apollo-soyuz pamphlet no. 3 sun, stars, in between

8/8/2019 Apollo-Soyuz Pamphlet No. 3 Sun, Stars, In Between

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Apollo-SoyuzPamphlet No. 3:Sun, Stars, In Between N78-271U9(NASA-EP-135) APOLLO-SOYOZ PAMPHLE T NO. 3:SON, STARS, IN BETWEEN (National Aeronauticsand Space Administration) 6H p MF A01; SOIHC se? of 9 volu.es CSCL 22A Dnclas


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This is one of a series of ninecurriculum-related pamphletsfor Teachers and Studentsof Space ScienceTitles in this series ofpamphlets include:EP-133 Apollo-Soyu z Pamphlet No. 1: The FlightEP-134 Apollo-Soyuz Pamphlet No. 2: X-Rays . Gamma-RaysEP-135 Apollo-Soyuz Pamphlet No. 3: Sun,Stars, In Betw eenEP-136 Apollo-Soyuz Pamphlet No. 4: Gravi tat ional FieldEP-137 Apollo-Soyuz Pam phlet No. 5 : The Earth from OrbitEP-138 Apol lo-Soyuz P amphlet No. 6: Cosmic R ay DosageEP-139 Apollo-Soyuz Pam phlet No. 7: Biology in Zero-GEP-140 Apollo-Soyuz Pamphlet No. 8: Zero-G T echnologyEP-141 Apollo-So yuz Pamphlet No. 9: General Science

Cover The Sun.the Solar Corona, and theZodiacal Light as seen from Soyuz


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For sale by the Superintendent of Documents,U.S. Government Printing Office, Washington, D.C. 20402(9-Part Set; Sold in Sets Only)Stock Number 033-800-00688-8


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Preface

The Ap ollo-Soy uz Test Project (ASTP ), w hic h flew in Ju ly 1975, arousedconsiderable public interest; first, because the space r iva ls of the late 1950'san d 1960's were wor king together in a joint endeavor, and second, becausetheir mutual efforts included developing a space rescue system. T he ASTPalso included significan t scientific experim ents, the resu lts of which can beused in teaching biology, physics, and mathematics in schools and colleges.

This series of pamp hlets discussing the Apollo-Soyuz mission and experi-ments is a set of curriculum su pplemen ts designed for teachers, supervisors,curriculum specialists, and textbook w riters as we ll as for the general pub lic.Neither textbooks nor courses of study, these pamphlets are intended toprovide a rich source of ideas, examples of the scientific method, pertinentreferences to standard textbooks, and clear descriptions of space experim ents.In a sense, they may be regarded as a pioneering form of teac hing aid. Seldomha s there been such a forthright effort to provide, directly to teachers,curr iculum-re levant reports of current scientific research. High schoolteachers who reviewed the texts suggested that advanced students who areinterested m igh t be assigned to study one pa m phle t and report on it to the restof the class. After class discussion, students might be assigned (withoutaccess to the pamphlet) one or more of the "Questions for Discussion" forformal or informal answers, thus stressing the application of what waspreviously covered in the pamphlets.

The authors of these pamp hlets are Dr. Lou Will iams Page, a geologist, an dDr. Thornton Page, an astronomer. Both have taught science at severaluniversities and have published 14 books on science fo r schools, colleges, an dthe general reader, including a recent one on space science.

Technical assistance to the Pages was provided by the Apollo-SoyuzProgram Scientist, Dr. R. Thomas Giuli, and by Richard R. Baldwin,W . Wilson Lauderdale, an d Susan N. Montgomery , members of the group atth e NASA Lyndon B. Johnson Space Center in Houston w hich organized th escientists' participation in the ASTP an d publish ed their reports of experimen-ta l results.

Selected teachers from high schools an d universit ies throughout th e UnitedStates reviewed th e pamphlets in draft form. They suggested changes inword ing, the addition of a glossary of terms u nfam iliar to studen ts, andimprovements in diagrams. A list of the teachers and of the scientific inves-tigators who reviewed th e texts fo r accuracy follows this Preface.

This set of Apollo-Soyuz pamphlets was initiated and coordinated by Dr.Frederick B. Tuttle, Director of Educational Programs, and was supported byth e NA SA Ap ollo-Soy uz Program Office, by Leland J. Casey, AerospaceEngineer fo r ASTP, and by William D. Nixon, Educat ional ProgramsOfficer, all of NASA Headquarters in Washington , D.C.

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Appreciation is expressed to the scientific investigators an d teachers whoreviewed th e draft copies; to the NASA specialists who provided diagramsan d photographs; and to J. K. Holcomb, Headquarters Director of ASTPoperations, and Chester M. Lee, ASTP Program Director at Headquarters,whose interest in this educational endeavor made this publication possible.

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TeachersAnd Scientific InvestigatorsWho Reviewed the Text

Harold L. Adair, Oak Ridge National Laboratory, Oak Ridge, Tenn.Lynette Aey, Norwich Free Academy, Norwich, Conn.J. Vernon Bailey, NASA Lyndon B. Johnson Space Center, Houston, Tex.Stuart Bowyer, University of California at Berkeley, Berkeley, Calif.Bill Wesley Brown, California State Universi ty at Chico, Chico, Calif.Ronald J. Bruno, Creighton Preparatory School, Omaha, Nebr.T. F. Budinger, University of California at Berkeley, Berkeley, Calif.Robert F. Collins, Western States Chiropractic College, Portland, Oreg.B. Sue Criswell, Baylor College of Medicine, Houston, Tex.T. M. D onahue, Universi ty of Michigan, Ann Arbor, M ich.David W. Eckert, Greater Latrobe Senior High School, Latrobe, Pa.Lyle N. Edge, Blanco High School, Blanco, Tex.Victor B. Eichler, W ichita State Univ ersi ty, Wich ita, Kans.Farouk El-Baz, Smithsonian Inst i tut ion, Washington, D.C.D. Jerome Fishe r, Emeritus, Un iversity of Chicago, P hoen ix, Ariz .R. T. Giuli , NASA Lyndon B. Johnson Space Center, Houston, Tex.M. D. Grossi, Smithsonian Astrophysical Observatory, Cambridge, Mass.We ndy Hindin, North Shore Hebrew Academy, Great Neck, N.Y.Tim C. Ingoldsby, Westside High School, Omaha, Nebr.Robert H. Johns, Academy of the New Church , Bryn Athyn, Pa.D. J . Larson, Jr., Grumman Aerospace, Bethpage, N.Y.M. D . Lind, Ro ckwell Interna tional Science Cente r, Thou sand Oaks, C alif.R. N. Litt le, Universi ty of Texas, Aust in , Tex.Sarah Manly, Wade Hampton High School, Greenvil le, S.C.Katherine Mays , Bay City Independent School District, Bay Ci ty ,Tex.Jane M . Oppenheimer, Bryn Maw r College, B ryn M awr , Pa.T. J. P epin, Univ ersi ty of Wy om ing, Laramie,Wyo.H. W . Scheld, NASA Lyndon B. Johnson Space Center, H ouston, Tex.Seth Shulman, Naval Research Laboratory, Washington, D.C.James W. Skehan, Boston College, Weston, Mass.B. T. Slater, Jr., Texas Education Agency, Aust in , Tex.Robert S. Snyder, N ASA George C. M arshall Space F light Center, Hu ntsvil le, Ala.Jacqueline D. Spears, Port Jefferson High School, Port Jefferson Station, N.Y.Robert L. Stewart, Monticello High School, Monticello, N.Y.Aletha Stone, Fulmore Junior High School, Aust in , Tex.G. R. Taylor, NASA Lyndon B. Johnson Space Center, Houston, Tex.Jacob I. Trombka, NASA Robert H. Goddard Space Flight Center, Greenbelt, Md.F. O. Vonbun, NASA Robert H. Goddard Space Flight Center, Greenbelt, Md.Douglas W inkier , Wade Hampton High School, Greenvil le, S.C.


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Contents

Section 1 l i t / : > r . . . v . ' ? < 17Section 2 Layers in the Sun . 3A. Cere of tfie&jjn:^ucles^ factions.* 3B. The Photosphere: Siirfacie fenlpfera'ture and Brightness 6C. Layers Above the Photosphere: Chromosphere

and Corona 8D. The Artif icial Solar Ecl ipse, Joint Experiment MA -148 10E. R esults of the Artif icial Solar Ecl ipse Experiment 14F. Questions for Discussion (Brightness, Sun's Core,Surface Temperature, Spectra, Corona) 16

Section 3 Stars and Gas Clouds in the Milky Way 19A. Stel lar Spec tra 19B. The Milky Way Galaxy 21C. Stellar Evo lution 23D. The Extreme Ultraviolet Survey, Experiment MA-083 23E. Resul ts of the EUV Survey Experiment 27F. Questions for Discussion (Spectra, lonization,Temperature) 32Section 4 How Much Helium in Space? 33

A. The Disc overy of Helium; Its Spec trum 33B. Formation of the Elements 33C. Interstel lar Gas Moving Through the Solar Sys tem 34D. The MA-088 Hel ium-Glow Expe riment 35E. Resu l ts of the Helium-Glow Experiment 39F. Quest ions fo r Discussion (Abundances of Elements,Doppler Shifts) 40Appendix A Discussion Topics (Answers to Questions) 41Appendix B SI Units and Powers of 10 45Appendix C Glossary 48Appendix D Further Reading 53

VII


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Tables and Figures

Table 2.1 Laws of Radiation 7MA-083 EUV Filters 28Schem atic Diagram of the Layers in the Sun 4Determination of the Total Power Output of the Sun 5Th e Planck Law of Radiation From an Opaque Surface 8Photographs of the Solar Corona(a) At Sunspot Minimum 9(b) At Sunspot Maximum 9Schematic Diagram of the Inner Corona, the Outer Corona,and the Zodiacal Light 10Orbit and Separation of Apollo and Soyuz for the Art i f ic ialSolar Eclipse 11Corona Photograph and Contour Plot 13Corona Photograph an d Contour Plot of Apollo R CSJet Firing 15Correct ion of Corona Intensi t ies Along th e Ecliptic 16The Electromagnetic Spectrum 20The Milky Way Galaxy 22The Extrem e Ultraviolet Teles cope 24Schematic Diagram of EUV Telescope Focusing 26Diagram of the EUV Telescope 27EUV Telescope Point ings for a Star Observat ion 29Typical EUV Count R ates Through One Filter 29EUV Spectrum of White Dwarf HZ 43 30The MA-088 View of R ed-Shifted Helium Glow 35The Helium-Glow Detector 37Schematic Diagram of MA-088 Electronics System 38

Table 3.1Figure 2.12.22.32.4

2.52.62.72.82.9

Figure 3.13.23.33.43.53.63.73.8

Figure 4.14.24.3

VIII


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1 Introduction

After 4 years of preparation by the U.S. National Aeronautics and SpaceAdministration (NA SA ) and the U.S .S .R . Academy of Sciences, the Apolloand Soyu z spacecraft were launched on July 15,1975. Two d ays later at16:09Green wich m ean time on Jul y 17, after Apollo maneu vered into the same orbitas Soyuz, the two spacecraft were docked. The astronauts and cosmonautsthen met for the first international handshake in space, an d each crew enter-tained the other crew (one at a time) at a meal of typical Ame rican or R ussianfood. These activities and the physics of reaction motors, orbits around theEarth, and weightlessness (zero-g) are described more fully in Pamphlet I,"The Spacecraft, Their Orbits, and Docking" (EP-133).

Thirty-four experiments were performed while Apollo and Soyuz were inorbit: 23 by astronauts, 6 by cosmonauts, and 5 jo int ly . These exper iments inspace were selected from 16 1 proposals from scientists in nine differentcoun tries. They are listed by nu mb er in Pamphlet I, and grou ps of two or moreare described in detail in Pamphlets II through IX (EP-134 through EP-141,respectively) . Each experiment w as directed by a Principal Investigator,assisted by several Co-Investigators, and the detailed scientific results havebeen published by NASA in two reports: the Apollo-Soyuz Test ProjectPreliminary Science Report (N AS ATM X-5 8173 ) and the Apollo-S oyuz TestProject Summary Science Report (NASA SP-412). The simplified accountsgiven in these pamphlets have been reviewed by the Principal Investigators orone of the Co-Investigators.

Low-density gas in the space between the stars has intereste d groun d-basedastronomers for many years. From spacecraft and rockets above the Earth'satmosphere, it is possible to make much better observations of gas near theSun because there is little or no scattered sunlight from th e atmosphere and allthe extreme ultrav iolet (light of very short w avel eng th) can be measu red.(Such l ight is absorbed by the atmosphere before it reaches telescopes onEarth.) Three experiments on Apollo-Soyu z exploited these advantages ofspace as tronomy.

The Artificial Solar Eclipse (Experim ent MA-148) was a joint experim entthat involved both astronauts an d cosmonauts . The Principal Investigato r wasG. M. Nikolskiy of the U.S.S.R. Academy of Sciences; he was assisted bytwo other R ussians and three A mericans. W hile the Apollo spacecraft backedof f from th e Soyuz vehicle (in line with th e Sun) an d then moved back towardSoyuz , th e cosmonauts photographed th e solar corona. These photographswere later studied an d measured by the Principal Investigator in Moscow.

The Extreme Ultraviolet (EUV) Survey (Experiment MA-083) countedhigh-energy ultraviolet photons coming toward th e Earth from various partsof the s k y . The Principal Investig ator fo r th is exper iment was Stuart Bow yerof the Univers i ty of California at Berkeley, who collaborated with many otherscientists at that universi ty and elsewhere. They used a special EUV telescopean d discovered several interesting EUV sources.


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The Interstellar Helium Glow (E xperiment MA-088) used similar detectorsand was also supervised by Stuart Bow yer. This experiment detected heliumgas from interstellar space flowing through the solar system.The them e of this pamphlet is the study of low-density gas in space: the gasat very high temperature in the solar corona, the "atmosphere" surroundingthe Apollo spacecraft, the outer layers of gas in superhot stars, and theinterstellar helium. The following three sections begin with backgroundinformation on the Sun, the stars, and the interstellar medium. Each sectiondescribes one experiment and its findings.


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2 Layers in the Sun

A

The Sun is a 1 390 000-kilometer ball of gas, m ostly hydrogen and helium .It s mass of 2 x 1030 kilograms dominates th e planets, th e comets, th e dust ,and the gas in the solar system. The Sun rotates about once a month in thesame direction as all the planets move in their orbitseastward; that is, itrotates counterclockwise as viewed from the north.

At the Sun's "surface" (the edge that you can see throug h dark glass1), thetemperature is about 5800 K (Fig. 2 .1). Astronomers call that surface, whic his 695 000 kilometers from the center of the Sun, the photosphere. As you cansee during a total solar eclipse, the photosphere is not really the edge of theSun. Below the photosphere, the S un's gas is opaque to visible light, whichcauses the photosphere to look as though it had a sharp edge. Above thephotosphere, transparent gases glow red for several thousand kilometers.Above that is the solar corona, which glows greenish white to at least1 000 000 kilometers above the photosphere. The brightness of the corona isabout one-mill ionth th e brightness of the photosphere. The corona isswamped by haze and blue-sky light except during a total solar eclipse, whenthe Moon covers the Sun for a minu te or so. (Both th e Moon and the Sun areabout 0.5 across as seen from Earth.) M ost of what we know about the solarcorona came from observations durin g total solar eclipses, which ar e visiblefrom small regions of the Earth's surface about twice a year.

Core of the Sun: Nuclear ReactionsThe Sun is the brightest object in the skyabout 1010 t imes brighter thanth e brightest star an d about a million times brighter than th e full Moon.Where does all its radiant energy come fro m? It is easy to show that chemicalreactions such as burning cannot provide th e vast amoun t of power radiatedby th e Sun. Astronomers have carefully measured th e solar energy passingthrough each square m eter perpendicular to the Earth-Sun line each second inal l wavelengths from ultraviolet through visible an d infrared rays, makingcorrections fo r absorption by the Earth's atmosphere (see Pamphlet II). Theymultiplied th e measured energy pe r square meter pe r second by the numberofsquare meters in a sphere th e size of the Earth's orbit around the Sun toge t the total solar power outputthe huge value of 3.83 x 10

26watts(Fig. 2.2). Even if the Sun were composed entirely of hydrogen (H) andoxygen (O), th e chemical reaction 2H + O H 2O would burn out in a mere4000 years. Geologists hav e evidence that the Sun has been shin ing at about

the same brightness for at least 4.5 billion years.

'Never look directly at the Sun with the naked eye. Direct sunlight can damage your eyes.


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Photosphere

10 000 000 KCore(nuclearreactions)

(Center) I120 000(Opaque)

695 000 700 000Radius, km

1 700000

Figure 2.1 Schematic diagram of the layers in the Sun.

In 1904, Einstein derived from hi s Special Re lativ ity Theory th e formulaE = m e 2 , w hich shows that mass m is equiva lent 2 to a very large amount ofenergy E (c is the velocity of l ight , 3 x 10 8 m/sec). Ph ysicists speculated thatmass was being converted to energy inside th e Sun, but it wa sn ' t until 1938that nuclear reactions were measured in the laboratory an d found to becapable of doing th is mass-energy conversion under th e high temperatures(10 000 00 0 K) and pressures kno wn to exist in the Su n's core. O ther starsar e similar to the Sun, although much farther away, and these nuclearreactions also explain their power output.

2See Project Physics, Sees. 20.1, 24.1 to 24.4, 24.9. (Throughout this pamph let , referenceswil l be given to key topics covered in these standard textbooks: "Project Physics," secondedi t ion, Holt , Rinehar t an d Winston, 1975, an d "Physical Science Study Committee" (PSSC),four th edi t ion, D. C. Heath, 1976. )


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Radius of Earth s orbitr = 1 . 5 0 x 1 0 8 k mMeasuredE, = 1.36 x 1 03 Wpassing through 1 m 2at Earth's distance

Determination of the total power output of the Sun:Ef = 4r r2^ = 3.83 x 1033 ergs/sec = 3.83 x 1026 wattsin all directions.

Figure 2.2

The loss of mass (0.71 percent) occurs when four protons (hydrogen nucle i)of 1.0081 atomic mass units each are compressed to form an alpha particle(helium nucleus) of 4.0039 atomic mass units. This does not happen all atonce but rather in a series of nuclear reactions that involve other atomicnuclei. The products of some of these reactions last fo r only a very short time .They can proceed only if the temperature is extremely high so that all theparticles have high velocities and high-energy impacts. The pressure alsomust be high enoug h to provide ma ny impa cts on each nuc leus per second.

Astronomers can compute th e temperature , th e pressure, and the densi ty atvarious depths inside the Sun (or inside any star of given size and surfacetemperature) , using basically the gas law and Newto n ' s Law of Gravi ta t ion .


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The gas la w 3 states that pressure is proportional to density times temperature.Newton's Law of Gravitation states that the deeper layers of the Sun arepulling outer layers down, so that the gas pressure on a centimeter of gas deepin the Sun is the weight of the 1 -square-centimeter colum n of gas above it (outto the surface of the Sun).

These calculations ar e complex. With the help of high-speed electroniccomputers, astronomers ca n estimate the pressure, density, an d temperatureat any depth in the Sun. This enables them to estimate at what depths in theSun th e nuclear reactions w ill "go" an d also to calculate th e energy releasedper second at the greater depths. They find that most of the nuclear energycomes from a central core, about one-sixth the radius of the Sun, whereapproximately 6 X 1011 kilogram s of protons are converted to helium nucleievery second. The density in th is 24 0 000-kilometer-diameter core is 100 to150 times the density of water (100 to 150 gm/cm 3); therefore, it containsabout 8 x 10 29 kilograms, or 40 percent of the Sun's mass. I ts temperature isbetween 10 000 000 and 15 000 000 K.

P ThePhotosphere: Surface Temperatureand BrightnessCalculations of conditions inside the Sun depend on the surface temperature,which is measured by applying the Planck Radiation Law to the spectrum ofsunlight. This law (Table 2.1 ) is a form ula relating the intens ity / of lightfrom a hot, opaque surface to the wavelength X of the light and the tem-perature T of the surface. From the complex Planck formula, two simplerformulas can be derived. The first tells about the color of a hot surface. Itstates that th e wavelength of max imum intens i ty \m is inversely proportionalto th e temperature T. The Sun is yellow, with Km equal to 5000 angstroms(500 nanometers) and the "color temperature" T c equal to 2.9 X 107/5000,or 5800 K. If its surface temperature were higher, \m would be smaller andth e Sun's color bluer. If the Sun were cooler, Km would be larger and theSun's color redder (Fig. 2.3). In this way, astronomers get the surfacetemperature of stars: red stars with a surface temperature of about 3000 Kand blue stars with a surface temperature of 30 000 K or more. These tem-peratures can also be estimated from absorption lines4 in the star's spectrum.Of course, the internal temperatures of the Sun and all the other stars aremu ch higher (they have to be about 10 000 000 K for the nuclear reactionsto occur).

"Project Physics, Sec. 11.5; PSSC, Sees. 17-1 to 17-5.'Project Physics, Sec. 19.1.


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Laws of Radiation(See Fig. 2.3) Table 2.1

Planck Law: th eintensity /^ atwavelength A

Ihcl\kT _

"M J secwhere T = surface temperature in kelvinh = the Planck constant, 6.62 x 10~ Jk = the Boltzmann constant, 1.38 x 10~23 J/Kc = the velocity of l ight, 3 x 10 s m/sec

Color law: wave lengthof m a x i m u m intensityAm> in angstroms

A,,, = 28 970 000/7

Brightness law: totalenergy E t (in joules)radiated from 1 squaremeter each second

, = oT*where tr = the Stefan-Boltzmann constant,5.67 x 10-" W/m 2 K4

The other formula derived from the Planck law tells about brightness. Itstates that the total energy E t radiated from a surface in all wavelengths oflight (ultraviolet, visible, infrared, and radio) is proportional to T 4. The datain Figure 2.2 show that each square meter of the Sun's photosphere (695 000-kilometer radius) radiates 6.3 x 107 watts. From th e brightness formula(Table 2.1), we find the brightness temperature TB to be 5800 K.Below the photosphere, the temperature increases toward the center of theSun. There are at least two layers we can discuss (Fig. 2.1). In the radiativetransfer layer, the energy is carried outward by photons of light that areabsorbed in the opaque gas, then reemitted, then absorbed after traveling afew millimeters, and so on. There are also one or two convective layers wherethe energy is carried outward by rising hot gas (as cooler ga s falls). Suchconvection curr ents are not simple. The Sun 's rotation, the strong magn eticfields near the equator, and temporary hotspots complicate the layers.


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\m =2500 A

aa ?

Visiblewavelengths

2000 4000 6000Wavelength , A

8000 10000

Figure 2.3 Planck Law of Radiation from an opaque surface. The areas under the curvesgive the total energy output in oules persecond from each square meter: f =5.67 x 10"8 W/m2 K4.

Q Layers Above the Photosphere:Chromosphere and CoronaGases above th e Su n's photosphere are t ransparen t , bu t they glow because oftheir high temperature. The red glow of the chromosphere (color sphere) ca nbe seen for a few seconds just af ter th e Moon covers the Sun in a total eclipse.The layer is only about 3000 kilometers thick, and the lower part is at asomewhat lower temperature than the photosphere. The red glow is ma inlydue to the emission of hydrogen at the 6563-angstrom (656.3-nanometer)wavelength . Astronomers ca n photograph th e chromosphere without aneclipse, using th e emission-line w aveleng ths on ly. Their photographs showhuge f lamelike "prominences" pushed out of the chromosphere by thepressure of s un lig ht. The gas density in the chrom osphere is about 10~ 5 k g /m 3(10~ 8 t ime s the density of wate r) , and the temp erature increases from 4500 Knear the bottom of the chromosphere to 100 000 K near the top (Fig. 2.1).

This increase in temperature with height cont inu es th rough th e greenishcorona to reach several mill ion ke lvin . A t first, it was puzzl ing to astronomers


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to find that the temperature of these very low density gases increases withheight and gets so high that x-rays are emitted (wavelengths less than 100angstroms, or 10 nanometers). It is now thought that sound waves carryenergy up from below th e photosphere to heat th e corona. The intense lightrays pass through th e transparent corona w itho ut heating it .Two photographs of the solar corona during a total eclipse are shown inFigure 2.4. The shape of the corona can vary widely, depending on thenumber of sunspots at the time of the eclipse. The corona often has longstreamers an d rays that ar e thought to match th e magnetic field of the Sun. Theouter edge of the corona is not as sharply defined as it appears to be in thephotographs; that is, a longer exposure will show more of the "outer corona."Previous space experiments on Skylab and Apollo missions showed that theouter corona merges into the "zodiacal light," a faint band that exten ds allaround the sky (Fig. 2.5). The zodiacal light near the Sun is part of the solarcorona. Farther away, it is visible sunlight reflected from dust in the solarsystemdust that is in orbit near th e common plane of the planets' orbitscalled the zodiac (the "ecliptic" in Fig. 2.5).

The solar corona photographed during a total eclipse (Yerkes Observatory Figure 2.4photographs).

(a) At sunspot minimum. (b) At sunspot maximum.

ORIGINAL PAGE BOP POOR QUALHY


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Outer corona Zodiacal light(extends aroundthe sky)

Zodiacal light(extends aroundthe sky)

Figure 2.5 Schematic diagram of the inner corona, the outer corona, and the zodiacal light.(Except for the Sun's photosphere, there are no sharp edges to any of theseregions.)

The Artificial Solar Eclipse,Joint Experiment MA-148The primary objective of the Artif icial Solar Eclipse Experiment was tophotograph th e outer corona an d measure it s brightness in white l ight atwave lengths from 4000 to 7500 angstroms (400 to 750 nanometers) . TheApollo spacecraft was used to eclipse the Sun for Soyuz. A secondaryobjective was to check the gas and particles around Apollo. Such a "con-taminating atmosphere," which leaks off every spacecraft, would show upwell on photographs of Apollo with th e bright Sun (and corona) behind it. Ofcourse, th e Apollo atmosphere wo uld interfere with measurem ents of thecorona on the photographs taken from Soyuzat least of the inner corona,close to the rim of Apollo.

The plan fo r separating Apollo an d Soyuz just after sunrise an d keepingApollo between Soyuz and the Sun for 8 min u te s is shown in Figure 2.6.Theastronauts could look back at Soyuz an d make sure that th e round Apolloshadow covered Soyuz. The cosmo nauts photographed throu gh a win dow onth e hatch that le d into th e Docking Module (seePamphlet I) wh en the twospacecraft were docked. Before un doc kin g, the astronauts attached a "lightbaffle" to the outside of that wind ow . This baffle shielded th e window from"earthlight" (sunlight reflected from th e Earth 's surface).

After the docking latches were opened, the Apollo Reaction ControlSystem (R CS, see Pamphlet I) jets were fired for 7 seconds to give Apollo a1-m/sec velocity away from Soyuz toward th e Sun. After coasting fo r about3.5 m inutes to adistanc e of 225 m eters from Soyuz, Apollo f ired the RCS jetsin th e other direction to stop Apollo's coast an d start it re turn ing to Soyuz.

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Orbit and separation of Apollo and Soyuz for the Artificial Solar Eclipse, Ex- Figure 2.6periment MA-148.

1. Orbital sunrise

Earth surface in sunlight Earth surface in darkness


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They redocked about 4 minutes later . At ma xim um separation (225 meters) ,Apollo looked twice as large as the Su n as viewed from Soyuz. It covered th eSun completely. However, there was a bright rim around Apollo that wascaused by diffraction of the strong sunlight at Apollo's edges. Also, duringon e interval of 142 seconds, earthshine was strongly reflected off the Apollounderside an d spoiled 24 of the Soyuz photographs.The Soyuz camera was automatic. It had an f/2.8 lens with a 90-millimeterfocal length an d gave photographs 50 millimeters square on special high-speed Kodak f i lm. The camera took a repeated sequence of six exposuresfrom 1/6- to 11-seconds' duration; nine such sequences (5 4 photographs)were obtained before and after the four sequences spoiled by earthshine. Thewide range of exposures was used because no one was sure in advance whatexposure t ime would be best. Th e outer corona appeared on 19 photographs.The film was calibrated with 12 exposures to light of known brightness in aMoscow laboratory and then developed. The density of the developed film(see Glossary) was measured on the corona photographs and on the laboratoryphotographs. Then th e corona brightness (intensity) was plotted.

The photographs show Apollo with its bright rim, the corona, the planetMercury, and two bright stars identif ied as Gamma Geminorum and AlphaCanis Minoris. The positions of the stars relative to the Sun at the time thephotographs were taken are kno wn ; thus , the Rus sian scientists could plo t theSun (behind Apollo) on each photograph and relate the measured coronaintensity to places on diagrams like the one shown in Figure 2.7. Thetemperatures of the two stars are know n from earlier observations; therefore,th e intensity of their images on the Soyuz film was used to calibrate th emeasured corona intensity (to 15 percent).

There were two other problems with the photographs: (1) light from insideth e Soyuz cabin was reflected from th e window just in front of the camera, an d(2 ) the came ra-lens eff iciency decreased from the center to the edge of eachphotograph. The Russian scientists took care of the first problem by measur-ing the reflected Soyuz cabin l ight in one o f the first photographs taken whenthe nearby Apollo completely shadowed the windo w from outside. Then theysubtracted th is cabin- l ight in tensi ty from th e measured corona intensit ie s. Themeasured intensi t ies also had to be corrected near th e edges of each photo-graph because of the reduced lens efficiency there.

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Corona photograph and contour plot. Each line on the contour plot shows the Figure 2.7position of one brightness level. Thephotograph was a 0.33-second exposurewhen Soyuz was 55 meters from Apollo.

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p Results of the Artificial Solar EclipseExperimentEight of the MA-148 photographs were accurately measured. Six of theseshow the outer corona and two show the effect of firing the Apollo RCS jets.Each of these photographs was scanned with a microdensitometer, a machinethat measures film density in each 0.1- by 0.1-millimeter square over theentire 50- by 50-m illimeter photograph (500 x 500 = 250 000 measurementson each photograph). These densities were then converted to intensities bycomputer.The r esult ing corona inten sities (and the Soyuz cabin ligh t and Apolloforeground contamination) are shown on the photograph and the contourplot in Figure 2.7. Each line on the contour plot runs through m easured pointsof the same in tens ity on the photograph, and the contour line is labeled withth e intensity value in units of 10~' of the Sun's surface intensity in whitelight. The highest contour (near the circle representing Apollo) is 7 units,and the lowest (at left) is 1.75 u nits. The image of Mercury (left center), thestar Gamma Geminorum (upper left), and the star Alpha Canis Minoris (top)are much brighter. Three straight lines through the Sun 's image (off centerbehind Apollo) show the southerly direction toward upper right, the westerlydirection toward upper left, and the ecliptic e ru nnin g through M ercury towardthe left. The straight border along the bottom of the conto ur plot in Figure 2.7is the edge of the light baffle outside the Soyuz window, and all intensitiesbelow it should be zero. The Soyuz cabin light has not yet been subtracted,nor has the correction for lens efficiency been made.

A similar contour plot and a photograph taken when the Apollo RCS jetswere firing is shown in Figure 2.8. The pro pellant gas and particles in the jetexhaust scatter su nlight from four long plumes. The Russian Principal Inves-tigator, G. M. Nikolskiy, notes that his experiment is "an effective way tostudy contam inants produced by RC S jet engines ."From Figure 2.7 and three others like it (w ith the RCS jets off ), the Ru ssianscientists subtracted th e Soyuz cabin light an d obtained intensities in the outercorona and zodiacal light from 2 to 12 from the Sun along the ecliptic. Figure2.9 shows the measured intensities from 2 to 17 (solid line), the Soyuz cabinlight (dotted line), and the corrected corona intensities (dashed line). Theformula fo r corona intensity I c that fits their measurements is

lc 4 x 10-'70 (R / R Q ) 2 i

whereJ?Q 's tne radius of the Sun and /Q is the surface brightness of the Sun.

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A l pha Canis Minoris South

Mercury

Corona photograph and contour plot when the Apollo RCS quad jets were on. Figure 2.8The photograph was a 0.33-second exposure when Soyuz was 109 metersfrom Apollo.

ORIGINAL PAGE mOF POOR QUAJJTV 15


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- 8 5

- 9 0

- 9 5

-10.0

-10.5

-11.0

\\ .Measun\sr intensitMeasuredisitiescorrected forcabin light

Measuredintensit ies

Figure 2.9 Correction of corona intensities along the ecliptic measured in the MA-148Experiment.

The dot-dash line shows measurements that were made from an airplane in1954. These measurements ar e three t im es fainter than th e MA-148 results.I t seems that the outer corona is much brighter than previously thought,although no explanation has been given by solar physicists.

Questions for Discussion(Brightness, Sun's Core, Surface Tem perature , Spectra, C orona)

1. The Sun is 1010 t imes brighter than th e brightest star in the sky. If starswere other sun s wit h the same power out put and size as our S un, how far fromEarth would they be?

2. If 6 X 1011kilograms of protons are converted to helium nuclei in theSu n's core each second, how long will it be before all the protons in the coreare used up? What will happen then?

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3. Blue giant stars have surface temperatures as h igh as 30 000 K. If theyare the same size as our Sun , ho w much more energy are they radiating persecond?

4. Just before an d just after totality in a solar eclipse, th e chromosphereshows briefly at one edge of the Moon. What kind of spectrum does i t show?

5. Flares ar e violent eruptions in the photosphere an d chromosphere. Theyfire charged particles out of the Sun at high velocity . W ould you expect a flareto have any effect on the corona above it?

6 . During th e Skylab m issions, x-ray photographs w ere taken of the Sunfrom orbi t . How large does the Sun look in such x-ray photographs?

7. Wha t was the expected advantage of photographing th e corona fromSoyuz compared wi th photographing it from the ground durin g a total eclipse?What disadvantages were discovered?

8. How wou ld you prevent cabin l ight from being reflected off the win dowinto the camera, as occurred on Soyuz?

9. Wha t can be learned about th e Sun's corona from th e corrected meas-urements of its brightness compared with earlier measurements from anairplane (Fig. 2.9)?

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3 Stars and GasCloudsin the Milky Way

Stellar SpectraAstronomers photograph th e stars. They also measure stellar spectra byspreading a star's light out into its different colors usin g a prism o r diffractiongrating. At wavelengths longer than visible red light, there are invisibleinfrared "heat waves" and radio w aves. At w aveleng ths shorter than visiblel ight , there ar e ultraviolet rays, x-rays, an d gamma rays (see Pamphlet II).Figure 3.1 shows th e complete electrom agnetic spectrum and the parts that getthrough th e Earth's atmosphere. Note th e three scales: wavelength X inangstroms and meters, frequency/in hertz , an d photon energy E in electron-volts. It is useful to remember that/ = cl X and E = /i/(where c is the velocityof light and h is the Planck con stant) and that E = 12 345/ X in angstroms.(See Pamphlet II.)

The spectrum of a star te lls much about it s temperature (Fig. 2.3), it smaterial com position , its motion, and even its size. Un til about 1945,astronomers could observe onl y that part of the spectrum that gets through theatmospherevisible light and a little more on each side (ultraviolet andinfrared). In the 1950's, radio telescopes extend ed the observed spectrum o vermost of the region show n in Figure 3.1 where the atmosphere is transp arent.No w, in the Space Age, we can observe the entire spectrum w ith instrume ntson rockets and spacecraft above the atmosphere. In addition to the generalchanges of intensity with wavelength /^ that indicate a star's surface tem-perature, the spectrum show s absorption lines gaps w here a gas has absorbedspecific wavelengths. Each atom an d each ion has a pattern of such lines;therefore, you can tell what elements are in the atmosphere of a star bymeasuring the lines in the spectrum. You can also estimate the temperatureby noting the elements that are io nized. For example , lines due to ionizedhelium (H e + ) show that th e temperature is about 20 000 K.

If th e star is receding from Earth at some speed v, the pattern of absorptionlines is shifted from th e normal wavelength X to a slightly larger valueX + AX . The "red shift" is AX = Xv /c. This "Doppler shift" goes the otherway (blue shift) fo r approaching stars (see Pamphlet IV).

A lum ino us cloud of gas b etween the stars is called a nebul a. Its spectrumshows emission lines (rather than absorption lines) at the same wavelengthsthat are characteristic of each atom (usu ally hyd rogen , ox yg en, or nitrogen).The emission energy comes from nearby stars whose ultraviolet light isabsorbed by the gas atoms, then reemitted.About 50 years ago, astronomers discovered n onl um ino us gas between thestars that gives "interstellar absorption lines." These ar e l ines due to calciumions (Ca+) and sodium atoms (Na) wh ich have different Doppler shifts fromother lines in the same spectrum. Such "differen t" lin es are caused by thegas in interstellar space between us and the star. It soon became clear that

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Figure 3.1 Tite electromagnetic spectrum and what gets through the Earth's atmosphere.

Atmospheric transparency

1.0

0.5 Ozoneabsorption

j I \\J I IMolecularabsorption

I I I I

Ionosphereref lection

J I I I010'5A 10'4A 10-3A 10'2A 0.1A 1A 10A 100A 1000A 104A 105A IC^A 107A IC^A 109A 1010A 10"A 1012A 10'3A 1014A 1016A

10'15m 10'14m 10"'%! 10'12m 10'11m 0.1nm 1nm 10nm 100nm 1/im 10Mm 100Mm 1mm 10mm 100mm 1m 10m 100m 1km 10km 100km

3 x 1 0

1 11 23 Me V 1 .23 MeV

19 Hz 3x1017 Hz

1

13x1016 Hz

ii1 I

1 2.3 keV 1 23 eV 1 23Photon energy

i i

Wavelength X1 1 I I I

3x1013 Hz 3x10" Hz 3x10 9Hz 3x10 7 H z 3x105Hz 3x10 3HzFrequency t

eV 0.01 eV

Gamma rays X-rays Ultraviolet fVisiblelight

Infrared Radio waves

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space between th e stars is not empty. Radio telescopes detected an abundanceof hydrogen, an d about 30 different molecules have no w been detected ininterstellar space. The strongest hydrogen absorption line , at a wav eleng th of1216 angstroms (121.6 nanometers), is found in the spectrum of every starobserved above the Earth's atmosphere. It is due to hydrogen along the lineof sight between us and the star.

P The Milky WayGalaxyThe Milky Way is a hazy band stretching all the way around the sky where th estars are much more n um erou s than in other parts of the sky. Thi s band is our"inside view" of the Milk y Way Galax y. I t shows us that the Galaxy is a hugedisk filled with stars, gas, an d dust (Fig. 3.2). The Sun and the Earth ar elocated about two-thirds of the distance from th e center to the edge of the d i sk .Wh en we look along the plan e of this disk, we see the band of stars that we callth e Milky Way.

There ar e more than 10 0 billion stars in the Milky W ay Galaxy and analmost equal mass of interstellar gas and dust. Astronomers have learned thisfrom counting th e stars on photographs and by measur ing th e rotation of theGalaxy. They find that the Sun and other nearby stars are mo vi ng at about 250km/sec in enormous orbits around the center of the Galaxy, which is 30 000light-years (2.8 x 10 17 kilometers) away in the direction of the constellationSagittarius (see Pamph let II ). (Sagittarius is a group of bright stars that look asthough they are close together in space; actu ally , they are spread o ut along thearrow in Figure 3.2.) From the size and period of these orbits, one cancalculate that the mass of ma terial (stars, gas, and du st) inside the Sun ' s orbitis about 4 x 10 41 kilograms or 2 X 10 11 solar masses. (B y Newton 's Laws,this is the mass needed to pull the Sun around su ch a large o rbit.) This pictureis confirmed by other galaxies, much farther away, which also have spiralarms, nebulae, and dust clouds in orbit around a nuc le us .

The interstellar gas is not uniform; it is more dense in some places than inothers. There are also clouds of dust, which obviously obscure the visiblel ight from more distant stars and nebulae (see Pamphlet II). This has ledastronomers to a theory of star formatio n. Stars are being formed r igh t now inou r Milky W ay Galaxy and in other galaxies. A cloud of gas and dus t getsconcentrated by chance in interstellar space. Then gravitatio nal force pullsthis cloud tog ether and increases the concentration of matter. As the materialfalls into th e center, th e kinetic energy of impacts among th e particles heatsth e material. When th e star gets to be about th e size of our S u n , th e centraltemperature reaches approxim ately 10 000 000 K , and nu clear reactionsbegin.

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Cross section 100 000 l ight-years

Galact ic plane(galactic latitude =0.

Figure 3.2 Top view andcross section of the Milky WayGalaxy, a gigantic rotating disk ofstars, gas, and dust, about 100 000 light-years in diameter and 5000 light-years thick, with a bulge or halo around the center of mass.

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Stellar EvolutionThe beginning of nuclear reactions in its core is just th e first step in the life of astar. After it settles down to "burning" hydrogen (converting protons tohelium nuclei), it has a long life without much change. The length of this lifedepends on the mass of the star. A giant star 10 or 20 times more massive thanthe Sun "burns" its hydrogen more rapidly and lasts a shorter timeperhapsonly 100 million years. A star the size of our Sun should last 10 billion years ormore, and a smaller star should last even longer.

As th e hydrogen is burned up, the star's core becomes a "dead" mass ofhelium, and the burning hydrogen is in a shell around this dead mass ofhelium, which grows ever larger. In a giant star, there is finally a collapse,then a huge "supernova explosion," wh ich blows most of the star's mass ou tinto an expand ing ne bula called a "supernova remnant" (see Pamphlet II).The core remains as a compact, high-de nsity Neutron Star. Before and duringth e explosion, a new set of nuclear reactions begins, "burning" th e helium"ashes" of the earlier hydrogen burning.

If th e original star was smallermore like our Sun in massit may gothrough a pulsating stage of expansion an d contraction or explode lessviolently. A less violent explosion leaves a White Dwarf star, whic h iscompact but less dense than a Neu tron Star. The White Dwarf is initially ho tan d cools very slowly fo r man y bil l ions of years. Man y White Dwarfs and afew Ne utron Stars have been observed by astronomers, an d there are probablymany more as yet undiscovered because they are so faint. Grav itat ional forceslowly takes over as such stars cool.How does a star die? The end of a star's life is not fully understood;however, it is theorized that, as a White Dwarf or a Neutron Star cools,gravitational force begins to take over. With no gas pressure to keep them"puffed up," grav itationa l force squeezes s uch stars into Black Holes withspace so curved aroun d the m that n either particles nor light can get in or out(see Pamphlet II).

The Extreme Ultraviolet Survey,Experiment MA-083By 1974, gamma-ray, x-ray, an d far-ultraviolet observations had been madefrom rockets and spacecraft above the Earth's atmosphere. Some of these aredescribed in Pamphlet II. The extreme ultraviolet (EU V) rays ar e betweenx-rays and far-ultraviolet rays at wavelengths from 10 to 1000 angstroms (1 to100 nanometers) (Fig. 3.1). Astronom ers had little hope of detecting EUV

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Figure 3.3 Front view of the EUV elescope after assembly at the University of Californiaat Berkeley (University of California photograph).

from stars because interstellar hydrogen strongly absorbs al l wavelengthsfrom 912 down to about 20 angstroms (91 .2 to 2 nanometers) . (The abun danthydrogen atom absorbs these wave lengths wh en it is ionized .) How ever, therewas evidence of gaps in the hydrogen clouds or of very low hydroge n densityin some directions where EUV photons might get through to Earth. The EUVmeasurements would fill in the complete electromagnetic spectrum an d mightlead to discoveries of some superhot stars.The need was to build a very sensitive EUV telescope that could detect faintEUV radiation from hot stars that were shrouded by interstellar hyd roge n.Stuart B owy er, the Principal Inv estiga tor at the Un iver sity of California atBerkeley, planned the use of the EUV telescope shown in Figure 3.3.BecauseEUV photons behave like x-rays, th e telescope could not be a lens or aconcave mirror as in an optical telescope. Instea d, it was a series of four ringmirrors shaped so that a beam of incoming EUV photons would be reflected toa common focus. The aperture (front opening) was 94 centimeters (about 3feet) in d iameter , and the effective area was about 10 square centimeters(because of reflection and filter transmission losses). Each ring was coatedwith a thin layer of gold to increase th e reflection of EUV photons at highangles of incidence.

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Figures 3.4 an d 3.5 show how the telescope focused EU V photons througha filter onto a channeltron detector using a special semiconductor ceramic as aphotocathode. The EUV photons released photoelectrons from th e photo-cathode, an d each electron was sucked into th e channeltron tube by about 100volts. The inside surface of the curved channeltron tube was coated with amaterial that released two or more electrons whenever an electron hit it.Therefore, about 108 electrons came out of the positively charged (3000 volt)end for each photoelectron th at entered. This short pulse was amplified andcounted. After 0.1 second, th e n u mb er of counts was telemetered to theMission Control Center (MCC) at the NASA Lyndon B. Johnson SpaceCenter (JSC) in Houston, where it was recorded on magnetic tape togetherwith th e time.

The telescope-channeltron combination had a field of view of 2.5 in thesky; that is, it counted EUV photons from all sources in a 2.5 circle. Thetelescope was mounted in the side of the Apollo Service Module an d could beaimed at any selected p oint in the sky by rollin g and turn ing the spacecraft (seePamphlet I). The pointing direction was radioed to JSC so that the scientistscould tell where the EUV telescope was pointed every second that it was inuse.To get an EUV spectrum, five filters were used. These filter s were locatedon a wheel in front of the detector (Fig. 3.5). Each filter was a thin film ofmaterial that let through only a selected band of wavelengths or photons ofselected energies, as shown in Table 3.1.

A six th "opaque" filter was used to check the background count, the falsecounts due to defects in the detector and electronics. These false countsamounted to 0.6 count/sec, which w as later subtracted from th e other EU Vphoton co unts recorded at JSC. The astronauts pointed the EUV telescope byrol l ing and turning Apollo, then they switched the telescope on. The filterwheel au tomatica lly placed each filter in front of the detector fo r 0.6 second,then moved to the next filter. Radio signals to JSC indicated whic h filter wasin place. The EUV telescope was pointed at a star for a few minutes, thenpointed 3 off the star fo r another fe w min u tes to get the sky background. If thestar was em itting EUV photons, they were measured by dete rmin ing thedifference between th e star and the background, as shown in Figures 3.6 and3.7.

The sensit ivity of each f i lter-telescope combination w as kno wn from pre-flight laboratory tests. The count rate for a star thus could be converted to thenumber of EUV photons arr iving each second. This calculation was doneseparately for each filter, so we know the number of 9-electronvolt photonsarriving each second, the nu mb er of 21-electronvolt photons, 46-electronvoltphotons, and so on up throu gh the values l isted in Table 3. 1. These numbersmake up the EUV spectrum of each star observed.

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Figure 3.4 Schematic diagramof EUV telescope focusing by the glancing-incidence mir-rors. The channeltron detector is also shown.

The EUV telescope was pointed at 30 different stars, 8 of them more thanonce, and at man y parts of the sky backg round for a total operating tim e of 40hours. Positive results were obtained on four of the stars, and the EUVbackground was measured over about one-third of the sky.

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Diagram of the MA-083 EUV elescope. Figure 3.5

Results of the EUV Survey ExperimentThe spectrum of a star in the constellation Coma Berenices called HZ 43is shown in Figure 3.8. (It is numbe r 43 in a list of White Dwarf stars pub-lished by Milton Humason an d Fritz Zwicky in 1950.) The peak intensity is ata photon energy of 46 electronvolts or a wavelength km of 270 angstroms(2 7 nanometers), approximately. Using th e color formula in Table 2.1,wecan estimate the surface temperature as T = 28 970 000/270 = 107 000 K.

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Table 3.1 MA-083 EUV Filters

Filter material Wavelengths admitted, AverageA (nm) photon energy,

eV

Parylene N 55 to 150 (5.5 to 15.0) 14 2Beryllium/Parylene 114 to 150 ( 1 1 .4 to 15.0) 10 0A l umi num plus carbon 170 to 620 (17.0 to 62.0) 46Tin 500 to 780 (50.0 to 78.0) 21Barium fluoride 1350 to 1540 (135.0 to 154.0) 9Opaque None 0

This is a very high temperature for the photosphere of a star. Other astron-omers measured the distance to HZ 43 as 200 li ght-y ears and show ed that it isa White Dwarf star of about 5000-kilometer radius (I /1 40 th th e size of theSun). The MA-083 Experime nt EUV measuremen ts show that it is the hottestWhite Dwarf known. (Measurements of the visual spectrum from ground-based observatories could no t show t his because the spectrum of a 100 000 Kstar is almost th e same as the spectrum of a 50 000 K star in the intervalfrom 3000 to 10 000 angstroms (300 to 1000 nano me ters) measured from theEarth's surface. See Figs. 2.3 and 3.1 . )

Three other stars were detected b y the EUV telescope. In each of these fourcases, there is, by chance, very little interstellar hydrogen along the line ofsight. We happen to see these four stars through gaps in the interstellarhydrogen clouds. In fact, th e combination of visual observations, EUVobservations, and x-ray observations (Pamphlet II) gives an estimate of thenumber of hydrogen atoms along the l ine of sightapproximately 0.02atom/cm 3 for HZ 43. Another W hite Dwarf, Feige 24 in the constella tionCetus, was found to be almost as hot as HZ 43; the hydrogen density in itsdirection is approximately 0.01 atom/cm3.

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EUV telescope pointings for a star observation. The telescope beam is 2.5in Figure 3.6diameter. There may be faint stars in the background of each pointing.

Pointed at star

Typical EUV count rates through one filter. Figure 3.7

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1010 0

10

01

0.011000

Photon energy, eV100 1000

I I II I I IAluminum plus carbon f ilter

Beryllium f ilter

Parylene N filter

Soft X-Ray Experiment

100Wavelength, A

10

100

10

0 1

00 1

Figure 3.8 EUV spectrum of HZ 43, a White Dwarf star in the constellation ComaBerenices.

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The third star detected was Proxima Centauri, the closest know n star to theSun , 4 light-years away . Proxima, in the constellation Centau rus, is very faintand cool (3000 K), so it is surprising that it emits 142-electronvolt photons ofwavelengths from 55 to 150 angstroms (5.5 to 15 nanometers), the mostsensitive EU V filter-detector combina t ion. The star was not detected during asecond pointing, which indicates that the EUV photons are emitted byProxima in bursts or "flares." Vis ible - l ight flares from Proxima, lasting lessthan an hour, had previously been observed by ground-based astronomers.The flares are thought to be due to some kind of explosion, possibly caused bynuclear reactions in a small region of the star's photosphere. Such explosionsmore than double the star's visual brightness for a short time, and the EUVmeasurements showed that temperatures in the Proxima flare were about 20t imes th e star's surface temperature.Another explosion was observed in a star called SS Cygni . The EUVtelescope was pointed at that star in the constellation Cygnus after otherastronomers telephoned the MA-083 scientists at JSC to say that SS Cygni hadsuddenly increased to 40 t imes its normal brightness on Ju ly 17 , 1975. (Theywere mo nitoring SS Cy gni because it is a "recurrent nova," exploding likethis at un predictable times.) Extreme-ultraviolet photons (142 electronvolts)were detected on July 20, bu t they could barely be detected 15 hours later andcould no t be detected at all on the third look 22 hou rs later. Wh en all the visualobservations of SS Cygni are collected and analyzed, it may be possible tocombine them with the EUV intensities to plot the history of a nova explosion.

The EUV telescope was pointed at 26 other stars. Most of them were W hiteDw arfs not very far aw ay, and all of them were expected to emit EUV li gh t.Non e of these stars were detected. From the kno wn sens itivity of the EUVfilter-detector combinations, this means that fewer than 50 EUV photonsfrom those stars enter a 1-square-m eter aperture each second. T his infor ma-tion is useful: it sets l imits on the temperatures of the stars and on thehydrogen density along the line of sight.The EU V sky-backgro und measurements for each of the observed starsalso give useful inform ation. These measurements average about 2 counts/secan d are approximately the same over one-third of the sky. No one yet knowswhere these EUV photons are coming f rom. They m ust originate nearby,because interstellar hydrogen absorbs EUV photons so effectively.

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F Questions for Discussion(Spectra, lonization, Temperature)10. If you plot th e n u mb er of photons at each energy (o r wavelength)

instead of the intensity in Figure 2.3, how would the curves be changed?11. The Sun's spectrum shows many absorption l ines of iron (Fe)and the

iron io n Fe+. The Fe atom is ionized to Fe+ by 7.83 electronvolts; that is ,collisions of 7.83-electronvolt energy knock one electron from the Fe atom.The Fe+ ion has a different set of absorption l ines. Explain how Fe and Fe+lines look in the spectra of stars cooler than th e Sun.

12. In a double star, one star orbits around anotherfirst approaching us,then receding. Suppose that such a pair of stars is on the other side of aninterstellar gas cloud. Both stars and the cloud contain sod ium . Describe howth e sodium absorption lines in the spectrum of one of the stars chang e durin git s orbit.

13. Hydrogen a toms ar e ionized by 13.53 elect ronvol ts . A 13.53-electronvolt photon (wavelength of 912 angstroms, or 91.2 nanometers)separates the electron from the proton in a hydrogen atom. What does a14-electronvolt photon (w avelen gth of 880 angstroms, or 88 nanometers) doto a hydrogen atom?

14. If a galaxy (l ike the M ilky Way Galax y in Figure 3.2) were not rotating,what differences would you expect?15. As hydrogen "burns" to hel ium in the core of a star, what happens toth e densi ty there?16. Wh ich is probably older, a cool or a hot White Dwarf star?17. How could the EUV telescope detect separately tw o stars less than

2.5 apart?18. If interstellar hydrogen were affecting the EUV spectrum of HZ 43 in

Figure 3.8, wo u ld th e actual temperature of that star be higher or lower than107 000 K?19. How do astronomers estimate th e radius of a White Dwarf like HZ 43?

(Hint: The brigh tness form ula (Table 2.1 ) gives the energ y radiated per squaremeter of surface area.)

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4 HowMuch Heliumin Space?

The Discovery of Helium; Its SpectrumThe element helium, an inert gas, was discovered in the Sun about 100 yearsago. (Its name is derived from Helios, th e Greek word fo r Su n . ) The discoverywas made from th e absorption lines of hel ium in the Sun ' s spectrum. Theselines did not match th e wavelengths of hydrogen, carbon, oxygen, iron, an dother elements measured in the laboratory. In 1895, helium gas was identifiedin natural gas on Earth.

Helium absorbs and emits several lines at visual wav elen gths . How ever, itsstrongest line, th e most sensitive detector of hel ium, is at 584 angstroms(58.4 nanometers) in the E U V . The helium ion He+ has a different set oflines, with th e strongest at 304 angstroms (30.4 nanometers) , also in theEU V. If you want to detect he lium or ionized heliu m in space, look for thesetwo lines. That is what was done in the Helium -Glow Experiment (MA-088)on the Apollo-Soyuz mission.

Formation of the ElementsWhy is the amou nt of helium in space important? The a nsw er to this questioncomes from a theory of how the 92 chemical elements we know were formeddur ing the 15-billion-year history of the unive rse. One idea is that the unive rsestarted as pure hydrogen and that helium (and all the other elements) wereformed by nuclear reactions in the cores of stars (Sec. 3C). A more recenttheory states that there was 25-percent helium at the beginning, when the"Big Bang" explosion caused all the matter to fly outward and form thegalaxies (and later th e stars in those galaxies) that we see today. Measuringtoday's ratio of helium to hydrogen in interstellar space may help to decidewhich of these two theories about how the universe started is correct.

A fraction of the original hydrogen present in the universe condensed intostars. In these stars, just as in the Sun, hydrogen was converted to helium. Inth e case of larger stars, nuclear reactions led to the formation of elements evenheavier than heli um , such as carbon, nitrogen, oxyg en, and iron. Eventual ly ,th e stars blew up in supernova explosions. These explosions released th eheavy elements formed in the core to interstellar space as gases. These gasesthen mixed wit h the original hydrogen (or hydrogen and h elium ) to form asecond generation of stars. These new stars then continued the manufacture ofnew elem ents. There m ay have been three or four such generations. It is thu sof interest to determine the proportion of heavier elements present in theuniverse and to match these proportions to those predicted by theory.

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c

Astronomers estimate the relative am oun ts of eleme nts in stars from thestrengths of thei r absorption lines in the spectrum of each star. In every case,hydrogen is most abundant. The Earth lost most of its atmospheric hydrogendurin g its early stages, but geologists can estimate wh at fraction of the Earth'smass is oxygen, si l icon, magnesium, iron, nickel, and so on. All theseobservations have been combined into a single table of the abundances ofelements, from hydrogen (atomic num ber 1) to uranium (atomic num ber 92).However, helium is probably more abundant in the cores of stars than on the irsurfaces. Thus, th e place to get the correct h elium/hydrog en abundance of theoriginal universe is in interstellar space.

Interstellar Gas Moving Throughthe Solar SystemBecause the Sun is mo ving at about 15 km/sec thro ugh the interstellar gas,there is an "interstellar wind" blowing into the solar system at 15 km/secfrom the forward direction (the direction of the cons tellation Cy gn us) . Someof the Sun's light is absorbed and reemitted by this gas in emission lines ofwavelengths of 304 angstroms (30.4 nanometers) fo r helium ions and 584angstroms (58.4 nanometers) for hel ium. These wav eleng ths are changed bythe Doppler shifta blue shift if we look i nto the win d or a red shift if we lookdownwind (see Pamphlet IV). The shift is AX = \v / c = 584 (15/300 000)angstroms = 0.03 angstrom (0.003 nanometer) for the 584-angstrom l ine ofhelium. The 30-km/sec velocity of the Earth in its orbit around the Sun canchange this Doppler shift, but Figure 4.1 shows that the Earth was movingacross the line of sight in July 1975, giving no additional Doppler shift. TheMA-088 scientists used th e 0.03-angstrom re d shift to separate interstellarhelium glow from a local glow of helium in the outer part of the Earth'satmosphere which was also stimulated by sunlig ht. (The local heliu m glowhad been photographed by a far-ultraviolet telescope on the Moon during theApollo 16 mission in 1972. It ext end s more than 5000 kilometers above th eEarth's surface.)

The expected direction of the in terstella r w in d, the positions of the Earth,and the Earth's orbital velocity around the Sun on July 20, 1975, wh en theMA-088 meas uremen ts were made are shown in Figure 4. 1. D ow nw ind fromthe Sun , there should be a co ncentration of the interstellar gas due to the S un'sgravitat ional attraction as the gas sweeps by. This "tail" w as viewed fromApollo-Soyuz throu gh the local Earth glow. The interstella r glow w asred-shifted 0.03 angstrom, whereas th e local Earth glow had no blue shift orre d shift (because Apollo-Soyuz was mov ing horizontally, neither toward the

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Local heliumglow

MA-088 view of red-shifted helium glow downwind from the Sun. Figure 4.1

local hel ium nor away from it). The MA-088 scientists canceled th e localglow by placing in front of the MA-088 detector a 10-centimeter-long tube ofhelium gas that absorbed all the local hel ium glow but allowed the red-s hiftedinterstellar helium glow from the Sun's tail to enter.

n The MA-088 Helium-Glow ExperimentThe helium -glow detector was similar to the one used for the MA -083 EU Vtelescope. There were four channeltrons with ceramic semiconductor photo-cathodes. Two of them were located behind he lium gas in tubes 10 centi-meters long at a pressure of 0.013N/cm 2 and a temperature of 300 K. Theends of the tubes were sealed with thin (300-nanometer) metal foil ( t in anda lu min u m) whic h served as f i l ters that were transparent to waveleng thsbetween 530 and 640 angstroms (53 and 64 nanometers) . Two other detectorswere located behind aluminum-foil filters covered with th in layers of carbon.These f i l ters transm itted 170 to 450 angstroms (17 to 45 nanometers) , includ-in g the 304-angstrom line of hel ium ions.

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Figure 4.2 shows th e MA-088 Experiment detector, a 35-centimeter cubethat weighed 23 kilograms. The detector was mounted on the side of theApollo Service Module (see Pamphlet I) near the EUV telescope and theMA-048 Soft X-Ray Experiment telescope (see Pamphlet II). The four largeholes (3.5-centimeter diameter) ar e openings for the four helium-g low detec-tors. The hole size, and the shields inside, limited the view of each detector toa 15 circle in the sky. All four detectors were pointed in the same direction.The two de tec tors w ith ou t he l iu m ce l ls recorded the 304-angs tromwavelength , both local and in terstellar ionized helium glow. The two detec-tors behind the helium cells recorded only the interstellar 584-angstromhelium glow when th e cells were filled with he l ium. The two cells werealternately filled for 5 seconds and then evacuated for 5 seconds. O ne cell wasfull while th e other was e mpty . The automatic filling was monitored by apressure gage, and records of the pressure and the tempe rature of the heli umgas in both cells were transmitted to JSC. When the cell was empty, thedetector recorded both local and interstellar helium glow at the 584-angstromwavelength . Whe n th e cell was full, it recorded only th e interstellar heliu mglow.

The electronics used to read the EUV counts from each detector every 0.1second are shown in Figure 4.3. The top two detectors without a he l iumabsorption cell are the 304-angstrom hel ium detectors . The schem atic "col-limator" l imits the view to 15. The ch anneltro n has 3000 volts from thehigh-voltage power supply (HVPS) on the output end, and this leads th e100-million-electron pulses to an amplif ie r and c oun t ing c i r c u i t . The"counts" accu mu lated each 0.1 second were compressed in analog form toreduce telemetry to JSC through the NASA Spacecraft Tracking and DataNetwork (ST DN) of radio stations. The lower tw o detectors "looked out"through helium absorption cells , and the gas filling-evacuating e qu ipme nt isshown schem atically. Otherwise, th e counting and te lemetry is the same asfo r the top two detectors.Telemetry received at JSC consisted of the fol lowing data fo r each 0.1second of t ime that th e MA-088 Exper iment was in operation: th e n u m b e r ofcounts recorded in each of the four detectors, th e helium pressure in each ofthe_two absorption cells, th e helium temperature in each of the two absorptioncells, an d "housekeeping" pressures and voltages. All these data had to becorrelated with records of the spacecraft attitude; that is , with th e direction inwhich th e heliu m- glo w detectors were pointin g, second by second. TheMA-088 Exper iment was on for all the MA-083 EUV Survey (Sec. 3) and

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The helium-glow detector. Thefour large holes are the viewports for the four Figure 4.2photometers.

MA-048 Soft X-Ray (Pamphlet II) pointings. Slow rolls of the Apollospacecraft were made so that the MA-088 helium-glow detectors sweptaround the entire sky and collected counts from almost all parts of the sky . The"tail of interstellar wind" behind the Sun (Fig. 4.1) w as scanned severaltimes. The direction of the Earth's shadow was also scanned because localhelium glow is small there.

ORIGINAL?**OF POOR37


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Figure 4.3 Schematic diagram of the MA-O88 electronics system.

D-A digital to analog


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Results of the Helium-Glow ExperimentThe MA-088 Experiment was operated on Apollo-Soyuz for a total of 10hours, wh ich includ es most of the time allocated for the MA-048 (Soft X-R ay)an d MA-083 (EUVSurvey) Experiments. The counts were recorded every0.1 second; therefore, 360 000 readings were received from each of the fourdetectors. The readings for each sky position were averaged, and correctionswere made fo r atmospheric absorption with a large computer. The pointingdirection was computed for each reading so that intensities could be plotted ona map of the sky. Basically, there are three maps: (1) the sum of local andDoppler-shif ted helium 584-angstrom glow, (2) the Doppler-shifted 584-angstrom glow from interstellar heliu m on ly, and (3) the sum of local andDoppler-shifted helium 304-angstrom glow. A fourth map was made bysubtracting map (2) from map (1) to get the local helium 584-angstrom glowonly . This fourth map fits the idea of the "geocorona," a glowing sphere ofhydrogen and helium around the Earth, about 100 000 kilometers in diameteran d wi th a "dimple" where th e Earth's shadow is (opposite th e Sun) (Fig.4.1). The MA-088 measurements show that th e local geocorona glow ac -counts for about 30 percent of the helium 584-angstrom intens ity in thenighttime sky.Although there are some narrow gaps, MA-088 observations cover theentire sky and show 10 to 50 counts/sec in the 584-angstrom readings. Map (2)of the Doppler-shifted 584-angstrom glow shows "patterns" around the skythat agree roughly with Figure 4.1, although th e Sun's ta il was not clearlydetected. The temperature of the interstellar gas should affect these patterns:th e higher the temperature, the more fuzzy the patterns should be. There areno sudden changes in the 584-angstrom intensity where the gas is at hightemperature, because random thermal motions of helium atoms ar e added tothe interstellar-win d velocity and "fuzz" the expected gas motions andDopp ler shi fts. By detailed stu dy of the fuzziness, the scientis ts hope to getthe temperature of the gas as well as the amount of helium.

The he lium 304-angstrom map (3) shows the glow of the local geocoronawell; it also shows the "tunnel" through it along the Earth's shadow (Fig.4.1). The scientists are stu dy ing the sharpness of this tu nn el to find outwhether it is an actual tunn el or jus t a dimple. They wan t to see whether thereis any heli um 304-angstrom light comin g in from outside th e geocorona. Thisis not easy because the tunnel edges are fuzzy; nevertheless, the investigatorsare con tinui ng their s tudies of the MA-088 helium-glow measuremen ts.

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F Questions for Discussion(Abundances of Elements, Doppler Shifts)20. If you go back in t ime to wh en the first stars were being formed in ourMilky Way Galaxy, would you find any carbon, nitrogen, or oxygen in

those stars? If one of the stars ha d planets, could life have evolved on anyof these planets?

21. It is stated in Section 4B that "the Earth lost most of its atmospherichydrogen during it s early stages." What major reservoir of hydrogen do westill have on Earth?

22. How do astronomers know that the Sun is moving at about 15 km/secthrough th e interstellar gas?

23. The Earth goes around the Sun in 12 months. If Apollo-Soyuz ha dwanted to get the maximum Doppler shift between the local helium glow andthe interstellar helium glow, in what month should the MA-088 measure-ments have been made? (See Fig. 4.1.)

24. The Apollo-Soyuz spacecraft orbited th e Earth once every 93 minutesat an altitude of 222 kilometers. Th e radius of the Earth is 6378 kilometers.What would be the Doppler shift of the 584-angstrom helium line viewedstraight ahead or straight back along Apollo's orbit?

25. What angles to the MA-088 line of sight (Fig. 4.1) would you need toknow to compute the Doppler shift of the 584-angstrom-wavelength inter-stellar w ind? Of the local heliu m glow?

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Appendix A

Discussion Topics (Answers to Questions)1. (Sec. 2F) The observed brightness b of a distant light source is pro-

portional to its lumin osity (power out pu t) L divided by the square of itsdistance (L/r2). If L is the same for the star and the Sun (whic h is not alwaysth e case), bsun/^star = '"2star^'"2Su n = 'O 10 .Therefore, the distance rstar =105 rsun for the brightest star. Fainter stars ar e more distant.

2. (Sec. 2F) The core of the Sun contains about 8 x 1029 kilograms ofmatter. If it was originally al l hydrogen and if 6 x 10 n kilograms ofhydrogen ar e converted to helium every second, it would take 8 x 1029/6 x 1011 = 1.3 x 1018 seconds, or 4.2 x 1010 years, to use up all thehydrogen. Before these 42 billion years had passed, the shr inking corewould change the Sun's structure. The Sun is estimated to be 5 billion yearsold now.

3. (Sec. 2F) The brightness formula in Table 2.1 shows that each squaremeter radiates power proportional to T 4. The Sun has a surface temperatureof about 6000 K. A star of the same size (same surface area) at 30 000 Kwould radiate (3 0 000/6000)4 = 54 or 625 t imes as much power as theSu n , most of it in mu ch shorter (ultraviolet) wa ve le ng ths . (Ac tua l ly , ho t bluestars are generally much larger than th eSun.)

4. (Sec. 2F) The brief spectrum of the chromosphere jus t before an d jus tafter totality is called th e "flash spectrum." I t consists almost entirely ofemission lines because th e chromosphere is a transparent ho t gas.

5. (Sec. 2F) The charged particles (ions) from a flare follow the magnetic-field l ines as they move away from th e Sun.Along this fl ightpath, th e ionscollide with other ions an d a toms in the corona, and the collis ions "excite"these particles to emit l ight . The flare particles thus make long, brightstreamers in the corona. Some streamers can be seen in Figure 2.4.

6. (Sec. 2F) Because th e Sun's corona is so hot,it em its x-rays. Therefore,the x-ray photograph s show the Sun to be as large as the corona, about threet imes larger (1.5 across or 4 million kilom eters in diameter) th an the visibleSun.

7. (Sec. 2F) The advantage was that the sky background from 222 kilo-meters above th e Earth's surface is greatly reduced because there are noclouds or haze. Thus, the faint inner corona is not swam ped with other l ight .The major disadvantage was the contam inatio n around Apo llo. Other disad-vantages that could have been avoided were th e cabin light reflected off theSoyuz window and the inferior lens used in the Soviet camera.

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8. (Sec. 2F) A cone of black paper or cloth that fitted tightly against th ewindow could have been used to prevent cabin light from reflecting off thewindow an d into th e camera.

9. (Sec. 2F) There is probably more material in the outer corona thanpreviously thought. This material may be dust or very lo w density gas.

10. (Sec. 3F) Photon energy E = hf = heIk is larger for short-wavelengthphotons than fo r long-wavelength photons. The curves in Figure 2.3 thuswould be lower at the left (short \) and higher at the right (long X ). Actually,the interval AE = (hc/k

2) AX affects this answer. The intensity /\ is theenergy arriving each second in waves of length X to X + AX. The photon

count N is the number of photons arriving each second wit h energies be-tween E and E + AE.11 . (Sec. 3F) The Fe lines would be stronger and the Fe+ lines weaker than

in th e Sun ' s spectrum. At lower temperature, there are fewer collisions ofenerg y greater tha n 7.83 electron volts because th e energy of particlemotion E = lh. mv2 = kT, where k is the Boltzm ann con stant (see Table 2 .1).

12 . (Sec. 3F ) Sodium in the interstellar ga s cloud absorbs l ight at 5890angstroms (589 nanometers) plus a constant Doppler shift AX = 5890 v,7c,where Vj is the gas-cloud recession (or approach) velocity an dc is the velocityof light. As the star approaches us, its sodium line would have a blue shift:AX = 5890 V y / c , where v s is the star's orbital veloc ity. When th e star stopsapproaching, AX = 0 ; when it is receding, A X = 5890 vs/c . The star'sabsorption line thus shifts back and forth, while the line of the interstellargas cloud remains at a fixed wavelength .

13 . (Sec. 3F ) Wh en a hydrogen atom absorbs 880 angstroms, whichrepresents a photon of higher energy than needed to ionize it, the electron isshot of f with kinetic energy equal to the excess: 14.0 13.5 = O.Selectronvolt .

14 . (Sec. 3F) A nonrotating galaxy should be spherical with no spiralarms. Stars would be orbiting in all different directions around the center ofmass, and interstellar gas would fall into that center. Such galaxies areobserved fa r from us and are called EO galaxies.

15. (Sec. 3F) After th e core temperature settles down in equi l ib r ium withradiation, the density rises because alpha particles (H e ++ ) are about four t imesthe mass of protons (H + ) , and the same number ar e packed into a cubiccentimeter .

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16 . (Sec. 3F) The cool White Dwarf is probably older. White Dwarfs allstart at about the same temperature an d cool very slowly.17 . (Sec. 3F) By sweeping across the two stars slowly, the EUV telescope(MA-083) would first count photons from the first star, then from the twotogether, an d then from the second star alone. From the times whe n the cou ntrate increased and decreased, one could tell approximately wh en the edge ofthe EUV telescope field of view crossed each star.18 . (Sec. 3F) The interstellar hydrogen would be absorbing more of theHZ 43 intens ity on the left side of Figure 3.8 (longer wavelengths) than onthe right side (shorter wavelengths). Therefore, the unaffected spectrum

would peak farther to the left (longer wavelength) and the temperatureT = 28 900 000/Xm would be lower.19. (Sec. 3F) The size of HZ 43 can be estimated from the brightness

formula in Table 2.1;that is, E t =aT* joules pe r second radiated at all wave-lengths from 1 square meter of its surface. If the distance r to HZ 43 can beestimated, the measured intens ity / (energy received in all wa velen gthsarriving on 1 square meter) can be multiplied by 4nr2 to get the total energyradiated. This is the area of the star's surface times^. So , 4irr*I = 4TrR 2aT 4and R = V//CT (rlT 2), where R is the star's radius.

20 . (Sec. 4F) The first stars in the Milky Way Galaxy are thought to haveformed from pure hydrogen or from hydrogen with a low percentage ofhelium. There were no heavier elements such as carbon, nitrogen, andoxyge n. W itho ut heavier elemen ts, solid planets probably could n't form, an dwithout carbon, nitrogen, and oxygen, no living organisms could haveevolved.

21. (Sec. 4F ) Water is the largest reservoir of hydrogen on Earth.There is a small amount of hydrogen gas high in the atmosphere andmore in oil and hydrocarbon gas deposits underground. Another largereservoir is water that is chemically locked in the rocks deep under theEarth's surface.

22 . (Sec. 4F) The spectra of many stars, al l around the sky, show theabsorption lines of the interstellar gas, primarily ionized calcium (Ca+) an dsodium (N a). Doppler shifts show an average blue shift in one half of the skyand an average red shift in the other half. Averages fo r various directionsin these halves of the sky and for nearby stars only show the maximum

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Doppler shifts from local interstellar wind coming from the direction ofthe constellation Cy gnu s (right ascension 21, declination +40; seePamphlet II).

23. (Sec. 4F) When the Earth's orbital velocity is almost directly intothe interstellar wind, the relative velocity of wind past the Earth is highest.In Figure 4.1, the Earth's orbital velocity (a vector) is about 60 below theupwind direction (toward the right). Because that vector turns 360 in 12months, or 30 per month, it wou ld have been as nearly upwind as possibletw o months earlier; that is, in May 1975. (Note: The vectors are not in oneplane.)

24. (Sec. 4F) The distance around the Apollo-Soyuz orbit is 2-rrr, wherer = 6378 + 222 = 6600 kilometers. Thus, the spacecraft's orbital speedv = 2-irrlT = 27r(6600)/93(60) = 41 470 kilometers/5580 seconds =7.43 km /sec relative to Earth. View ed straight ahead, the 584-angstrom linefrom th e local heliu m glow (fixed relative to Earth) would be blue shifted by584(v/c) = 584(7.43/300 000) = 0.014 angstrom. Straight back, it would bere d shifted by the same amount.

25 . (Sec. 4F) The Doppler shift is proportional to the component of therelative vector velocity along the line of sight. The Earth has one smallcomponent of its 30-km/sec orbital velocity, and the interstellar wind in the"solar tail" ha s another large comp onent. In each case, th e component alongth e line of sight is v cos 0, where 0 is the angle between th e vector v andthe line of sight. So 0 l and 02 are needed, where B l is the ang le between theEarth's orbital velocity and the line of sight and 6 2 is the angle between thesolar-tail velocity and the line of sight. The orbital velocity vs of the Apollo-Soyuz spacecraft around the Earth adds a third, very small, component v scos 0 3, where 93 is the angle between v s and the line of sight.The local helium glow is at rest relative to the Earth. The component ofV y along th e line of sight v s cos 63 is also needed to get the Doppler shift of thelocal helium glow.

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Appendix BSI UnitsPowers of 10

International System (SI) UnitsNam es, sym bols, and conversion fac tors of SI un its used in these pamp hlets:Quantity Name of unit Symbol Conversion factor

Distance meter m km =0.621 milem = 3.28 ftcm = 0.394 in .mm = 0.039 in.Mm = 3.9ox 10'5 in. = 104 Anm = 10 A

Mass kilogram kg 1 tonne = 1.102 tonskg = 2.20 Ibgm = 0.0022 Ib = 0.035 ozmg = 2.20x 10-6Ib = 3.5 x 10'5 ozTime second sec yr = 3.156x 10 7 secday = 8.64 x 10 4 sechr = 3600 secTemperature

AreaVolumeFrequency

Density

Speed, velocity

Force

kelvin

square metercubic meterhertz

kilogram pe rcubic meter

meter per second

newton

K

m 2m 3Hz

kg/m 3

m/sec

N

273 K = 0 C = 32 F373 K = 100 C = 212 F1 m 2 = 10" cm 2 = 10.8 ft21 m 3 = 10 6 cm 3 = 35 ft31 Hz = 1 cycle/sec1 kHz = 1000 cycles/sec1 MHz = 106 cycles/sec1 kg/m 3 = 0.001gm/cm 31 gm/cm 3 = density of water1 m/sec = 3.28 ft/sec1 km/sec = 2240 mi/hr1 N = 105 dynes = 0.224 M

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Quantity

Pressure

EnergyPhoton energyPowerAtomic mass

Customary UnitsQuantity

Wavelength oflightAccelerationof gravity

Name of unitnewton per squaremeterjouleelectronvoltwattatomic mass unit

Used With the SI UnitsName of unit

angstrom

g

Symbol Conversion factorN/m 2 1 N /m 2 = 1.45 x 10'4 lb/in2

J 1 J = 0.239 calorieeV 1 eV = 1.60 x lO '19 J; 1 J = 10 7 er gW 1 W = 1 J/sec

am u 1 amu = 1.66 x lO'27 kg

Symbol Conversion factor

A 1 A = 0.1 nm = lO'10 mg 1 g = 9.8 m/sec2

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Unit PrefixesPrefix

teragigamegakilohectocentimillimicronanopico

Abbreviation

TGMkhcmMnP

Factor by which unitismultiplied

1012109

106103102

i o - 2i o - 3i o - 8i o - 9io-lz

Powers of 10Increasing Decreasing102 = 10010

3= 1 000

1 0 < = 10 000, etc.Examples:2 x IO 6 = 2 000 0002 x IO 30 = 2 followed by 30 zeros

i o - 2 = 1 / 1 0 0 = o.oii o -

3= 1 / 1 0 0 0 = o . o o iIO-4 = 1/10000= 0.000 l.etc.

Example:5.67 x 10'5 = 0.000 056 7

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Appendix C

GlossaryReferences to sections, Appendix A (answers to questions), tables, andfigures are included in the entries. Those in italic type are the most helpful.absorption line a gap or dip in a spectrum, caused by the absorption of

specific wavelengths by a gas between the l ight source and the observer.(Sees. 2B, 3A, 3F, 4A, 4B; App. A, nos. 11, 12, 22) See Project Physics,Sec. 19.1.

angstrom (A) a uni t of wave length used by physicists for more than 80 years;1 angstrom = 10 ~' meter or 0.1 nanometer . (Sec. 3A)

Apollo spacecraft a three-m an spacecraft originally designed for trips to theMoon. It consisted of a Command Module (CM) attached to a ServiceModule (SM). For the Apollo-Soyuz mission, a Docking Module (DM)was attached to the CM.

background a uniform intensity over a large region of the sky . (Sees. 3D, 3E;App. A, no. 7; Figs. 3.6, 3.7) "Instrumental background" (Sec. 3D) is thecount rate due to instrument defects when no photons are entering thedetector.

"Big-Bang" cosmology th e theory of the origin of the univers e wh ich statesthat th e universe was created about 15 billion years ag o when a giantexplosion emitted high-frequency radiation and pushed matter apart. Thatmatter is now condensed into stars in galaxies wh ich ar e receding from th eEarth. The more dista nt ones are receding faster because th ey started fasterin the Big Bang. (Sec. 4B)

channeltron a curved vacu um tube that gives a 100 -million-electron outpu tpulse fo r each electron input . (Sees. 3D, 4D; Figs. 3.4, 4.3)constellation a group of stars, such as the Big Dipper, that defines an area

(generally 10 or 20 across) in the sky . Most of the constellatio ns werenamed by the Greeks (Gemini, Cygnus, Sagittarius) . (Sees. 3B, 3E, 4C;App. A, no. 22)

corona (solar) a vast, low-dens ity cloud of gases surrounding th e Sun. It stemperature increases outward to more than 1 000 00 0 K. (Sees. 2, 2C,2D, 2E; App. A , nos. 5 to 7, 9; Figs. 2.1, 2.4, 2.5, 2.7, 2.9)

count one pulse of current or voltage from a detector, indicat in