Chapter12.ppt

Chapter12Thedeathsofstars Fig 12 1ThisschematicH Rdiagramsummarizesthethreemajormodesofstellardemise 死亡 Low massreddwarfslivelonglivesandbecomewhitedwarfs Sun likestarsbecomegiantswithcorescollapsetobecomewhitedwarfs Themostmassivestarsbecomesupergiantsandexplodeassupernovae 12 1Reddwarfs Fig 12 1 Main sequencestarswithM 0 4solarmass theyarecool smallandfaint lowerrightportionofthemainsequenceThestarsareconvectivefromtheircenterstotheirsurfacesgasisconstantlymixedHydrogen heliumashesdistributeduniformlyinsidethestarnohydrogen burningshellwillnotformgiants Lowmassburnshydrogenveryslowlyverylonglivesonmainsequence 100 sbillionsyears Theyarenotmassiveenoughtoburnhelium nothotenoughatthecore contractslowlywhenfuelsareusedupgettingsmaller hottermovefromrighttoleftonH Rdiagrambecomewhitedwarfsreddwarfs 12 2Sun likestars0 4solarmass massofthestar 4solarmassesIntheredgiantphase thestar scoreishotenoughtofuseheliumintocarbon triple process andthentooxygenlayersofheavyelementsuptocarbonandoxygenThecoreisnothotenoughtofusecarbon oxygenintoheavierelementsBurningofhydrogen heliumintheshellsreleaseslargeamountofenergythestarexpandsfurtherandlosesalargeamountofenergy Thesurfaceofagiantisheldbygravityweaklyalotofmassintheouterlayersispushedawaybystrongstellarwind streamofhighenergyparticles andradiationpressure Fig 12 2OurcustomaryH Rdiagrammustbeextendedtohighertemperaturestoshowtheevolutionofastarafteritejectsaplanetarynebula Centralstarsofplanetarynebulae filledcircles arehotterandmoreluminousthanmostwhitedwarfs opencircles Thedottedlineshowstheevolutionofa0 8solarmassstarasitcollapsestowardsthewhitedwarfregion Presumablythecentralstarsofplanetarynebulaecooltobecomewhitedwarfslongafterthenebulaehavedissipated Planetarynebulae Fig 12 3 areexpelledouterlayersofgiantstars pushedoutbysuddenburstsofenergywhenfuelsareignited e g heliumburning Asmall denseandhotcoreisrevealedatthecenter nuclearreactionsextinguish 熄灭 andluminositydropsrapidlysurfacetemperatureishighbutluminosityislowbecauseofitssmallsizeresidesonthelower leftsideoftheH Rdiagram becomesawhitedwarfGasinthenebulaisionizedbyultravioletradiationfromthecentralhotstarlightisemittedwhenionsrecaptureelectrons forminganemissionnebula Fig 12 3Aplanetarynebulaistheejectedsurfaceofamedium massstar Thecoreofthestarcollapsesintoahotfaintobjectthatfinallybecomesawhitedwarf Planetarynebulaecanhavecomplicatedshapes whichmaydependontheirmassdistributionbeforetheexplosions 12 3Whitedwarfs Fig 12 4 Fig 12 4 Left AgroundbasedphotographoftheglobularclusterM4 Right HubbleSpacetelescoperevealsmanyfaintstars enclosedbywhitecircles inthecluster Thesefaintstarsarewhitedwarfsformedintheirfinalstageofstellarevolution Globularclusterscontainplentyofwhitedwarfsbecausetheyarerelativelyold Insideamain sequencestar theexpandingthermalpressurecounterbalancesthecontractingforceofgravity Whenfuelsinastar scoreareexhausted nomoreheatisproducedthermalpressureisweakenedgravitywins thestarcorecontractsthesmalleritgets thestrongerthegravitationalforce inversesquarelaw iscontractiongetsfasterandfasterwhenthedensitybecomesveryhigh 106gcm 3 electrondegeneracypressuretakesoverandresistsgravitybecomesawhitedwarf Electrondegeneracypressure Whendensityisveryhigh electronsarepressedtogether Butquantummechanicspredictsthatelectronsdon tliketobesqueezedtogether theyexertstrongoutwardpressuretoresist PropertiesofawhitedwarfAstarinequilibrium veryhot small Earth size andithasaveryhighdensity 1solarmass 1teaspoonofmatter 15tons Stronggravityonitssurface 105timesthatofEarth s e g SiriusB 天狼星 1solarmass 0 76Earth sradius surfacetemperature 32 500K density 3 106gcm 3 Awhitedwarfradiatesenergyslowlybecomescoldanddarkafterbillionsofyearsbecomesablackdwarf endswithaquietdeathChandrasekharlimit Themaximummassofawhitedwarfthattheelectrondegeneracypressurecansupport 1 4solarmasses Thedegeneracypressurecannolongerresistgravitybeyondthislimit i e thestarwillfurthercollapse seeChapter13 14 12 4SupernovaMassofthestar 4solarmassescontinueburningofheavierandheavierelementsuntiliron themosttightlyboundedelements Fig 12 5 cannotproduceanymoreenergybyfusingironaninert 不活动 ironcoreisformedinwhichnoenergyisgeneratedthestarcorecannotresistthecontractingforceofgravitycollapsesrapidly Fig 12 5Schematicinteriorofamassivestarapproachingtheendofitslife Thiscoolgiantstar aboutthesizeoftheorbitofJupiter containsacoreaboutthesizeoftheearthinwhichconcentriclayersfuseheavierfuelsaroundthegrowingironcore Notevenelectrondegeneracypressurecanstopgravitythecorecontractsfasterandfasterdensitygoeshigherandhigher Whendensity 1014gcm 3 nucleons neutronsandprotonsinatomicnuclei arepressedtogether exertingstrongpressuretoresist reactinglikeahardcore innercorerebounds buildinguppowerfulshockwaveswhichpropagateoutward triggeringnuclearreactionsintheouterlayers Anextremelyhighenergyexplosionresults equivalentto1028MtonofTNT completedestructionofthestar souterlayersinaveryshorttimesupernova suddenincreaseinthebrightnessofastarbymanymagnitudese g gueststar inTaurusrecordedbyChineseastronomersinShungdynasty 1054AD itwasvisibleduringdaytime anditfadedafteramonthandvanishedintwoyears Supernovaremnant Nebulousremnantofasupernovaexplosione g CrabNebulaM1istheremnantofthesupernovain1054Asmall extremelydensecoreofneutronmattermayremainaftertheexplosion neutronstar 12 5EvolutionofbinarystarsFig 12 6Evolutionofaclosebinarysystem a Oneofthestars right ismoremassive b Themoremassivestarevolvesintoagiant transferringmasstoitscompanion whichthenbecomesmoremassivebutisstillonthemainsequence c d Thegiantdiesandbecomesacompactobject e g awhitedwarf e Massistransferredtothecompactobjectfromthecompanion which f lateralsobecomesagiant Fig 12 6 TwostarsrevolvearoundeachotherundertheirmutualforceofgravityMorethanhalfofallstarsintheuniversearemembersofbinarysystems Whenonememberofthesystemswellsintoaredgiant matterinitsouterlayersislooselyheldandmaybetransferredtoitscompanion theevolutionpathsofbothstarsarechangedAlgolparadox Insomebinarysystems thelessmassivestarisagiantwhilethemoremassiveoneremainsonthemainsequence Explanation Themoremassivestarleavesthemainsequencefirstandswellstoagiant spilling 溢出 materialsovertoitscompanion Fig 12 6 thecompanionbecomesrelativelymoremassivebutisstillonthemainsequenceIfbothmembersswellintogiants theycouldmergetogethertoformonerapidly spinningsupergiant Ifonememberbecomesawhitedwarf orothercompactstars itcanformanaccretiondisk 吸积盘 Masstransferringfromtheco