hottest "O" stars and now ending with the coldest Y dwarfs. The entire scheme includes the classes: O, B, A, F, G, K, M, L, T, Y. Our yellow sun belongs to the G class of stars. M stars are colder than our sun, and reddish in color.

While the O through K classes are all considered stars, M and L objects are a mixture of stars and brown dwarfs, and T and Y objects are all brown dwarfs. The term "brown dwarfs" was chosen because at that time, astronomers didn't know what colors these objects would actually have at the visible wavelengths our eyes see, and brown is not a true color of light (there are no "brown photons"). Astronomers now know that T dwarfs would appear reddish, or magenta, to the eye. But they are not certain what color Y dwarfs are, since these objects have not been detected at visible wavelengths. The purple color shown here was chosen mainly for artistic reasons. In addition, the Y dwarf is illustrated as reflecting a faint amount of visible starlight from interstellar space.

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