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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 Evolutionofa1solarmassstar 1. Protostar 2. mainsequence 3. RGB 4. HorizontalBranch 5. AGB 6. PlanetaryNebula 7. WhiteDwarf Dowehaveallthisright? Howdowecheckallthisout? (1)Starclustersareperfectbecausetheycontainstarsinmanyof theevolutionaryphases.Cantesttimescale,surface temperature andluminosity predictions.After30yearsoftesting,itlookslikewe understandthebasicevolutionofstarsverywell. (2)MypersonalfavoritetestisthemeasurementofradioactiveTc inAGBstars. Technecium43 Tcisanelementwithnostableisotopesandthelongest-lived isotope(Tc98)hasahalf-lifeof4.2millionyears. ModelsforAGBstars,predictthatTcwillbesynthesizedinbetween shellflashesandconvected tothesurface. In1952Tcwasdetectedforthefirsttimeinastarandnowis routinelyfoundinthespectraofAGBstars.Thisisdirectproofof nucleosynthesisinstarsandapowerfulverificationofstellarmodels. EvolutionofCloseBinarySystems BeforegoingontotheevolutionofmassivestarsandsupernovaeII, we’llthinkabouttheevolutionofclosebinarysystems. TherearemanymultiplestarsystemsintheGalaxy,butforthevast majority,theseparationofthestarsislargeenoughthatonestar doesn’taffecttheevolutionoftheother(s). TheAlgolMystery Algolisadouble-linedeclipsingbinarysystemwithaperiodofabout 3days(veryshort).Thetwostarsare: StarA:B8,3.4Mo main-sequencestar StarB:G5,0.8Mo `subgiant’star Whatiswrongwiththispicture? Algol Themoremassivestar(A)shouldhaveleftthemainsequenceand starteduptheRGBbeforethelessmassivestar(B). Whatisgoingonhere? Thekeyistheshort-periodorbit. TheAlgolStory OriginallythesystemcontainedStarAat1.2Mo and StarBat3.0Mo. Betweenthetwostarsisapointwherethe gravitationalforcesofthetwostarsbalance.Thisis calledaLagrangepoint. L1 LagrangePoints Thereare5Lagrange pointsintheEarth/Sun system.L1,L2andL3are unstableonatimescale of23days L3isapopularspotfor Vulcan. L2istheproposedorbit forJamesWebbST L4andL5arestableand collectstuff LagrangePoints Youshouldbealittleconfused abouthowthisallworks. TheLagrangePointsareonly obviousinarotatingreference frame. Algolcont. BacktoAlgol.AsStarBevolvesandexpandsasitheadsuptheRGB. WhenitsradiusequalsthedistanceoftheL1point(calledtheRoche Radius)thematerialinStarB’senvelopefeelsastrongerattraction toStarAandthereismasstransferredfromBtoA. MassTransferinBinaries InthecaseofAlgol,StarBtransferred2.2Mo ofmaterialtoStarA. StarA:1.2Mo ->3.4M☉ StarB:3.0Mo ->0.8M☉ MassTransferBinaries ThinkaboutthecontinuedevolutionofAlgolandyouhavethe explanationfornovae. Iftheoriginalprimarytransfersmostofitsmasstotheoriginal secondary,youareleftwithamassivemain-sequencestaranda heliumWD. WhentheoriginalsecondarystartstoevolveuptheRGB,ittransfers somematerialbackontotheheliumWD. Novae AsthefreshhydrogenaccumulatesonthesurfaceoftheheliumWD itislikeaninsulatingblanket-- thetemperaturerisesto107Kand thereisaHydrogenfusionexplosion. Thestarbrightensbyanywherefromafactorof10toafactorof 10,000. Insomecases,thistakesastarfromtoo-fainttoseetobrightenoughtoseesotheseobjectswerecalledNova-- newstar. Novae NovaVel1998(3rdmagnitude) Novae NovaPersei becameoneofthe brighteststarsintheskyin1901. Looktherenowandseethe expandingshellfromthe explosion.Thevelocityofthe materialis~2000km/sec Novae NovaCyg(1992)illuminateda cloudofnearbyHydrogengas. Theexpandingshellofthenova couldbeseenafewyearslater withHST. Novae NovaCygin1994. Mostnovaare`recurrent’. Everyyearthereare20- 30novae observedintheGalaxy.`Naked eye’novaoccurmorelikeoneper decade. MassTransferinBinaries Thescenariothatleadstonovaexplosionscanproduceaneven wilderphenomenon. Intheearly1900s`novae’weresometimesobservedinother galaxiesandwereusedtohelpsetthedistancestogalaxies. But,whenitbecameclearthateventhenearestgalaxiesweremuch furtherawaythananyonehadthoughtthissuggestedthatthe extragalactic`nova’weremuchbrighterthanGalacticnova-- the termsupernova wascoined. Supernova Supernovaareveryluminous-- as brightasthecombinedlightofall thestarsinasmallgalaxy! Theyriseinbrightnessvery quicklyandthenfadeover timescalesofmonths. Supernova Earlyonitwasrealizedtherewere twodistincttypesofSN. SNIhavenohydrogenintheirspectra andareseeninalltypesofgalaxies SNIIhavehydrogenandareonlyseen inspiralgalaxiesandnearstarformingregions SupernovaSpectrum TypeIa Si NoH He TypeIb NoHe TypeIc NoSi H TypeII SupernovaI Nohydrogeninthespectra Seeninalltypesofgalaxies Seeneverywherewithingalaxies(haloanddisk) Maximumbrightness:6x109L☉ Adecadeago,15- 20werediscoveredperyear,lastyear166 SupernovaI ThereisarobotictelescopeupatMt. Hamiltonthatdoesanautomaticsearchfor SNeveryclearnight. Takeimagesoflotsofgalaxies,digitally subtractthem,lookforanyresidual. SupernovaI Whatisgoingonhere?Ittookalongtimetosortthisout. RememberWDmasstransferbinariesandtheChandrasekar limit. WhatwouldhappenifmasstransfernudgedthemassofaWD abovethe1.4M☉ limitfordegenerateelectrongaspressure? SupernovaI WhenaWDexceedstheChandrasekar limitthereisaviolentversion oftheheliumflash. Thetemperatureskyrocketsandwithinasecondafusionchain reactionfuseselementsalltheuptoradioactivenickel. Thisstarhasexplodedinarunawaythermonuclearcatastrophe! SupernovaI WhatisRIGHTaboutthistheory? (1)Willseetheseobjectsin`old’ populations. (2)Modelsforthedetonationofa 1.4M☉ WDgivetherighttotalenergy (3)Thepredictedamountofradioactive Ni56 intheexplosionfitthelightcurve perfectly Luminosity(solarunits) 109 Yellowline:theorywithNi56 Diamonds:data 103 Redline:modelswithoutNi56 0300600900 Timefromexplosion(days) SNI What’sWRONGwiththistheory? Fiveyearsago,theanswerwentlikethis. TheaccretedmassofaRedGiantontoaWDwouldbehydrogen rich,yetthesignatureofSNIisnohydrogen.Obvioussolutionisto havethemergeroftwo0.7MoheliumWDs.Problemwas,didn’t haveanexamplesofclosehelium-WDpairs! Now,wedo. EvolutionofStarsMoreMassivethantheSun High-massstars,likeallstars,leavethemainsequence whenthereis nomorehydrogen fuelintheircores. Thefirstfeweventsaresimilartothoseinlower-massstars– firsta hydrogen shell,thenacoreburninghelium tocarbon,surroundedby helium- andhydrogen-burningshells. EvolutionofStarsMoreMassivethantheSun Starswithmassesmorethan2.5solarmassesdonotexperiencea heliumflash – heliumburningstartsgradually. A4-solar-massstarmakesnosharpmovesontheH–Rdiagram– it movessmoothly backandforth. EvolutionofStarsMoreMassivethantheSun Thesequencebelow,ofactualHubble images,showsaveryunstable redgiant starasitemitsaburstoflight,illuminatingthedust around it: EvolutionofStarsMoreMassivethantheSun Astarofmorethan8solarmasses canfuseelementsfarbeyond carbon initscore,leadingtoaverydifferentfate. ItspathacrosstheH–Rdiagramisessentiallyastraightline – itstays atjustaboutthesameluminosity asitcoolsoff. Eventuallythestardiesinaviolentexplosioncalledasupernova. EvolutionofStarsMoreMassivethantheSun Insummary: TheEvolutionofHigh-massStars Forstarswithinitialmain-sequencemassgreaterthanaround6Mo theevolutionismuchfasterandfundamentallydifferent. 1Mo 3Mo 15Mo 25Mo 10 x 109 years 500 x 106 years 15 x 106 years 3 x 106 years MassiveStarEvolution Thecriticaldifferencebetween lowandhigh-massstarevolution isthecoretemperature. InstarswithM>6Mo thecentral temperatureispotentiallyhigh enoughtofuseelementsallthe waytoIron(Fe) NucleosynthesisinMassiveStars Fusingnucleitomakenewelementsiscalled nucleosynthesis. Temperature 15 million K 100 million K 600 million K 15000 million K etc Fusion reaction H -> He4 He4-> C12 C12-> O16 (Mg24) O16-> Ne20 (S32) etc MassiveStarNucleosynthesis Ina25Mo starnucleosynthesisproceedsquicklytoFe(whyitstops therewewillgettoinaminute). Themostcommonreactioniscalledthe`alphaprocess’anditis fusingHe4 toexistingnuclei.Thisprocessisreflectedinto abundanceofvariouselementsintheUniversetoday. NucleosynthesisinMassiveStars C+He->O WhatisspecialaboutFe? Feisatthepeakofthe`curveofbindingenergy’ Fe Aneasierwaytothinkaboutthisisinthe mass/nucleon foragivennucleus: Nucleosynthesis Fusinglightelementstogetherresultsinmorenuclearbinding energyandlessmasspernucleon.Whenthemassdisappears,itis convertedtoenergysolight-elementfusionproducesenergy. But,whenfusinganyelementtoFe,younowneedtoPROVIDEsome energytobeconvertedintomassandNaturedoesn’tliketodothis. Ontheotherhand,elementsheavierthanFecanbreakapartandgo tolessmass/nucleonandreleaseenergy. Stage Central T Duration (yr) H fusion 40 million K 7 million He fusion 200 million K 500 thousand C fusion 600 million K 600 O fusion 1.2 billion K Ne fusion 1.5 billion K 6 months Si fusion 2.7 billion K 1 day 1 CoreCollapse ThefusionchainstopsatFeandanFecoreveryquicklybuilds. WithinadayofstartingtoproduceFe,thecorereachesthe1.4Mo Chandrasekar limit. Onatimescalelessthanasecondthecoreimplodesandgoes throughaseriesofeventsleadingtoatremendousexplosion. CoreCollapse 1) ExceedtheChandrasekar limit 2) Temperaturereaches10billionK 3) Fenucleiphotodisintegrate,coolingthecoreand speedingthecollapse 4) Thegravitationalpressureissohighthat neutronization occursconvertingtheelectronsand protonsintoneutronsandreleasingablastof neutrinos 0.1sec 0.2sec CoreCollapse Thecoreisnowsolidneutronsandatnucleardensity.ThisisaVERY stiffballofneutrons. Theouterlayersofthestarfallin,encountertheneutroncoreand `bounce’backsettingoffashockwavethatpropogates outward blastingtheenvelopeintospaceat50millionmilesperhour. SupernovaII Thisisawildevent. Intheexplosionthemodelspredict: Manyrareelementswillbe manufacturedinnon-equilibrium reactions Arapidlyexpandeddebrisshell Anextremelydenseballof neutronswillbeleftbehind SupernovaII Anyreasonstobelievethis story? Many! 1) SNIIhavebeenseeninmany galaxiesinthelast100years andalwaysnearstar-formation regions: Guiltbyassociation! SNII 2)Predictedpeakluminosityof108L☉ isobserved 3)Predictedexpansionvelocityof10,000to 20,000km/secisobserved 4)IntheGalaxy,whenwepointourtelescopesathistoricalSN,we seechemically-enriched,rapidlyexpandingshellsofgas SN1987a Therewasamajorbreakthroughin1987. 165,000yearsagoinanearbygalaxycalledtheLargeMagellanic Cloud,astarblewupasaSNII. Thefirstindicationwasaneutrino`burst’.About10billionneutrinos fromSN1987apassedthrougheveryhumanonEarth.Neutrino detectorscaughtabout14ofthem. 99%ofaSNIIenergyisreleasedasneutrinos. SN1987a Thesecondindication,about4hoursaftertheneutrinosarrivedwasa newnaked-eyestarintheLMC SN1987a Forthefirsttime,theprogenitorstarofaSNIIwasidentified: 20MoSupergiant-- bingo! ThefinalpredictionofSNIItheoryisthatthereshouldbeavery denseballofneutronsleftbehindinthecenterofaSNIIremnant. Morelater. HistoricalSupernovae Therearemorethan2500SNthathavebeenseeninothergalaxiesinthelast 100years.Basedonotherspiralgalaxies,abigspiralliketheGalaxyshouldhave about: 0.5SNIpercentury 1.8SNIIpercentury HistoricalSN WemissmanyintheGalaxybecauseofdustobscuration. FromradiosurveysforSNremnants,wehavediscovered49 remnantsforaninferredrateof3.4SN/century. Thereareseveral`historicalsupernovae’-- brightnewstarsthat appearedintheskyandwererecordedbyvariouspeople. HistoricalSN 1006,1054,1181,1572,1604and1658wereyearswhenbright`guest stars’werewidelyreported HistoricalSN Forallthegueststars,pointamoderntelescopeat thepositionandseearapidly-expandingshellof material. Intwocases,theremnantwasdiscoveredbforethe historicalevent HistoricalSN The1054ADeventwassobrightitcastshadowsduringtheday-- this isthepositionoftheCrabNebula HistoricalSN ThenearestSNremnantisthe`Gum’nebulafromaround9000BC. FourtimescloserthantheCrab,itwouldhavebeenasbrightasthe fullmoon. Amysteryis`Cas A’-- thiswasaSNatabout1600AD,shouldhave beenverybright,butnorecordsofitexist. SupernovaeintheGalaxy WearelongoverdueforabrightGalacticSupernova. Forawhile,anearbySNwasavalidcandidateforthesourceofthe demiseofthedinosaurs. Therearetheproductsofshort-livedradioactiveisotopeslockedup inprimitivemeteoriteswhichsuggestaSNinthevicinityoftheSolar Systemabout100,000yearsbeforetheSunformed.ASNmayhave triggeredthecollapseoftheproto-Sun. Wewillgettothis SupernovaSpectrum TypeIa Si NoH He TypeIb NoHe TypeIc NoSi H TypeII Core-Collapse Supernova Exploding WhiteDwarf