the supernova a stellar spectacle

8/8/2019 The Supernova a Stellar Spectacle

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h e S u p e r n o v a(NASA-EP-126) THE SUPERNOVA: A STELLARSPECTACLE (National Aeronautics and SpaceAdministration) 43 p MF A01; SOD HC $1.30as NAS 1.19:126 CSCL 03A G3/89

N77-15942

Unclas11530

onal Aeronautics an d


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THE SUPERNOVA

A Stellar Spectacle

A curr iculum project of the Am erican Astron om ical So ciety,prepared with th e cooperat ion of the

Nat io n a l Aeronaut ics and Space Adm inist ration and the National Science Foundat ion

by

W . C. StrakaDepa r t m en t of Physics, Jackson State UniversityJackson, Mississippi

Nat ional Aeronaut ics and Space Administ rat ionW ash in g ton , D C 20546 September 1976

Fo r sale by the Superintendent of Documents, U.S. Government Printing OfficeW ashington, D.C. 20402 - Price $1.30Stock No. 033-000-00654-3/Catalog No. NAS 1.19:126


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IntentionallyLeft Blan


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P REF A CE

In the past hal f century as t ronomers have provided mankind wi th a new view of theuniverse, with gl impses of the na tu re of infini ty and eternity that beggar th e imagination.Part icularly, in the past decade, NASA's orbi t ing spacecraft as well as ground-basedastronom y have brou ght to m an's at tent ion heavenly bodies, sources of energy, stellarand galact ic phenomena, about the nature ' O f ' w h i c h the world's scientists can onlysurmise.Esoteric as these new discoveries may be, astronomers look to the anticipated SpaceTelescope to provide improved understanding of these phe nom ena as well as of the newsecrets of the cosmos which they expect It to unveil . This instrument, which can observe

objects up to 30 to 100 times' fainter than those accessible to the most powerfu lEarth-based telescopes using similar techniques, wil l extend the use of variousastronomical m ethod s to m uch greater distance s. I t is not impossible that observat ionswith this telescope will provide glimpses of some of the ea rliest galaxies wh ich werefo r m ed , and there is a remoter possibility that i t wil l tel l us som ething abo ut the- edge ofth e universe.I

The researches of the past 10 years, plus th e possibility of even more fundam enta ldiscoveries in the nex t decade , are fascinat ing l aymen and firing th e imaginat ion ofyouth. NASA's inquir ies into public interest in the space program show that a majo rsource of such interest is stel lar and galact ic astronomy. NASA's enabling Act , th SpaceAct of 1958, lis ts a prim ary purpose of NAS A, "the expan sion of hum an know ledge ofphenomena in the atmosphere and space"; the Act requires of N A S A that "it provide forthe widest pract icable and appropriate dissemination of information concerning i tsactivities and the results of those activities."

In the light of the above, NASA is publishing fo r science teachers, part icularlyteachers of secondary school chem istry, physics, and Ea rth science, the fol lowing fou rbooklets prepared by the American Astronomical Society (AAS) with th e cooperat ion ofN A S A :The Supernova, A Stellar Spectacle, by Dr . W . C . Straka,Depa r t m en t o f Physics, Jack son State Univ ersi ty, Jackso n,Mississippi.Extragalactic Astronomy, The Universe Beyond ourGalaxy, by Dr . Kenneth C. Jacobs , Depar tment ofAstronomy, Universi ty of Virginia, C harlot tesvi l le, Virginia.Chemistry Between the Stars, by Dr . Richard H. Gammon,Nat iona l Radio A st ronomy Observatory, Char lot tesvil le ,Virginia.

iii


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A t o m s in As t r onom y , by Dr . Paul A . Blanchard ,Theore t ica l Studies Grou p, NA SA Godd ard Space FlightCenter , Greenbel t , Maryland .The Nat ional Science Foundat ion has cooperated in this project by fund ing for theA A S a High School As t r onom y Educa t ion W or ks hop in June 1974 at the Universi ty ofRichmond in order to give the manuscr ip t s a thorough pedagogic rev iew in t e rms ofcurricular relevance and classroom use. The resul t ing publ icat ions provide exci t ingaccounts of recent discoveries in the cosmos, and of the na t u r e of the scient i f ic thoughtand techniques by which scient is ts are t ry ing to understand these discoveries .N A S A expresses it s appreciat ion to the authors and to the m em ber s of the AASTask Gr oup on Education in Astronomy (TGEA), whose enthusiasm and energy carriedth e project to complet ion, par t icular ly to Dr . Gerrit L. Verschuur , Director of the FiskePlanetar ium, Universi ty of Colorado, w ho served as Director of the project ; D r. D o n a t G .

W e n tz e l, As tronom y Prog ram , Universi ty of M aryland , ini t ia tor of the project ; Dr . PaulH. Knappenberger , J r . , Di rec tor , th e Science Museum of Virginia and Ch a i r m an of theTGEA , w ho served as W orkshop Di rector, and Herm an M. Gur in , Execut ive Officer of theAmer ican As t ronomica l Society. To those w ho were enrol led in the W o rk sh o p and toothers wh os e j udg m en t s and suggestions helped give th e manuscr ip t s th e necessaryscienti f ic and curr icular val idi ty, NAS A is gratefu l .

Apprec ia t ion is also expressed to the National Science Foundation for its suppor t ofth e W o rk s ho p , to the Universi ty of Ri ch m ond for its cordial and efficient service as hostto the W o rk sh o p, and to the NASA Goddard Space Fl igh t Center for its assistance inmaking possible th e publ ica t ion of this project .Technical M onito r for the project was Dr . Na ncy W . Boggess, Staff Ast ronomer ,Astrophysics Divis ion, Office of Space S cience, NA SA. C oord inators of the project wereD r. Nancy G . Roman, Chief , As t ronomy/Rela t iv i ty , Office of Space Science, NASA, andDr. Freder ick B. Tuttle, Director , Educat ional Programs Divis ion, Office of Publ ic Affairs,NASA. Assis t ing until his ret i rement in June 1974, was Mr. Myrl H. Ahrendt ,Instruct ional Mater ials Officer , Educat ional Programs Divis ion.

Sep temb er 1976 Na t iona l Ae ronaut ics and Space Adm inis t ra t ionW ash in g to n, D C

IV


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SCIENCE AND CURRICULUM ADVISORS TO THE PROJECT

High School Astronomy Education Workshop

Workshop Faculty and Administration:D r. Paul H. K n ap p en berge r , Jr., Director, Science Museum of Virgin ia , Richmond ,Virginia.Mrs. Rebecca M . Berg , Ast ronom er , S t rasenburgh P lanetar ium , Rochester Museum andScience Center, Rochester, N ew York .D r. Richard A . Berg, Astronomer, Space Science Center, University of Rochester ,Rochester, N ew York.D r. Char les D . Smith , Ast ronomer , Science Museum of Virginia, Richmond, Virginia.D r. Gerrit L. Verschuur , A st ronomer , F iske P lanetar ium , Univers ity of Colorado ,Boulder, Colorado.Dr . Donat G . W e n tz e l, Astronomer , Universi ty o f M ary land , Co l lege Park , Mary land .Workshop Participants:Joseph H. Bean , Jame s Madison Jr. High School, Roanoke, Virginia.Mrs. Jean n e E. Bishop, W estlake High and Parkside Junior High School, W estlake, Ohio .Mrs. Am y Catherine Clark, James Madison High School, Vienna, Virginia.George H. Davis, W ashing ton-Lee High School, A rlington , Virginia.Edmund Thomas DeJulio , Haverling Central School, Bath, N ew York .Leonard R. Ference, Christ iana High School, Newark , Delaware .Jan e P . Geoghegan, Thomas Jefferson High School, Richmond, Virginia.D r. Fred J. Giessow, Mary Insti tu te, St. Louis, Missouri .Melvin L. Glover, Maggie L. W a lk e r High School, Richmond, Virginia.A . P a t to n H am m o n d , III,Hidden Valley Inte rm edi ate School, Roano ke, Virginia.D r. Darrel B. Hoff, University of Northern Iowa, Cedar Falls, Iowa.Lois J . Hrubik, Fairf ield Middle School, Richmond, Virginia.John W . Kritz ar, Edison High School, Alexa ndria , Virginia.


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Dr. Don H. Lucas, W ood wa rd-C lyde Co nsul tan ts , A t lan ta , Georg ia .Jean C . Lusard i , W oodbr idge Senior High School, W oodb r idg e , V i r g in ia .Dr . M . L . M eeks , Hays tack O bserva tory , W es t ford , M assachuse t t s .Mrs. Helen K . M oncu r e , Th omas Mc Kean Hig h Sch oo l, W i lming to n , D e lawar e .Joseph B . Pe t t i ford , H oward Middle School, Alex and r ia , Vi rg in ia .David B. Pinckney , Montoursv i l le Area High School , Montoursv i l le , Pennsylvania .Eugene W . Skinner , Fa i r fax C oun ty Pub l ic Schools , A lexandr ia , Vi rg in ia .Steven E. Sm ith , Ar l ing ton Pub l ic Schools P lane ta r iu m , Ar l ing ton , Virg in ia .Norman Sper l ing , Pr ince ton D ay School, Princeton, N ew Jersey.Robert C . Tate, Harper P lane ta r ium, At lan ta , Georg ia .Chr is G. V aganos , Hayf ie ld Secondary School, Alexan dr ia , Vi rg in ia .Denn i s E . W ym an, Lee-Davis High School , M echanicsv i l le , Vi rg in ia .Harvey W . W y nn , W akef ie ld High School . Arl ington, Virginia .Michae l Zeil ik, II, Univers i ty of New M ex ico , A lb uq ue r q ue , N ew M e x i c o .

Other Advisors to the ProjectPeter A. Basi le , W elsh Valley Junior High School , N arberth , Pen nsy lvan ia .Pax ton K . Black , Brookland Middle School, Richmond, Virg in ia .Henry J . Con ger, Norview High Schoo l , N orf olk , Virginia .Don Dalo ise , Mount Baker Secondary School , Cranbrook, Br i t i sh Columbia , Canada .George D . House, Arl ington Senior High School , Arl ington, Massachusetts .Capt . Gera ld P . Joyce, Gran by High School , N orfo lk , Vi rg in ia .Gregory K anaskie , Henderson Senior High School, W es t Ches te r , Pen nsy lvania .Mrs. Helen L . Kubico , T homas M cKean High School, W i lmin gton , Delaw are .Joh n M un le y , L ake Taylor High School , Norfo lk , Virginia .James T . Mur ph y , Read ing M em o r i a l High School , Reading , M assachu se t t s .Douglas Paul , Arm our Heigh ts Pub l ic School , Toronto, Onta r io , C anada .

V I


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John R. Percy, University of Toronto, Toron to , Ontar io , Canada .R u th K . Sanders, Lakeside High School, Atlanta, Georgia.Mrs. Jane T. Selden, Douglas Freeman High School, Richmond, Virginia.John Stolar, Ridley South Junior High School, Norristown, Pennsylvania.

VII


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STATEMENT OF PURPOSE

The p urp ose of this brochu re is to discuss the most spec tacu lar event in the life of astar, the supernova, related objects and the i r impor tance in as t r onom y and science ingeneral . The only thing in the un iverse more spectacu lar than this explosive f lareup of thestar in its dea th throes is the quasa r (which some astronom ers think m ay be closelyrelated) . N o t only are supernovae thought to m a r k the end of a star, bu t they m ay also beth e mechanism through which th e elements created in the nuclear reactor that is the coreof a star are dispersed and mixed i n to the raw mater ials of in te r s te l l a r space-the mater ia lfrom which the new generat ion of s tars wil l be formed. In the process of the explosion,the heaviest of the elements are also formed. It is from these elements from the interiorsof the stars tha t the planets and life i tself are made.

Some of the most fascinat ing and puzz l ing objects in the un iverse are also associatedwith supernovae. Am ong these are the pulsars and X-ray objects , the neutron s tars andblack holes, the Crab Nebula and the vas t ne tworks of f i l aments , and perhaps evengamma-ray burs ts and gravi ty-wave events .

The general organizat ion of the bro chure is indicate d by the table of con tents : theoutburst as observed and according to t h e o r y , th e s te l la r rem nan t , the nebular remnant ,and a summary of some of the unsolved puzzles . Some suggested discussion topics andstudent projects are given, with several levels of diff icul ty, so that the teacher may choosema ter ial app ropriate for the par t icula r class .

W . C. Straka

via


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CO NT E NT SPage

S T AT E M E NT O F P U R P O S E viiiI. I N T R O D U C T I O N 1II . THE STAR 7

A. Observat ions 7B. Theor ies Concern ing th e O u t b u r s t 16C . Theories Concerning th e Leftover Star 18III . THE S H E L L E XP E L L E D 21

A . Observat ions 21B. Theories 24C . Side Effects 26IV. PUZZLES AND PARTIAL ANSWERS 29

A . Detect ion 29B. Coincidence 30V. S U M M A R Y 31S O U R C E S O F F U R T H E R I N F O R M A T I O N 32S U G G E ST E D D I S CU S S I O N T O P ICS AND P R O JE CT S 3 3G L O S S A R Y . . . . . . 3 6

IX


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CHAPTER IINTRODUCTION

O ne even ing, when I was co n tem p la t in g as usual th e celes-t ial vault , whose aspect was so famil iar to m e, I saw, withinexpressible astonishment, near th e zen i th , in Cassiopeia,a rad ian t s tar o f ext raord inary magni tude . S t ruck wi th su r-prise, I could hardly believe m y eyes. . .T y ch o Brahe, November 1572

Every year , one or two stars flare more brightly than they have appeared in the skybefore. M ost of these can only be seen through the telescope, but every few gen eration sone of these novae, for so Tycho called his new star , shines more brightly than any otherstar in the sky, perh aps brightly enou gh to be seen in broad da ylight . The name nov a is aLat in word mean ing new. Y et these "new" stars last only a short t ime, fading from sightin perhaps a year. T he ancient Chinese astronomers called these newly appeared bu ttemporary objects "guest stars," a more accurate term. Somet imes long af ter th e star hasfaded astrono me rs can d etect throu gh the telescope a cloud of debris at the location of anova looking as if i t had com e from a huge explosion .

Unt i l th e 20th cen tu ry , all novae were thought to be the same or nearly th e same.Then Edwin Hubble showed that the great nebu la in Andromeda must be another galaxylike our own M i lk y W ay . A s a result , astronomers realized that th e nova discovered in theAn drom eda nebu la in 1885, S And rome dae, must have been much b igger than any other.Such gigantic novae came to be called supernovae .

Ordinary novae are fo u n d in telescopic obse rvations several t imes a year . Occasion-ally one m ay beco m e visible to the unaided eye . In reali ty , these "new stars" are not newat all. Instead they are the f lareup of a very o ld star , the last grand fling of the star beforeit d ies. Some may flare up several times, but these recurrent novae are less bright than theothers. Since the brightness that a star appears to have is d e te rm in ed by i ts actual in t r ins icbrightness, the distance of the star from Earth , and any dust o r gas that l ies betw een thestar and the Ear th (p roducing a d imming cal led interstellar absorption}, m an y n o v aep ro bab ly r em ain h id d en from us by a combinat ion o f thei r remoteness and the dust andgas clouds (which are called nebulae}.

W hil e an ordinary nova m ay brighten by as m u c h as 10 000 times, a su p ern o v a m ayflare to a bil l ion t imes i ts original luminosity . This extreme br ightness , combined wi th thefact tha t supernov ae are rare (only six have been recorde d in our galaxy in the past 2000years , and a seven th has been deduced) , leads us to the conclusion that a su p ern o v a m u s tbe a very d i f fe ren t kind of t h in g from an ord inary nova . Ast ronomers now recogn ize that ,whi le both the nova and supernova are explosions of stars, the ordinary nova explosion ison ly in the outer layers of the star . T he supernova, on the other han d , is an explosion inthe in ter io r regions , wi th specta cu lar effects on the outer layers as well .


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After th e nova o r supernova fades , as t ronom ers have somet imes de tec ted c louds ofdebris expan din g outwa rd. This agrees w ith th e idea that an explosion has taken place .The supernova is f o u n d to expel much more mater ia l than th e ordinary nova, againsuggesting that the causes, as well as the end resul ts , may be basical ly differe nt .A s th e as t ronomer looks about th e sky, he can a l so de tec t many nebulae that haveth e appearance of explosions . For ex am pl e , th e Crab Nebula in the constel lat ion ofTauru s (f ig . 1) has that appea rance. The C ygnus Loop (fig . 2) , a lso called the V eil N ebu la,is an in t r ica te ne twork of f i laments arranged in a circle. It also appears to have come froman explosive event near it s center .The supernova, as is obvious from the p recedin g paragraphs, is one of the mo stspec tacula r and impor tan t events in the universe. It is the most spectacular thing thath appens in the life of a star . A s will be seen in the rest of this brochure , th e supernovaaffects almost everything of interest to the ast ronom er . C hapter II will discuss w hat isknown about the explosion and what is left of the s tar afterw ard ; chapter III will discuss

th e cloud of debris that is t h r own off in the o u t b u r s t ; and chapter IV will list some of theastronomical puzzles and myster ies s t i l l surrounding supernovae.In addi t ion to the part they play in the life of the universe, supernovae have playedan impor tan t par t in the deve l opm en t of scientific thought. Like many other celest ialevents , these s tars which suddenly become br ight were regarded with superst i t ion byancient and pre-Renaissance p eop le and were considered to hav e ast rological s ignificance.F or example , accord ing to Sung Dynas ty records , th e repor t of the head as t ronomer ofth e cour t in 1054 says:

Prostrat ing myself , I have observed the appearance of aguest star; on the star there was a slightly iridescent yellowcolor. Respectful ly, according to the disposi t ions fo rEm perors , I have progno st icated, and the result said: Theguest star does not infringe upon Aldebaran; th i s showsthat th e Plentiful One i s Lord, and tha t th e count ry hasgrea t wor th . I request that this be given to the Bureau ofHistoriography to be preserved .(Translated by J . J .L . Duyvenbak, quoted in Gorensteinand Tucker , Scient i f ic American, July 1971, p. 77 . )

Such records of brightness , color , and location are valuable to both th e as t ronomerand th e histor ian. But care mus t be taken, since astrologers predict only good things.Indeed, they had good reason to do so, since in many ancient cul tures bearers of badt idings were often executed. There is evidence that th e Chinese ast ronomer m ay haveal tered th e color of the star, fo r example , f rom the yel low-orange predict ing s ickness toi r idescent yel low, the Oriental royal color .

W h e n Tycho recorded his observat ions of the supernova of 15 72 , he also recordedthat the people of his is land of Hven feared that this was a s ign of imp end ing plague,famine, or other disaster .


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Figure 1. The Crab Nebula (Messier 1) of 4 wavelengths . This object in Taurus is the r e mn a n t ofth e supernova of 1054. (Hale Observatories pho tograph )


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Figure 2. The Cygnus Loop. The r e m n a n t of a supernova which happened 30000to 40000 years ago. A possible X-ray pulsar has been found near the center of theLoop. (Hale Observatories photograph)

--tUGINAL PAGE ISOF POOR QUALITY!


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Before the Renaissance , the heavens were thought to be unchanging and imm utable .The Sun, M oon, and plan ets moved ag ainst the back grou nd of the stars , but in a fairlypred ic tab le m anne r . Com e t s and meteors were thought to be in the a tmosphere and thuspart of the imperfect Earth . The observat ion of Tyc ho and of Johannes Keple r (be t te rknown for his laws of planetary motion) of new temporary s tars helped to change this.No t only did the Sun, M oon, and pla nets mov e, but the "unchanging" s tars could chang e!

More recent ly, a supernova played an important but negat ive role in the progress ofour ever-growing view of the universe . In 1885, the nova know n as S An drom edae wasdiscovered in the Grea t Nebula in Andromeda (a l so known as M 3 1 ) . At the t ime, i t wasthought tha t the whole universe w as con t a i ned in the Milky W ay galaxy . The greatphi losop her Im m an ue l Ka nt had proposed in 1755 that there might be other galaxies l ikeour own ("island universes"), but there was no evidence to suppor t h is theory.The idea that S Andromedae was an ordinary nova hindered acceptance of thetheory of "island universes." If S Andr om edae we r e a normal nova, then it , and t h us th eGrea t Nebula in Andr om eda , wou l d still be within the boundar ies of the Milky W a y .Thus, th e Grea t Nebula and the other nebulae l ike it ( the sp i ra l nebu lae) would jus t benea rby clou ds of ma terial and not "island universes." These theories sparked a de ba tetha t wa s not resolved un t i l the 1920's . T he resolut ion, by Edwin Hubble, prove d thatthere were indeed o the r galaxies and that S Andromedae was a previously unrecognizedand spectacular object , the supernova.Even now there are man y th ings about supernovae tha t a re unexpla ined . Throughoutthis brochure we shal l look at some of the at tempts at explanat ion and indicate wheresome of the u nk no w n puzz les l ie .

There are more things in heaven and ear th, Horat io, thanare dreamt of in your phi losophy.

-Hamlet, I, v, 166The miracle occurs not in cont radic t ion to Na t u r e , but incontradict ion to w h a t w e know abou t N a t u r e .

S t . Augus t ine


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C H A P T E R IITHE STAR

A. Observat ionsOne of the major prob lems in ast ronom y is the wide gulf betw een the observat ionsof supernovae and the theories that have been tried. The theories can explain some of thedetails, but no theory has yet been able to explain completely what is h appen i ng .An important beginning step is to know how many supernovae occur . Is it some-thing that happens to a few stars only or is it someth ing tha t h appens to every star?Ordinary novae happen several t imes each year, but only six supernovae have beenseen from Earth in our own galaxy during all of recorded history. Table 1 s um m ar i z e ssome of the informat ion known about these six.

Table 1S u p e r n o v a e O b se rved b y Naked Eye

Date( A . D . )

18 5

369

1006

1054

1 57 2

1604

Constellation

C e n t a u r u s

Cassiopeia

Lupus..:'''

,.- '-i.' ,* " ;"Taurus (Crab / '

N e b u l a )JiJttff'if

Cass iope ia

O p h i u c h u s

A p p a r e n tBrigli tness

Br ighte r t h a nV e n u s(-6)Br ighte r t h a nMars o r J u p -i t e r (-3)'Brign/er than^VenuH-5)"v

' W-jff ' * ' ' *


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The column labe led "apparent br igh tness" gives th e brightness of the supernova atit s m ax i m um va l ue , as seen by the observers. The objects to which th e brightness iscompared are a m o n g th e brightest in the skythe plane t s Venus , Mars, and Jupi te r attheir br ightest and the br ightest s tar in the nigh t sky, Sir ius . The n um be r in parenthe ses isthe apparent magni tude on a scale used by astronomers and may be used in classes wherethe teacher has discussed the ast ro nom ical m ag ni t ud e scale.

The table shows that all of the supernovae seen were very bright. Also, they \vere allfairly close to the Ear th . B y compar i son , th e center of our Milky W ay galaxy is abou t 8 to10 kpc aw ay from us (1 kpc is abou t 3 x 1 0 1 8 m ) . The galact ic longi tude is measuredfrom the direct ion to the galact ic center . All of these supernovae, in other words , tookplace in our pa r t of the galaxy .These tw o facts tell us that a supernova had to be very bright fo r astronomers tonot ice i t in the d ays before there w ere telescopes . A lso, superno vae tha t take place veryfa r away from us in our o wn galaxy are not seen.I t i s interesting to no te tha t the superno va of 1006 wa s recorded by A rabian andEuropean as t ronomers , as well as those in the Far East, while the one in 1054 (firstrecorded on July fou rth o f that year) w as not . O n the other han d, the sup erno va of 1054is recorded in the rock carvings of In dians in the w estern par t of No rth America (f ig . 3) .These In dian s have no w ri t ten his tory.Tycho recorded enough informat ion in his book D e Stella Nova about th e supernovaof 1572 tha t ast rono me rs can draw an accurate graph of the br ightness of the s tar as i tp rogressed f rom Ju ne 1572 un t i l it faded out ear ly in 1574. H e also included descr ipt ionsof th e color.Kepler , the discoverer of the laws of planetary motion, and Gal i leo, who fi rs tapplied the t e lescope to as t ronomy, both recorded the supe rnova of 1604, begin ning inOctober . An in te res t ing compar i son of cu l tures may be foun d in the qui te accura terecords of this supernova kept by Chinese ast ronomers .In add i t ion to the s ix tha t are reco rded in the his tor ical and prehistor ical records, asup erno va also occu rred in the co nstellation of Cassiopeia in a bou t 1670. Un fo r t una t e l y ,there are no records of this event . W e know about i t today because rad io as t ronomersdiscovered the debris as one o f the m ost pow erful radio sources in the sky. This deb ris islocated at a galact ic longi tude of 110 and an est imated dis tance of 2 .8 kpc.In order to f ind out more about these rare objects , as t ronomers obviously must findmo re su pernovae. The only possible w ay, unless we are very lucky and one happens closeby, is to look at o ther galaxies . W e already me ntioned the superno va tha t hap pen ed in1885 in M31 (Andromeda) .Fritz Zw icky began a search for supern ovae in other galaxies in the 1930's. M a n yphotographs of o the r galaxies are taken each year . Then the new photographs are care-fully com pared w i th o lder ones to see if there are any stars that have become brighter.This project has bee n v ery successful , leading to the discovery of several supernov ae eachyear . O ne of the br ightest is show n in figure 4.


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Figure 3. Rock drawings in Arizona, believed to depict the supernova of 1054.(left, William C. Miller; righ t, R. C. Euler)


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SnO"0t

M5

O0

c1rtf-S


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Even though this has been very successful, such a search requires many hours of hardwork. Several other projects have been proposed to make this work easier and faster .These projects would make use of computers to aid in finding l ikely galaxies and inmaking th e necessary before and af te r comparisons . A te levis ion c amera w ould be hookedto the te lescope to m a k e the image more readi ly vis ible to the eye wi t h ou t the necessityof developing th e photographic plates . The most sophist icated proposal would have hadth e computer t ake over all the tasks, including point ing the t e lescope , s tudying th etelevision image, and m aki ng th e necessary comparisons . None of these projects is in theope rat ing s tage at present .

Historical records tell us that there is one supernova every 270 years or so in ourgalaxy, but we find that there are many more supernovae in other galaxies , perhaps asm any as one every 20 or 25 years. This leads us to an apparent puzzle . Why are therefewer supernovae in the M ilky W ay than in other galaxies? Is our galaxy uniq ue, or isthere some reason why we canno t see a l a rge number of the supernovae that do occur? Ifth e estimate of one every 25 years in other galaxies is correct , then we are seeing only 10pe r cen t of the supernovae in the Milky W ay. W e will discuss this puzzle in ch ap t e r IV .

The next thing we can find from a s tudy of the supernovae that have been foun d inour own and other galaxies is where they occur .As seen from the Earth, our M ilky W ay, a typical spiral galaxy, appears as a hazyband of l ight around th e sky. W e ar e located in the disc, inside or at the edge of a spirala rm, and abou t 8 to 10 kpc from th e center . Because we are in the m i ds t of a ll the stars,gas, and dus t , our vision is somewhat obscured (see projects at end of brochure) . It issimilar to being in the middle of a thick forest ; we may not be able to see a house that is100 m away , but we can look into the sky and see the Sun, which is 150 million km

away . So too in our galaxy w e canno t see the center which is perhaps 10 kpc a w a y , but inanother direct ion we can see other g alaxies tha t are 50 000 kpc a w a y .The supernovae w e have found in historical records are all located near th e band thatmarks the Milky W ay in the sky. Many of the supernovae in other spiral galaxies havebeen found in or near th e main d i scs of the galaxies, too. This tells us tha t th e k i nds ofstars that become supernovae are most ly th e types of stars found in the discs of spiralgalaxies . A very few supernovae have been found in the spiral arms themselves . These arel ikely, then , to come from th e types of stars found in the spiral arms.Observat ions of how the brightness of the light rises and falls dur ing th e supernova

outburs t have led to the conclusion that there are at least tw o dist inct types of super-novae. These are simply cal led supernovae of Type I and supernovae of Type II . Figures 5and 6 show graphs of the rise and fall of the l ight, bf'the tw o types . The impor tan tdifference to note is tha t af te r abou t 50 days th e Type I s upe r nova fades fairly slowly,while the Type II pauses briefly, then fades rap id ly . Type I supernovae are f o u n dthroug hou t the discs of spiral galaxies , while Typ e II are fou nd in the spiral arms. Thesupernovae found so far in elliptical galaxies are all Type I.

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MAXIMUM

+2

ca< +4

c+6

+8

0 100 200 D A Y SFigure 5. Type I supernova comp iled from various sources.

M A X I M U M -

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100 200 D A Y S

Figure 6. Type II supe rnova com piled f rom various sources.12


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There is also a difference in the m ax im um brightness reached for each of the twotypes of supernovae. Type I reaches a m ax im um brightness 3 x 10 9 t imes that of the Sun.Type II reaches 9 x 10 8 t imes the Sun's luminosi ty . Obviously, i f the Sun should becomea supernova , th e Ear th would be vaporized. (However , the Sun isn't th e kind of star thatbecomes a s upe r nova . )Th e s upe r nova will not a ppea r that br ight to us because dis tance diminishe s theapp arent b r ightness , and there is ofte n dust and gas in the w ay to block our par t of thelight. But the supernova can somet imes out sh ine , for a br ie f m o m e n t , all the other stars inthe galaxy put together (f ig . 4) .

It is i m p o r t a n t to look at the supernova 's color and a t w h a t it s s pec t r um is like atdi ffe ren t t imes, since the color can tell th e as t ronomer someth ing a b o u t the t e m p e r a t u r eand the spectru m wil l te l l him the speed of the ex pan din g ma ter ials , the chem icalspresent , their qua nt i t ies , e tc . , wh en properly interp reted. (The spe ctrum is also used totell which t ype the sup ernov a is dur in g the ea r ly s tages of the outb urst . )T he color of supernovae tends to be yel low to red, indicat ing a fairly l ow t em pe r a -ture for an as t ronomica l objec t , perhaps 4000 to 6000 K. Of course, this is j u s t th etem per atur e of the ou tside o f the exp and ing m ater ial (we canno t see the inside), and i t i snot the t e m p e r a t u r e of the star before the ou tbu rs t . U n f o r t u n a t e l y , no one has yetobserved a star before it becomes a supernova. Some have been seen a few days before th emaximum br igh tness w as reached, but then th e explosion w as a l ready unde r wa y .The low tempera ture means tha t most of the rad ia t ion com ing f rom the supe rnova i svisible l ight , wi th only a small proport ion in the inf ra red , u l t rav io le t , and X-ray par ts ofth e spec t rum. Even though it is only a small fraction of the to ta l l ight coming from th e

star, the a m o u n t of X-ray is probably mi l l ions or bi l l ions of t imes greater than th eamount of X-ray coming f rom the Sun . Observa t ions from some of the present andp lan n ed NAS A s a t e l l i te s and space probes will give us those answers .The spec t rum of a supernova i s ex t remely complex and hard to in te rpre t . There a rebroad and narrow emission bands at the same t ime. There are also many complex darkfeatures. Some fea tures can be ident i f i ed as being caused by h y d r o g e n , h e l i u m , ox yg en ,and some o ther e lements , wi th major differences be t ween Types I and II .The Do ppler shift (see Glossary) of the l ines in the spe ctrum of the supernova showst h a t the rate at w hich the gas is ex pa nd ing is t ruly e xp los ive -a bou t 8000 km/s for TypeII supernovae. This i s many t imes greater than the escape veloci ty from th e star. The

w i d t h of the broad emission bands indicates that the d i f fe ren t par ts of the gas are m ovin gin many direct ions at once, with a wide range of veloci t ies . This disordered motion isreferred to as turbulence.The spec t rum of a supernova does not rem ain the same, but shows many changesdur ing the yea r or so that i t is observed. The astron om ers w ho at t em pt to devise thetheories to explain what is happening seem to have an impossible task, but some progresshas been m ade , as we shall see be l ow.

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After the explosion is over and the m aterial that has been throw n o ff has spread o utinto space , what is left over? Supernovae in othe r galaxies are so far away that even th elargest telescopes do not al low astronomers to see what is left a few years or decade s afterthe explos ion . But we can consider which types of objects known to the as t ronomermigh t be the r emains of the star. The first t ype of object suggested is called a whi t edwarf. The first white d wa rf to be observed w as the co m pan ion to Sir ius . These stars are awhi t e color, indicat ing a h igh tem pe ratu re , perhaps 10 000 K; yet they are very fa int .Usually som ething very hot glows very brightly. These stars m ust then be very small .

The typical white dw arf is just as massive as any other star , ranging from one-half toone and one-half t imes as massive as the Sun, w hich is an average star. The Sun's mass is 2x 10 3 kg, a b o u t 300 000 t imes more massive than th e Earth. But the whi t e dwar f has at iny radius, about the same as that of a planet , so al l of the matter is squeezed veryt ightly together . If we could obtain a teaspoon of whi te dwarf mater ia l , w e would findthat i t would weigh a ton. The electrons have been str ipped off of the atoms, and thenuclei and electrons have been crowded as closely together as possib le . W hen m at ter is inthis compac ted cond i ti on , we say t ha t it is degenerate .

The second possibility is an object referred to as a pulsar. W hen these were firstdiscovered by the radio astrono me rs in 1967, they show ed up as a series of pulses at veryregular intervals . The start led radio astronomers at fi rst thought t h a t they had picked up aradio beacon from some othe r civi lizat ion in the galax y, and the p ulsars were first cal ledLGM's (Lit t le Green M en). Bu t very qu ickly the astronom ers real ized that ar t i ficial radios ta tions couldn ' t p roduce s ignals l ike a pulsar ; i t had to be a natura l pheno me non .M a n y years before, some theoret icians had proposed another kind of degeneratem a t t e r that might be called super-degenerate. In this kind of mat ter , th e mater ia l is so

t ightly squeezed that th e electrons are forced into th e protons . T he negative and positivecharges cancel each other, and a neutron is formed. These neutrons are then squeezed asclosely together as possible. The star that resul ts is called a neut ron star. Such a star mightexplain th e pulsar . The mat ter is so dense that i f we could exchange th e teaspoon ofsample whi te dwarf for a sample teaspoon of neut ron star, we would find that i t weighed10 6 tons.Pulsars are found by the regular pulses they send out in the radio part of thes pec t r um . A t tempt s have been made to find if they also pulse in other par t s of thespect rum. Al though they are very fa in t and thus difficult to observe, two have beenfound to pulse in the visible part of the spect rum. B y means of rockets and satellites sent

above the Earth's atmosphere, these tw o plus on e other have been found to have X-raypulses as well .There are a hund red or so pulsars kn ow n. The first one identified was in the C rabNebu l a . T he star t h a t turned out to be the pulsar had been picked ou t previous ly bys tu d y in g the w ay in w hich the n ebula was expa ndin g and t rac ing the expansion back-wards .Pulsat ion is very rapid . The fastest p ulsar is the one in the C rab Neb ula, which pulsesabo ut 30 t im es a second. Figure 7 shows the pulsar when i t is shining and whe n i t is not

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Figure 7. The pulsar in the Crab N ebula , M - l . The top picture shows th epulsar during a pulse. The lower picture is betw een pulses . (Lick Observ-a tory photograph)

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shining. In figure 1, the pulsar is the lower one of the pair of stars near the center of thenebula .Pulsars are concent ra ted toward th e disc of our galaxy. Because radio waves can gothrough th e dus t and gas c louds , we are able to see that they are concent ra ted toward the

center of the galaxy j u s t as the stars are.Another possibi l i ty of what might be the remains of a star is found a m o n g a n u m b e rof objects that are seen with NASA satel l i tes that look at the X-ray par t of the s pec t r um .U h u r u , one of the m o s t famous of these NASA satel l i tes , has found several of theseobjects. It is t h oug h t by some as t ronomers that some X-ray sources might be a type ofobject known as a black hole (discussed in a subsequent subsect ion) .O ne X-ray source, Hercules X-l , is associated with a variable-brightness binary s tar ,HZ Herculis. It seems l ikely that Her X-l and HZ Her are the remains of a supernovaoutburst in a binary s tar system which s tayed together . However , in this case, the X-ray

source appears to be due to a neutron s tar rather than a black hole.There seem to be a n u m b e r of "runaway" stars around th e galaxy that could be theremains of binary s tars that did not stay together after th e supernova explosion. Supposewe look at what would happen to a binary s tar i f one of the two companions lost par t ofit s mass suddenly . If it loses to o much mass , th e gravity of the tw o is no longer enough tohold them together . Both of the stars "run away" from their original location. There areeven a few pulsars that seem to be runaway stars (such as the one k n o w n as P S R 1 1 3 3 + 1 6 ,which travels at 300 km /s), giving support to this idea.Some other observat ions that relate to supernovae are the gamma-ray burs t s

observed by some of NASA's satelli tes, gravity waves, and cosmic rays. These will not bediscussed here, but the t eacher m ay wish to r e fe r to articles in Scientific American, Skyand Telescope, Astronomy, or othe r periodicals.B. Theories Concerning the Outburst

It is not yet k n o w n why a superno va occurs . There have been several theories in thepast tha t proposed that th e nuclear react ions in the center of a s ta r might suddenlyincrease in an uncont ro l led manner . This would lead to the release of a h ug e am oun t ofenergy that could resul t in an explosion of the entire star .A nor m a l star's energy is suppl ied by the conversion of hydrogen in to he l ium. If it

were not for this nuclear reaction, th e star wou ld jus t cool off. The Sun, fo r ex am pl e ,wo uld cool off in 2 x 10 7 years , but hydrogen fusion (into the heavier hel ium) will keepthe Sun hot for from 5 to 10 x 10 9 years .W h e n all the hydrogen in the center of the star is conver ted to hel ium, changes takeplace to permi t th e fusion of hel ium a toms to form carbon. W h e n all the hel ium avai lablein th e star's core is exhaus ted , it appears l ikely that a sort of "energy crisis" occurs ,leading to the supernova explosion.

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The carbon de tona t ion model sugges ts tha t when the fusion of ca r b on into stillheavier e lem ents beg ins , the nuc lear reac t ions proceed ca tas t rophica l ly , p rec ip i ta t ing anexp los ion in the core of the star. The explos ion th rows the outer layers of the star intospace , p roducing the ou tb u r s t we see as the supe r nova .This neut ron star fo r ma t ion mode l was or ig ina l ly deve loped to expla in how a neu-t ron s ta r might form . W hen the nuc lear reac t ions conver t ing carbon in to heavier e lemen tsbegin , a la rge number of par t ic les ca l led neut r inos a re produced . These neut r inos car rya wa y so much ene r g y so r ap id ly , a cco r d ing to th i s theory , tha t the core of the starcollapses in on i tself . This rapid inward collapse is cal led an z'mplosion. As the electron sare forced in to the n u c l e i of the a toms, forming neut rons (and thus the neut ron s ta r ) , ahuge a m ou nt of en ergy i s prod uce d . This energy can only be used up by expel l ing theoute r parts o f the s tar . This expulsion is the viole nt explosion that we see as the super-nova .The basic di f fe rence between these two theor ies is that the ca r b on de tona t ion mode l

has an immed ia t e ex p los ion p r oduced by the vio len t s ta r t ing of the carbon reac t ions ,whi le the neut ron s ta r format ion model has a sudden loss of neut r inos , l ead ing to animplos ion tha t resu l t s in the v io len t expuls ion of the outs id e of the s ta r .A third theory of how superno vae happ en i s tha t two s ta rs , for exa m ple , wh i tedwarfs , might co l l ide . This co l l i s ion would produce the explos ion , and the force of theimpact would produce the neut ron star. The problem with this theory is that collisionsbetween s ta rs are very ra re ; there m ight be only one such collision in the whole life of ourga laxy . But we k n o w that at l eas t seven supernovae have happene d in the last 2000 years .A recen t suggest ion is tha t the two stars, a white dw arf and a red giant (a very late

stage in a star's life jus t be fore the w hi te dw arf stage) might be fou nd as a b inary pa i r . Bya process s imilar to the Moon ' s p r oduc ing t i de s on Ear th , the wh i t e dwar f mig h t bedragged closer and closer to the red giant un t i l the white dwarf actual ly fal ls in, t h usp r o d u c i n g th e necessary coll is ion.A ll three o f these theor ies have suppor t , bu t n o n e is comple te ly sa t i s fac tory . Moreresearch wil l have to be car r ied out to d e t e r m i n e , for e x a m p l e , the d i f fe r ences b e tweenType I and Type I I supernovae .One result of supernovae in al l three theories is the formation of the heavier ele-ments . The process of f o rm i n g the heavier e lem ents and the i r var ious i so topes i s re fe r red

to as nucleosynthes is . Some nu c leosy nthes is takes p lace in the norm al nuc lear reac t ions inthe stars. T he fusion of h yd r og en to form he l ium h as a l r eady b een men t ioned , as has thefo r ma t ion o f c a r b on f rom th e fusion of he l ium. But format ion of some of the heavierelements requires energies much greater than can be found in the cores of nor ma l stars-energies tha t can only be found du r ing the superno va explos ion .Thus , supernova explos ions affect the chemis t ry of the universe in two w a y s : bycreat ing the heavies t of the e l emen t s and by st irr ing the e l emen t s fo r med b o th in the starsand in the outbu rs t i t se l f in to the m ater ia l s tha t wi l l form the new s ta rs , p lane ts , and ,pe r h aps , life.

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C. Theories Concerning the Leftover StarAlthough th e wh i t e dwar f may be the result of an ordinary nova, it seems veryunl ikely tha t a supernova could produce one. This conclusion is reached because th eexplosion of a supernov a is mu ch too violent and there are too m any white dwar fs . The

num ber of white dw arfs is , howev er , just r ight for the num ber of ordina ry novae.The n eutron s tar seems a m uch bet ter can didate fo r the afte rm ath of a supern ova.The ext remely h igh dens i ty of the neutron s tar could only be produced by s om e t h i ng asenerget ic as a supernova . But how does a neutro n s tar produce the pulses that we see inthe pulsar?There have been m any theories suggested for the exact way in which a pulsar works .The best ones make use of a magnetic field. Many stars, including the S un and the mostmassive stars, have magnetic fields. W h e n th e star's core collapses to form th e n e u t r o n

star, i t a lso concentrates the magnet ic f ield, maybe by as much as a bi l l ion t imes .If the s tar was spin nin g before the explosion (and m ost s tars do spin), i t will besp inn ing a f t e r wa r d as wel l . B ut , jus t as a f igure skater speeds up her spin by d r a w i n g herarms in close, th e star will speed up its spin as it col lapses to form th e neutron s tar . (Thisis cal led conserv ing ang ular mo me ntum .) The star will be spinning several t imes a second .If the m ag ne t i c field is t i l ted s l ight ly compared to the sp in , the direct ion of the field willsweep aro und through space. Charged par t icles , such as electrons, will be accelerated bythe m agn etic field. In being acce lerated, the electrons w ill radia te radio waves, visiblelight, and al l the other par t s of the s pec t r um .Figure 8 shows the t i l ted magnet ic field and the cone of radiat ion produced. As thecone sweeps past th e Ear th , w e will see a flash. Thus, th e pulsar can be thought of assomewhat l ike a l ighthouse, with it s beam of light periodically sweeping past th e observer(i f he is in the right locatio n).The black hole is the m ost exot ic of the possibi l it ies. There is mu ch argu me nt amo ngscient is ts over whether or not they can exist . N o black hole has ye t been observed, andastron om ers are n ' t cer tain w hat to look for . (Some astronom ers think that an X-raysource in Cyg nus , C yg X -l , m i g ht be a black hole .)According to Einstein 's general theory of relat ivi ty, gravi ty is real ly a warping andben d in g of space . It is possible to have a region of space completely warped and curved inon i t self . Black holes cannot be unde r s t ood by using the law of gravi ty fo u n d by N e w t o n .

The black hole has such s t rong gravi ty that i t even t raps l ight , hence i ts name. No l ightcan escape a black hole, and any ray of l ight that comes too close will be t r apped as well .If a neut ron s ta r has more than twice th e mass of the Sun, then it m us t con t i nuecol lapsing into a b lack hole . Einstein 's theory predicts , in fact , that any resis tance to thecollapse will i tself create more gravity that will inevitably crush the star to an i n f in i t edens i ty . A region forms in space-t ime, which has zero volume and in f in i te ly st rong gravi tya t its bounda r y . Any t h i ng , wh e t h e r it be m at t e r o r l ight , that ventures to o close to theblack hole is s wep t in . This point of no re turn is abou t 15 km from th e center of theblack hole, as seen by the observer on the outs ide .

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Theory predicts tha t m at t e r fall ing in to a black hole should give off a large nu m be rof X-rays. Thus, some X-ray sources could be black holes. I t m ight be possible to iden t i fythem with some cer tainty i f they are par t of binary s tar systems.

R A D I OW A V E S

ft*

M A G N E T I C FI ELD P A T T E R N O F R A D I A T I O NS ENT OU T

Figure 8 . A mo del of a pulsar . In this mo del , the radiat ion is sent out in a cone-shapeddis t r ibut ion . A pulse is heard each t i m e th e cone sweeps past th e direct ion to the Ea r t h .

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CHAPTER IIITHE SHELL EXPELLED

A. ObservationsA r ound th e sky a r e a nu m b e r o f neb u lae that appea r t o h ave come f r om somev io len t event . Some o f these may b e th e deb r i s th r ow n out by the supernova explos ions .Perhaps not al l of them are the debris i t sel f , but a shock wave spreading th rough space asa r ipple spreads across the surface of a pool whe n a pebb le i s d rop ped in .A couple of doze n such remn ants have been ident i f ied . In four cases the nebula i s a tthe same location as a known supernova . These supernovae were th e ones tha t occur red in1006 (Lupus) , 1054 (Crab Nebula) , 1572 (Tycho's), and 1604 (Kepler's) .The C r ab N eb u la in the cons te l la t ion o f Taurus w as discovered by John Bevis in1 7 3 1 . I t was d iscovered independent ly by the "comet hunter" Char les Messier in 1758.A nnoyed b y th i s and other neb u lae that look l ike comets when seen th rough a te lescope ,Messier compi l ed a ca ta log o f just over 100 objects to avoid . T he C r ab N eb u la b ecame th efirst object on his l ist , and i s now ca l led Mess ie r 1 or M - l ( the An dro m eda ga laxy i s the31s t ob jec t , and is ca ll ed M 31) .T he other three supernova remnants tha t have been i den t i f i ed wer e a ll f o u n d by therad io as t ronomers . S o fa r , the only one o f these three visible is Kepler ' s supernova . Thereare a lso o ther n ebu lae tha t look as i f they were produc ed by explos ions , bu t which a ren o t associated with the his tor ica l supernovae . One of the m o s t b eau t i f u l objec ts in thesky is the n e t w o r k of f ine f i laments a r ranged in a circular form in the constel lat ionCygnus . This nebulos i ty is k n o w n as the C y g n u s Loop, or the Vei l Ne bula ( f ig . 2 ).There are a n u m b e r o f other -objects that a re s imi la r in appea r ance to the C y g n u sLoop . A large nebula in Aur iga which has on ly a c a ta l og n u m b e r , Shajn 147 (fig. 9 ),cons is t s of an in t r ica te ne twork of filigree spread across 5 degrees of the sky . Anotherremnant , found in Gemini , i s shown in f igure 10 .Ph o tog r aph s t aken of some of these objects over a per iod of years show them to begrowing in size. Carefu l m easu r em en t s h ave a ll owed a s t r onomer s t o t r a ce t h e m o t ionsb ack to the center of the n e b u l a in a few cases. This gives the age of the nebula and , thus ,the date of the supe r nova . Mos t of t h em h appened 10 , 000 or more years ago .A s o m e w h a t d i f f e ren t evidence of a pas t supernova i s foun d in the imm ense GumNe bu l a (nam ed for C ol in G um , the as t ronom er who d iscovered i t ) . Th is ob jec t i s anex t r eme ly ex t ens ive ne twor k of neb u lo s i t y , appea r ing to s t re tch 40 degrees across the skyin the southern par t of the Milky W a y . It spi l ls over in to a n u m b e r o f d i f f e r e n t cons te l la -

tions, but is centered on Vela and Pu ppis , the sail and dec k of the ship Argo. Someas t ronomers feel that par t s o f the ne bula m ay ex tend over as m uch as 90 degrees of thesky! This nebula appears large because it is close by . N e a r the cen t e r of this complexobject is a f ine f i l amen ta r y neb u la that i s very much l ike the Cygnus Loop or Shajn 147.

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shock wave . The mater ia l a t the ou ter edge sudden ly becomes hundreds o f t imes moredense . The t em p era tu re d ro p s from a million degrees to almost no thing .T he n e x t stage has th is new ly fo rmed , co ld , dense shel l con t inu in g to m o v e o u tw ard ,carried only by i ts m o m e n t u m ( t h e r e is no longer heat to push it o u tw ard ) . It sweeps upth e in ters te l lar mat ter befo re it like a snowplow. As i t gathers more mass , th e shell slowsd o w n .F inal ly , when th e speed has slowed to a b o u t 10 or 20 k m / s , th e shell just d issipatesand the ho le created in the in ters te l lar med iu m s lowly fills back in .Complex computer p rograms us ing th is theory have been wri t ten . The c o m p u t e r isa n o t h e r t oo l for the as t ronomer , jus t l ike th e telescope. I t speeds up the task of d o in g thea r i t h me t i c (a l though the co m p u te r o n ly d o es w ha t it is to ld) .B u t is the theory successfu l? The co m p u te r ca l cu la t io n s say that young supernovaremnants shou ld show th e original debris thrown off the star . M ed iu m - aged r em n an t s

should show sharp shock f ro n t s su r ro u n d in g hot regions. O ld remnan ts shou ld show co lddense shells.W e saw tha t two young supernova rem na n ts , the C rab Nebu la and C as A, show largea m o u n t s of the chemicals that come f rom th e in teriors of stars. The older remnan ts , suchas Shajn 147 and the Cygnus Loop , show composi t ions l ike th e rest of in ters te l la r space .O ld n e b u l a e are f o u n d to have high densit ies in the f i l aments , but the t e m p e r a t u r e s arenot as low as p red ic ted . Some of the yo ung re m nan ts have high densi t ies in the shel ls . CasA doesn ' t show a complete shel l , but m an y b lo bs of m ate r i a l . The Crab doesn ' t lookspherical , and yet this was a basic a s s u m p t i o n t ha t a spherical shell was thrown off thestar .An o the r d e f i c i en cy is t ha t th e computer ca lcu la t ions d o n ' t i n c lu d e m agn e t i c fields.W e k n o w f ro m th e observat ions ment ioned before tha t there are m agn e t i c fields in theCrab Nebu la , am o n g others .The last fault with the theories to be m en t io n ed he re is t h a t they do no t exp la in thef i laments . N o n e of the r e m n a n t s k n o w n looks like th e soap bubbles p red ic ted by thetheor ies . Some at tempts have been made to explain these f i l aments . O ne theory suggestst h a t the f i l am en t s m ay be f o rm ed w hen the clouds of g as and dust that are k n o w n to bescat tered throughout space are compressed by the shock wave or shell .

W h e n a man has no ideas at al l he starts working with ac o m p u t i n g m a c h i n e. -P. Debye

C. Side EffectsOne of the m ajo r p ro b lem s of as t ro n o m y h as been to e x p l a i n how s tars (and p lanets)are fo rmed . The var ious theor ies say t h a t , once th e con tract ion of a cloud of m ate r i a lbegins, grav i ty wi l l con t inue the con tract ion to form a star . T he problem has been how toget the con tract ion s tar ted in the first place .

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Perhaps the supernov a r em na nt nebu la can help to solve this problem . W e have seentha t the superno va rem na nt form s a high-densi ty region at i ts oute r edge. Furtherm ore,the dense region is cold. This m ay be j us t wh a t is needed to star t the cont rac t ion to forma star . W e might also note that the a m o u n t of m at t e r in such a s upe r nova r em na n t nebu lais enough to form several thousand stars. In one sense, the n , the death of one s tar in asu p ern o v a may resul t in the bir th of hundreds or even thousands of new stars. Thesupernova is, in a w a y , b o t h the beginning and the end of a star's life.

Dust grains and molecules may also form in these cool dense shells. (See thebrochure in this series t i t led Chemistry Between th e Stars, by R. H. G a m m o n . )The heaviest e lements can only be formed d urin g the supe rnov a outb urst . Thesee lem en t s and those formed during the normal course of a star 's life (carbon, ni t rogen, andoxygen) wi l l be dis t r ibuted in to space by the shell of debr i s th rown off the star . W e haveal ready m ent ion ed tha t the rem nants of the C rab Nebula and C as A have been observedto have large amounts of these very elements .The most common e lements in the universe are hydrogen, he l ium, and the C-N-Ogroup (carbon, n i t rogen , and oxygen) . T he biologists and chemis t s tell us t h a t the basice l e m e n t s of life are carbon, n i t rogen , oxygen, and hydrogen . Hydrogen is all around andwas present from th e beg i nn i ng , but the rest were formed in the stars. Thus, we and al llife on Earth may be the products of supernovae.

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CHAPTER IVPUZZLES AND PARTIAL ANSWERS

There are many puzz les tha t remain to be explained. In some cases, we may neverk n o w th e answer. In other cases, th e answer will be so obvious that we may ask why noone t h oug h t of it before .W e have seen several cases in which the theory was incomplete , sometimes onlybecause the m odels were s imple ones. By adding the co mp licat ions later , m any of thetheories can be improved. But othe r observat ions rem ain une xpla in ed .

A. DetectionSeveral puzzles exis t concerning th e de tec t ion o f supernovae and the i r r emnants .O ne puzz le is why the supernova of 1054, which was so well descr ibed in China andJ apan , was not seen in Europe and Arabia . European and Arabian as t ronomers had seenth e supernova of 1006,which seems to have been of the same br ightness .O ne suggestion is tha t th e Europeans were so convinced tha t th e heavens wereimmutable (unchanging) tha t such a change w as ignored or dismissed. This does notexpla in , however , why the Arabian as t ronomers did not see i t .Another suggest ion is that perhaps there was bad weather dur ing tha t year ; bu t thereis no record of any such bad weather over the whole of Europe, and the Arab countriesare no t ed fo r their clear skies.Another supernova tha t was not seen was the outburs t o f Cassiopeia A . By 1670,there were a number of large telescopes . Two supernovae had been observed during thecentury preceding (1572 and 1604).,Marfyt>'ther astronomical events were recorded. Yetno record has been found anywhere on Earth Orrh is ' supe rnova . W hy was it no t seen?'i ' '"ilV/ > -uds would 'dina the/ .supernova, and i t might have got tenf. If so, then some*a's t ronornefs est imate a 50 percent

//^t ;t ' -V~ *'; '' ': --'iJfc' *^ t "' -' "ki . f,< . .-" - ' * i 3 " : ' "

i > -In ters te l lar clouds would 'dina the/ .supernova, and i t might have got ten only as br ightas the planet Jupi tef . If so, then some*a's t ronornefs est imate a 50 percent chance of thestar 's being missedH. = - * . i - /In more recent t ime, almost all' of th 'S.^nown pufsai have been detecte d only asradio pulsars . Only in on e case has pulsat ien Defini tely been detec ted o pt ical ly, w ith onemore possible case. In bo th of these (the Crab Nebuja pulsar and the one in Hercules (HZHerculis)), X-ray pu lsat ion s have also been detec ted. There is one case in which an X-raypulsar has been detected where no radio pulsar i s known to exist in the cente r of theCy gn u s Loop.

If th e theories are correct, there should be an opt ical pulsar for each radio pulsar .W hy do we not see t h em ? In many cases, peculiar blue stars have been seen at the radiolocat ion , but they d o not p ulsate . Perhaps the theory is wro ng. Perhaps there is som ethingw e d o n ' t k n o w .

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Anot h e r puz z l e is the lack of supernovae in our own Milky W ay galaxy. This hasal ready been m en t i oned . W e find t h a t other galaxies have from two to five supernovaeeach century. Yet we know of only seven in our galaxy in the last 2000 years. W hy sofew?One possible a nsw er is this . If we dra w a map of ou r galaxy and put the kn ow nsupernovae on it (table in ch. II), they all l ie in our part of the galaxy (see the suggestedprojects) . The gas and dust clouds may prevent us from seeing the others .

B. CoincidenceIf all the var ious th ings w e bel ieve to have come from supernovae real ly do , thenthere should be a num ber of co inc idences among them . For exam ple , the rem na nt nebu-lae, pulsars (or neutron s tars) , and black holes are al l supposed to resul t from supernovae.Thus, they should be close to one another in space.Stel lar associat ions are found in the par ts of the spiral arms with the highest concen-trat ions o f gas and d us t . They are group ings of the young est , hot test , br ig htest , mostmetal-r ich stars in the galaxy. These are not s table groupings . The s tars scat ter soon afterthey are form ed. Since the superno vae of Type II are thoug ht to com e from the mostmassive, metal-r ich s tars , then pulsars , X-ray s tars , and supernova remnants should befound close to or in such associations. However, there are none to be found, except fo rth e Vela pulsar .O ne clue to this lack of relics of supernovae is tha t associat ions disperse rapidly.Another c lue is the runa w ay s tar s ment ioned ear li e r. The mo re m assive s tars are a lmos t allbinary . Perhaps when th e supernova occurs , the two stars fly apar t so rap id ly tha t the

pulsar or X-ray s tar ( the neutron s tar or the black hole) will quickly leave the association.The next puzz le is tha t , except for two definite cases and one possibi l i ty, there areno X-ray sources in the centers of supernova remnants . The two cases are the pulsar in theCrab Nebula and the Vela pulsar . The possible case is the objec t at the center of theCyg nus Loop. If neutro n s tars pro du ce X-rays and b lack holes are seen as X-ray s tars , whyare there no more than these? Again, a possible , but unl ikely, answer is the runaway stars .A curious puzzle has been found by the radio ast ronomers while looking at super-nova r em nan t nebu l ae . If these nebulae real ly come from supernovae and pulsars are alsothe resul t of supernova e, then pu lsars should b e fou nd at the cen ters of supe rnova

remn ants . Also, supernova rem na nt neb ulae should be found a round pulsars. Yet , in onlytwo cases is there a pulsar in a sup erno va remn an t . These two are the C rab Neb ula and theVela pulsar in the G um Neb ula . Some X-ray as t ronomers may have found an X-ray pulsarin the Cygnus Loop, bu t this has not been confirmed by other ast ronomers . Al l the otherh u n d r e d pulsars a re not near such nebulae , and the nebulae known are not near pul sar s .

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C H A P T E R VSUMMARY

In this brochure we have examined a few of the aspects of one of the most spectac-ular pheno men a in the universe . W e have seen tha t i t consists of many d i f fe ren t things.There is a f lareup of the star i tself; a shell of material expelled into space; and a shockwave spreading th rough the in te r s te l l a r me dium .Each of these is a violent occurrence. The s tar flares to a bil l ion times br ig hter thanthe Sun, so that no system of planets surrounding i t could survive. If such a supernovawere to occur in the nearest s tar to the Sun, Alpha Centauri , it woul d be a h und r ed t im esbrighter than th e full M o o n in the night sky, or one-sixteenth as bright as the Sun itself.More important , the amount of X-ray and ul t raviolet l ight bombarding our atmospherew o u ld put the sh ie ld ing lay er of the ionosphere to a severe test .At the same t ime, th e supernova is i m por t an t to the universe . By i ts occur rence , th ee lem en t s created in the nuclear furnace in the cores of s tars are mixed into the rawmater ia l f rom which new stars , planets , and life itself are formed . In the violence of theexplosion, th e heaviest of the elements, including most of those on which our civilizationdepends , a re crea ted . The shock wave provides th e ini t ia l squeeze needed to form thosenew stars and plane t s on which new life m ay form.In supernovae and their remnants , science is confronted wi th many grea t puzz les .There are several conflicting theories to expla in many of the ph enom ena . There areobservat ions that seem to con t r ad i c t one another . The resolut ion of these apparent con-

flicts will eventual ly do much to advance science.

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SOURCES OF FURT HER INFORMAT IONIn add i t i on to the references given in the t ex t and the suggested exercises, the

fo l lowing sources m ay prove useful .Resour ce L e t t e r EM A A - 1 : Edu ca t iona l Materials in A s t r o n o m y an d A st r oph ys i c s , R .Ber endz en an d D . DeVor k in . A mer i can Journal of Physics, v. 41, p . 783, 1973 . Avai lableas a repr in t for 50 cents and re turn enve lope f rom Arnold A . St rassenburg , Execut iveOffice, American Assoc ia t ion of Physics Teachers , Drawer A W , Stony Brook, N .Y .11790 .

Photographs and slides:(Hale Observa tor ies )

California In s t i t u t e of Technology Books tore1201 Eas t C a l i forn ia Bou levardPasadena , Ca l i forn ia 91109(Lick Observa tory)

Lick ObservatoryUnivers i ty of C a l i forn ia , Santa C ruzSanta C ruz , Ca l i forn ia 95060A t i ny s amp le of the man y t ex t s ava i l ab l e :

Abell , Exploration of the Universe , Hol t , Rinehar t , and W ins ton (several ed i t ions) .Baker and Freder ick , As t ronomy, Van Nos t rand ( severa l ed i t ions) .B r a n d t and M ar an , N ew Horizons i n A s t r onomy , F r eem an , 1972 .Go ldsmi th , From the Black Hole to the Infini te Universe, Ho lden D ay, 19 74.W y a tt , Principles of A st r onomy , A l lyn an d Bacon (several edi t ions) .

Magazines (in m any s ch oo ls and pu b l ic l ib ra r ies ):A s t r o n o m yScience N ewsScientific AmericanSky and Telescope

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SUGGESTED DISCUSSION TOPICS AND PROJECTS

Chapter I1. Many ancient people (par t icular ly in Europe) bel ieved that the heavens werei m m ut ab l e . The effect of supernovae discoveries on this belief can be discussed and canbe fi t ted into a larger s tudy of how M an's view of his universe has expan ded (see M.Munitz, Theories of the Universe).2. Discuss th e scient i f ic method.

Chapter 111. Prepare a map of the locations of supernovae in the Mi lky W ay galaxy . If possible,obtain polar graph paper . Otherwise, a compass, protractor , cent imeter ruler , and pencilm ay be used. At the center of the paper , mark a po i n t to be labeled "Sun." Find the linemarked "0" on the graph paper o r draw a vert ical l ine downward from th e poin t marked"Sun." This is the direction to the center of the galaxy . Mark a distance of 10 cm alongthis l ine from th e "Sun" and label this point "Galactic Center." Yo ur scale is now 1 cm =1 k p c . W i th th e compass, draw a circle centered on the "Galactic Center" of rad ius 10 cm(passing through the Sun). This represents th e Sun's orbi t around th e Galaxy . Mark th eposi t ions of the supernovae l is ted in table 1 (ch. II) in the fo l lowing man ner . The super-nova of 185 A.D . is a t 312 galact ic longi tude. M easure this angle counterclockwise withthe protractor centered on the Sun (or find the l ine marked 312 on the graph paper) .Measure from the Sun a dis tance of 2 .5 cm, corresponding to the 2 .5-kpc dis tance. Labelthis point "185 A.D.'" Do the same for the o the r supernovae plus Cassiopeia A (given inth e text) . Are the supernovae in the same par t of the galaxy as the Sun?2. A n ev ening session poin t ing o ut br ight s tars and plane ts wil l give the s tuden ts anidea of the br ightness of the supernovae. Point ing out constel lat ions will show thelocat ions .3 . A map of pulsars as seen from Earth is included in a 1968 Anthony Hewish ar t icleon p ulsars publish ed in Scientific A m eric an. A similar ma p is in Sky and Telescope, July1970. Discuss th e concent ra t ion of the pulsars to the band of the M i lk y W a y . A re most ofthe s tar s a l so in the Mi lky W ay band ? The concent ra t ion of supernovae to the M i lky W ayband is discussed in the text.4. Some articles on X-ray stars are in Sky and Telescope, Apri l 1973, and ScientificAmer ican , July 1972. A pop ular d i scussion of whi te dwar fs and neutron s tars is in Skyand Telescope, J anua r y 1971 . A n article t i t led "The Death of a Star" may be found inth e W orld Book Encyclopedia Science Year, 1968, p. 70 . An article on black holes ,"Gravi tat ional Col lapse," appears in Scient i f ic American, November 1967.Other relevant ar t ic les that may be used fo r class reports appeared in Sky andTelescope, No vem ber and Decem ber 1970 and Janu ary and February 1971 .

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5. The teacher can discuss nuc lear react ions on a level ap p ro p r i a t e to the class.Descriptions may be found in many general science books and in t roducto ry physicalscience texts . Formation of the e lem en t s in stars should be em p has ized in physics classes.6 . For physics classes, th e rate of spin of a neu tron s tar , as ev idenced by the pulsarper iod , can be used as an i l lus t ra t ion of a n g u la r m o m e n t u m . The Sun's rotation period isabo u t 25 d ay s , at the p resen t r ad iu s of 700 000 km. Calcu la te the period i f the Sun wereto collapse to the 700-km rad ius typ ical of the neu tron s tar (it is close to the period ofth e Crab pulsar) .7. To demonstra te absorp t ion , fill a rectangu lar aquar ium or o the r tank with trans-paren t sides with wate r . The length should be at least twice th e width . Mix a dark dye,such as black ink, into the water unt i l it appears modera te ly dark (but not so dark youcanno t see through i t ) . W i th the l ights in the room turned out, shine a f lashlight throughth e tank a long th e shor t side, then th e long way . Compare the brightness of the flashlightin each case with the uno bsc ured f lashlight . I f a photo grap hic or othe r l ight meter isavailable, me asure the diffe ren ce in brightness. I f an aqu arium is not available, clear waterglasses will do. Make su re th e w ate r is darkened to the same degree in each glass. Discusshow this i l lus t ra tes th e effect of dust clouds in space on starlight and the brightness ofsupernovae and thei r remnan ts .

Chapter III1. Slides and pictu res from Hale and Lick Observatories may be used to trace outnetworks of f i l am en t s and general ly examine supernova remnan ts . (Are th e f i lamentsconnected wi th each other?) Hale sl ides of in teres t include cata log numbers 7 (CrabNebu la , color S-22), 56, 57, 1 0 2 , 1 1 1 , 64, 108, 109 (color S-20), and 159. Lick catalognumbers include N3a, N3b, N3c, N 5 (color X I 7 ) , N 5 a , S I 7 , and S30. See also th eresource let ter by B eren d zen and DeVork in . A n article on "Supernova R e m n a n t s " m aybe found in Scientif ic American, July 1971.2. Teachers of physics classes can give a problem of conservation of momentumbased on an e xp an din g supernov a shell coll id ing inelastically with in terste llar gas. Haves tu d en t s find th e speed of several radii. A typical shell is one solar mass with an initialspeed of 8000 km/s . In ters te l lar mat ter has a densi ty of abo u t 1 hydrogen a tom per cubiccen t im e te r .3 . At var ious t imes s tar t ing in late 1945, photographs of nuclear exp los ions (a tom

and hy d ro gen bo m b tests) have appeared in Life and o t h e r magazines . F ind some of theseshowing the exp los ion step by step before the cloud fo rms. Compare these p ic tu res to thephotographs of su p ern o v a r em n an t s in this brochure and the Hale and Lick slides (seen u m b e r 1 above) .

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Chapter IVThese topics should be discussed in more detail . For example , is Europe ' s wea therl ikely to have been bad al l du r ing 1054? Is i t reasonable that our galaxy would have fewersupernovae (o r otherwise be so unique) than any other galaxy, including galaxies thatlook th e same? How fast would a runa w ay pulsar have to go to get outside th e ex pand i ngshell (physics classes can work a problem based on the orbital speed obtained fromKepler ' s third law)? Could the pulsar ' s beam be point ing in another direct ion?

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G L O S S A R Y

a tom smallest par t icle of an element , consis ts of a n u c l e u s and e l ec t r ons in orbi t aboutthe nucleus .

black hole object w ith extremely high surface gravi ty . Gravi ty is high en ough to t raplight and crush matter beyond recogni t ion.disc (of galaxy) flat disc-shaped region of galaxy containing most of s tars , on whichspiral pat tern is superimposed.Doppler shift change in wavelength of l ight or sound caused by motion toward (blueshift) or away from (red shift) observer.electron negat ively charged subatomic par t icle that normally moves about the nucleus

of th e a t om .galactic longi tude longi tude of a s tar or other object in the sky m easured in degrees (l ikelongi tude on Earth) . The "equator" is the plane of the Milky W a y . The startingpo in t (0) l ike th e Greenwich mer id ian is the direct ion toward th e center of thegalaxy as seen from Earth .ga l a xy l a r ge grouping of stars, typically 10 6 to 10 11 stars.inters tel lar mat ter gas and dus t in the galaxy between th e stars.kpc ki loparsecMilky W a y our own galaxy, a spiral galax y.nebula- cloud of inters tel lar gas or dust . Sometimes a galaxy.neut r ino a neutral par t icle smaller than a neut ron , having a mass approaching zero.neu t ron a subato m ic p ar t icle w ith no charge and the same mass as a pro ton. Par t of thenuc leus .neut ron star a s tar of extremely high densi ty composed ent i rely of neutrons .nucleus (of atom) heavy par t of an atom, made of neutrons and protons, with a posi t ive

charge. The electrons go around th e nucleus ,(of galaxy ) central con centrat ion of s tars , gas, and dust in a galaxy .parsec 3 x 1015 m.pro ton a subatomic par t icle with a posi t ive charge, about 1800 t imes heavier than theelectron.pulsar a pulsat ing radio source . Probably a rotat ing neutron s tar .

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quasar any of a number of s tar l ike objects t h a t emi t imm ense quant i t i es of energy andtha t appear to be ext remely d i s tan t from the Earth .solar mass 2 x 10 3 kg.spiral arms the part of a spiral galaxy where the gas, dust, and young stars are concen-t ra ted , dis t r ibuted in a spiral pat te rn thro ug hou t the gala xy.w hi te dwarf a s ta r tha t is very dense and smal l , having exhaus ted all of its n u c l e a r fuel.