StellarEvolution SurveyofAstronomyA110 AftertheMainSequence • Goals: – HowlongcanstarsremainontheMainSequence? – WhathappensasHydrogendepletes? – HowareGiantsformed? • ZeroAgeMainSequence(ZAMS) – Protostars becomeMSstars. • Whenthestartscorehydrogenburningandreacheshydrostatic equilibriumitiscalledazeroagemainsequence star. • DuringitslifeontheMSHisconvertedtoHeandtherearesmall changesintheluminosity,radiusandtemperature. • Hydrogenisusedupandthecoreshrinks(fewerparticles) temperature,pressureanddensityrise. • Increasedpressure increasenuclearreaction(moreenergy). Pressurefromphotonspushesouterlayers expands. • FortheSun:Radiusincreased6%,Luminosity40%,Temperature 300K. StellarEvolution SurveyofAstronomyA110 – Hydrogenburninganddepletion • Theincreasedtemperatureheatsmaterialsurroundingthecoreand startshydrogenburninginouterlayers. • Moremassivestars(O,B)burnHrapidly(~3x106 years). • Lowmassstars(M)burnslowly(~2x1011 yrs). • Massdeterminesformationtime,luminosity,temperatureandlifetime. • MStimeislongestperiodofastarslife. • WhentheHydrogenisdepleted(convertedto4He)thecorestops producingnuclearenergy(shellsaroundthecorehavenotdepletedand continueburning- shellhydrogenburning). • Coredoesnotgenerateenoughpressure,temperaturetocountergravity andshrinks. • Shrinkingcorereleasesgravitationalenergy(Kelvin-Helmholtz) reheatsthecore. • Thisincreasestheshellhydrogenburning(energyreleasedheats the outershells). 8 8 • Coreof1M¤ starcompressesbyone-third,Tincreasesto10 Kover10 years. StellarEvolution SurveyofAstronomyA110 • MainSequenceLifetimes – Energyoutput(Einstein) • E:Totalenergyreleasedbystar f:Fractionofmassconvertedtoenergy 2 M:Massofstar E = f Mc c:Speedoflight – Luminosityvs lifetime • LuminosityofastaroveritsMSlife E • L:Luminosity L = t:lifetime t • SubstitutingMassforEnergy – Luminosityvs mass • MSmassrelatedtoluminosityL M3.5 fMc2 • Substitutingformass t = L • Largermassshorterlifetime 10 • 1M¤ 10 years 1 ∝ • 25M 3x106 years t ¤ M 2.5 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 • FormingaRedGiant Figure21-4 – Hdepletionandcorecontraction • Shellburningspreadsoutwardsthroughthelayersofthestar. • Heatfromthecontractingcoreandshellburningincreasethe internalpressureofthestarcausingittoexpand(x100). • Asthestarexpandsitcools(astheairletoutofatireiscooltothe touch).Whenitcoolsto3500Kitemitsintheredspectralrange (828nm). • Itisthenknownasaredgiant. • Thelargerthesizeofthegiantthelessgravitationalpullonthe -7 -1 outermaterial.Masslossincreasessubstantially(10 M¤ yr -14 -1 comparedto10 M¤ yr forSun). • FortheSun: Hydrogendepletion:5x109 yrs RedgiantRadius:1AU Temperature:3500K Luminosity:1300L¤ • Redgiantsarecoolbutveryluminous(largeradii). StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 • HeliumBurning – IncreasedcoretemperatureignitesHe • Asaredgiantthecoreissmall(30,000km),compactandalmost pureHe. • ForHetoigniterequireshighT(2protonsperHeincreases repulsiveforces). • HydrogenburningshellincreasesthemassofHeatthecore. • Heignitesat108 K. – Triplealphaprocess • Helium“burns”releasingenergy(inthesamewaythatHydrogen reactionswork). 4 He + 4 He → 8 Be • 8Beisveryunstable(breaksdownto4He)A3rd4Hecancollide beforebreakdown. 8 Be + 4 He → 12 C + • Three4Hehaveformedtoproduce12Candaphoton(Gammaray). StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 – Triplealphaprocess • Anadditionalreactioncanoccurand12Ccanbeconvertedto16O releasingenergy. 12 C + 4 He → 16 O + • Heliumburningproducesmoreenergyandcreates heavier elements(metals). • Knownascoreheliumburningandlastsabout20%aslongasthe MSlifetime. • Theseelementsareessentialfortheproductionoflife. – HeliumFlash • Howtheheliumburningstartsdependsonthemassofthestar. • Inhighmassstars(>2M¤ )heliumburningstartsgraduallyas T® 108K. • InlowmassstarsHeburningissudden(afewminutes).Calledthe heliumflash. • Inidealgases pressure,temperatureanddensityarerelatedsimply • P:pressure ρ:density P = ρ kT T:temperature • Pressureincreases Heats Expands Cools(safetyvalve). StellarEvolution SurveyofAstronomyA110 – HeliumFlash Figure21-5 • Inredgiantthecoreisverycompressed • Atomsareionizedandthefreeelectronscannotbe compressedanymore(Pauli exclusionprinciple). • Coreactsasadegenerateelectrongas(pressureis independent oftemperature). • Heliumignites,temperatureincreases(pressuredoesnt increasetocompensate)andarunawayeffectoccurs. • Eventuallytemperatureincreasessothattheelectronsare nolongerdegenerateandcoreexpands. • Energygeneratedisusedincoreheatingandexpansionand absorbedbyouterlayersofthestar(wedont seetheflash). • Heburningcausesluminositytodecrease. • Thecoreexpands,coolsanddoesnt heatthesurroundingH shells(energyproductiondrops).Outershellscontract. • AfterHeflashstarishotter,smallerandlessluminous. StellarEvolution SurveyofAstronomyA110 • Lifeofastarcluster Figure21-6,21-8 – EvolutionofftheHRdiagram • Formationtime,timeontheMSandevolutionofftheMSaremass dependent. Time(yrs)Process <5x103 Ostarscontracttoprotostars 105 OstarsonMS 3x106 BandAstarsjoinMS 3x107 OstarsHdepletedandbecomeredgiants 7x107 Lowmassstars(M)onMS 108 G→ M starsleftonMS • Theturnoffpointofthemainsequenceindicatestheageofagroup (cluster)ofstars(color-magnitudediagram). • HorizontalbranchstarsarepostHeflashlowmassstarsundergoing coreHeandshellHburning(movebacktoredgiantphase). StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 StellarEvolution SurveyofAstronomyA110 LifeofaStar MolecularCloud <100K,lowdensity G r a v i t y Protostar 3000K,HighLuminosity,Large H i g n i t i o n ZeroAgeMS CoreT=106K,Small H u s a g e MainSequence 7 H 10 K,stable d e p l e t i o n RedGiant Hdepleted,Large,Cool LowMass<2M¤ HighMass>2M¤ HeliumFlash HeliumBurning Core108K,suddenHeburning Core108K,gradualHeburning StellarEvolution SurveyofAstronomyA110.