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PHYS420:Astrophysics& Cosmology Dr RichardH.Cyburt AssistantProfessorofPhysics Myoffice:402cintheScienceBuilding Myphone:(304)384-6006 Myemail:[email protected] Mywebpage:www.concord.edu/rcyburt Inpersonoremailisthebestwaytogetaholdofme. MyOfficeHours TR5:30-17:00am W4:00-6:00pm Meetingsmayalsobearrangedatothertimes,byappointment DouglasAdams Hitchhiker’sGuidetotheGalaxy StellarEvolution Whenhydrogenfusionstartsattheendoftheprotostar stage,astar isbornonthe`zero-agemainsequence’. Ashydrogenisbeingconvertedintoheliuminthecoreofastar,its structurechangesslowlyandstellarevolutionbegins. StellarEvolution ThestructureoftheSunhas beenchangingcontinuously sinceitsettledinonthe mainsequence. TheHydrogeninthecoreis beingconvertedinto Helium. StellarEvolution Starsbegintoevolveoffthezero-agemainsequence fromday1. Comparedto4.5Gyr ago,theradiusoftheSunhas increasedby6%andtheluminosityby40%. 4.5Gyrago Today StellarEvolution InthecaseoftheSun(orany1M☉ star)thegradualincreasein radiusandluminositywillcontinueforanother5billionyears. Whilehydrogenfusionisthedominantenergysource,thereisa usefulthermostatoperating.IftheSuncontractedandheatedup, thefusionrateswouldincreaseandcausetheSuntore-expand. StellarEvolution Astheheliumcoregrows,itcompresses.Helium doesn’tfusetoheavierelementsfortworeasons. (1)with2p+pernucleus,theelectricrepulsion forceishigherthanwasthecaseforH-fusion.This meansthatheliumfusionrequiresahigher temperaturethanhydrogenfusion-- 100millionK (2)He4+He4 =Be8.Thisreactiondoesn’trelease energy,itrequiresinputenergy.ThisparticularBe isotopeisveryunstable. StellarEvolution AstheHeliumcorecontracts,itreleasesgravitational potentialenergyandheatsup. Hydrogenfusioncontinuesinashellaroundthe heliumcore. Onceasignificantheliumcoreisbuilt,thestarhas twoenergysources. Curiously,asthefuelisbeingusedupinthecoreofa star,itsluminosityisincreasing EvolutiontoRedGiant Asthecontractingheliumcore growsandthetotalenergy generatedbyGPEandthe hydrogenfusionshellincreases. Lgoesup! AsLgoesupthestaralso expands. ElectronDegeneracy Electronsareparticlescalled`fermions’(ratherthan`bosons’)that obeyalawofnaturecalledthePauliExclusionPrinciple. Thislawsaysthatyoucanonlyhavetwoelectronsperunit6-D phase-spacevolume inagas. ElectronDegeneracy Whenyouhavetwoe- perphase-spacecellinagasthegas issaidtobedegenerate andithasreachedadensity maximum-- youcan’tpackitanytighter. Suchagasissupportedagainstgravitationalcollapseby electrondegeneracypressure. Thisiswhatsupportstheheliumcoreofaredgiantstarasit approachesthetipoftheRGB. HeliumFlash Heliumcoreissupportedagainstgravitybyelectrondegeneracy Whendensityandtemperaturearehighenoughforthe`triple-alpha’ (3He->C)reaction. Electron-degenerategasesdonotexpandwithincreasing temperaturesotheonsetofheliumfusionisarunawaythermonuclearreaction. Asthetemperaturegoesup,increasein`phasespace’lifts degeneracyandstarsettlesintoHeliumfusiononthehorizontal branch RedGiants HydrostaticequilibriumislostandthetendencyoftheSunto expandwinsalittlebitatatime.TheSunisbecomingaRedGiant. Willeventuallyreach: L->2000L☉ R->0.5AU Tsurface->3500k RedGiant 100Ro 108years L 3Ro,1010years Temperature SunasaRedGiant WhentheSunbecomesaRedGiantMercuryandVenuswillbe vaporized,theEarthburnedtoacrisp.LongbeforetheSunreaches thetipoftheRGB(redgiantbranch)theoceanswillbeboiledaway andmostlifewillbegone. Themost`Earthlike’environmentatthispointwillbeTitan,amoon ofSaturn. RGBEvolution AstheSunapproachesthetipoftheRGB CentralTCentralDensity Sun15x106k102 grams/cm2 RedGiant100x106k105 grams/cm2 Forstarsaround1M☉,withtheseconditionsinthecoreastrange quantummechanicalpropertyofe- dominatesthepressure. Heliumfusion/flash Theheliuminthecorecanstarttofusewhenthedensityand temperaturearehighenoughforthe`triple-alpha’reaction: He4 +He4 ->Be8 Be8 +He4 ->C12 TheBerylliumfallsapartin10-12 secondssoyouneednotonlyhigh enoughTtoovercometheelectricforces,youalsoneedveryhigh density. HeliumFlash TheTempandDensitygethighenoughforthetriple-alphareaction asastarapproachesthetipoftheRGB. Becausethecoreissupportedbyelectrondegeneracy(withno temperaturedependence)whenthetriple-alphastarts,thereisno correspondingexpansionofthecore.Sothetemperatureskyrockets andthefusionrategrowstremendouslyinthe`heliumflash’. HeliumFlash Thebigincreaseinthecoretemperatureadds momentumphasespaceandwithinacoupleof hoursoftheonsetoftheheliumflash,theelectrons gasisnolongerdegenerateandthecoresettles downinto`normal’heliumfusion. Thereislittleoutwardsignoftheheliumflash,but therearrangment ofthecorestopsthetripupthe RGBandthestarsettlesontothehorizontalbranch. HorizontalBranch Horizontalbranch RGB HorizontalBranch Starsonthehorizontalbranchhavesimilaritiestomainsequencestars Heliumfusioninthecore Hydrogenfusioninashell TheSecondAscentGiantBranch Horizontal-branchstars(likemain-sequencestars)begintouseup theirfuelinthecore. Inthiscase,thestarisbuildingupaCarbon core.Forstarsnear1Mo thetemperaturenever getshighenoughforCarbonfusion. Thecorebeginstocontract,releasinggravitationalpotentialenergy andincreasingthefusionratesintheHeandHfusionshells.Does thissoundfamiliar? AsymptoticGiantBranch CarbonCore Heliumfusionshell HydrogenFusionshell AsymptoticGiantBranch ThisislikethetransitionfromthemainsequencetotheRedGiant Branch. StarsevolveofftheHBupandrightintheHR-Diagramonatrack parallelandabovetheRGB.Now,theenergygenerationismuch moreerratic.Thetriple-alphaprocessratescaleswithT30(!).AGB starsundergo`Shellflashes’. Asymptoticbranch Horizontalbranch Temperature PlanetaryNebulaStage ThetripuptheAGB(or`secondascentgiantbranch’)gets terminatedwhenthestar’souterenvelopebecomesdetachedand beginstodriftoffintospace.(!!) Theformerenvelopeshinesinthelightofemissionlines. Astheenvelopeexpandsandbecomestransparenttheveryhotcore oftheAGBstarcanbeseenatitscenter. PlanetaryNebulae Theouterenvelopeexpandingoutasashellappearsasa ringinthesky. PlanetaryNebulae TheemissionissimilartothatfromHIIregions.Ultravioletphotons fromthehotformerAGB-starcoreionizeatomsintheshell.On recombination,photonsproduced. PlanetaryNebulaeShells Theejectionmechanismfortheshellisacombinationofwindsfrom thecore,photonpressure,perhapstheshellflashesandthelarge radiusofthestar. Theshellexpandsintospaceatrelativelylowspeed(20km/sec). Approximately50%oftheAGBstarmassisejected. PlanetaryNebulaeShell Theshellexpandsandisvisibleforabout30,000yearsgrowingtoa sizeofmorethanalightyear. TheshellisenhancedintheabundanceofHe,Carbon,Oxygen (becauseofconvectionduringtheAGBphase).Thisisoneofthe meansbywhich`GalacticChemicalEvolution’proceeds. Thereareabout30,000PNintheGalaxyatanytime. PlanetaryNebulaeCentral`Star’ TheobjectinthecenterofthenebulaistheformercoreoftheAGBstar. (1)Itishot!T>150,000kinitially (2)Supportedbye- degeneracy (3)Mass~0.6M☉ (4)Radius~6000km(Earth) (5)Density~109 kg/m3 Athimbleofmaterialatthisdensitywouldweightabout5tonsonEarth. PlanetaryNebulaeCentral`Star’ Thecentral`star’isn’tastarbecauseithasnoenergysource.Thisis awhitedwarf. Supportedagainstgravitybye- degeneracy. Lotsofresidualheat,noenergysource,awhitedwarfislikeahot ember.Asitradiatesenergyintospace,thewhitedwarfcoolsoff. ThereisanupperlimittothemassofaWDsetbye-degeneracy. 1.4M☉ istheChandrasekar Limit. WhiteDwarf • Energysource:none • Equilibrium: e- degeneracyvsgravity •Size:6000km(Earth) WhiteDwarfs WDsappearintheHR-Diagramintheupperleftand VERYrapidlyevolvedownwardandtotheright. Luminosity Whitedwarf coolingcurve Temperature WhiteDwarfs Atleast15%ofthestellarmassinthesolarneighborhoodisinthe formofWDs.Theyareverycommon,thoughhardtosee. WhiteDwarfCosmochronology TheWDsinthesolarneighborhoodhaveaninterestingstorytotell: ThisdropoffinWDs atlowLandTis because ofthefiniteageof the Galaxy #ofWD high Luminosity(orTemp) low WhiteDwarfsintheGalaxy Wethinkthatallstarswithinitialmain-sequencemassless thanaround6M☉ becomewhitedwarfs. WhenwelookatthenumberofWDsatdifferentluminosity (ortemperature)therearesomeinterestingbumpsand wigglesANDadramaticdropoff attheLuminositythat correspondstoacoolingageof11Gyr. Evolutionofa1solarmassstar 1. Protostar 2. mainsequence 3. RGB 4. HorizontalBranch 5. AGB 6. PlanetaryNebula 7. WhiteDwarf PN AGB Heflash HB RGB ZAMS WDcooling Hot------Temperature------ cool Evolutionof1Mo Star Protostar Grav. contraction 5x107 years Main Sequence Core H fusion 10x109 years Red Giant Core contraction and shell H fusion Horizontal Branch Core He fusion and shell H fusion AGB Core contr + He fusion + H fusion White dwarf none 5x108 years 5x107 years 1x106 years A very long time Evolutionof1M☉ Star Thetimespentinaparticularevolutionaryphaseisrelatedtothe numberofstarsofthattypeweseeintheskyofthattype.(although youhavetobecareful) WhentheSunisanAGBstar,itsenvelopewillextendouttotheorbit ofMars,theH-fusionshellwillreachtheorbitoftheformerEarth. 1M☉ main-sequencestarbecomesa0.6M☉WDmademostlyofC withalittleH,He. Evolutionof4M☉ Stars Forstarslessthan6M☉ theselastslidesdescribetheevolution prettywell.Therearesomedifferencesinthedetailsthatdependon theinitialmain-sequencemass. Forstarsthatstartwith4M☉,itgetshotenoughinthecoresto(1) avoidtheheliumflashand(2)tostartcarbonfusion. TheWDremnantcontainsNe,MgandSiandtheamountof enrichedmaterialreturnedtotheISMislarger. ObservingStellarEvolutioninStar Clusters ThefollowingseriesofH–R diagrams showshowstarsof thesameage,butdifferent masses,appearasthecluster asawholeages. After10millionyears,themost massivestarshavealreadyleft themainsequence,whilemany oftheleastmassivehavenot evenreachedityet. ObservingStellarEvolutioninStar Clusters After100millionyears,adistinct main-sequenceturnoff beginsto develop.Thisshowsthehighestmassstarsthatarestillonthe mainsequence. After1billionyears,themainsequenceturnoff ismuchclearer. ObservingStellarEvolutioninStar Clusters After10billionyears,a numberoffeaturesare evident: Thered-giant,subgiant, asymptoticgiant,and horizontal branchesareall clearlypopulated. Whitedwarfs,indicatingthatsolar-mass starsareintheirlastphases,alsoappear. ObservingStellarEvolutioninStar Clusters Thisdoublecluster,handchiPersei,must bequiteyoung – itsH–Rdiagramisthatofa newborncluster.Itsagecannotbemore thanabout10millionyears. ObservingStellarEvolutioninStar Clusters TheHyadescluster,shownhere,isalsoratheryoung;itsmainsequenceturnoffindicatesanageofabout600millionyears. ObservingStellarEvolutioninStar Clusters Thisglobularcluster,47Tucanae,isabout10–12billion yearsold,mucholder thanthepreviousexamples.
