evoluon of high mass stars - texas a&m...
TRANSCRIPT
Evolu&onofHighMassStars
Astronomy101
HighMassStars
• O&BStars(M>4Msun):– BurnHot– LiveFast– DieYoung
• MainSequencePhase:– BurnHtoHeincoreviaCNOcycle– BuildupaHecore,likelow‐massstars– Lastsforonly~10Myr
MaximumMass:60-100Msun
• Ifastaristoomassive,thecoregetssohotthat:– Radia&onpressureovercomesgravity– Starbecomesunstable&disrupts.
• Ul&matelimitisnotpreciselyknown
• Suchstarsshouldbeveryrare.• Massivestarsliveontheedge...
RedSupergiantPhase
• AYerHcoreexhaus&on:– InertHecorecontracts&heatsup– HburninginashellaroundtheHecore– Huge,puffyenvelope~sizeoforbitofJupiter
• MoveshorizontallyacrosstheH‐Rdiagram:– Takes~1MyrtocrossH‐Rdiagram
CrossingtheSupergiantBranch
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10 -2
10 -4
Temperature (K)
Lum
inos
ity (L
sun)
Red Supergiant
RedSupergiantStar
Inert He
Core
H Burning Shell
Cool, Extended Envelope
Not to Scale
HeliumFlash
• CoreTemperaturereaches170MillionK• IgnitesHeliumburningtoC&O:– RapidPhase:~1Myr
– Heburninginthecore– Hburninginashell– StartbuildingaC‐Ocore
• StarbecomesaBlueSupergiant.
BlueSupergiant
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10 -2
10 -4
Temperature (K)
Lum
inos
ity (L
sun)
Blue Supergiant
Helium Flash
HeCoreExhaus&on
• WhenHerunsoutinthecore:– InertC‐Ocorecollapses&heatsup– H&Heburningmovesintoshells– BecomesaRedSupergiantagain
• C‐OCorecollapsesun&l:– Tcore>600MillionK– density>150,000g/cc
• IgnitesCarbonBurningintheCore.
EndofHeliumBurning
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10 -2
10 -4
Temperature (K)
Lum
inos
ity (L
sun)
CarbonBurning:
• Nuclearreac1onnetwork:12C+12Cfusesto:– 24Mg– 20Ne+4He– 16O+2×4He
• BuildupaninertO‐Ne‐Mgcore
• Veryinefficient:– Makesmanyneutrinos– Lastsonly~1000yearsbeforeCrunsout.
EndofCarbonBurningPhase:
Inert O-Ne-Mg
Core
C Burning Shell
Red Supergiant Envelope
He Burning Shell
H Burning Shell
IntermediateMassStars
• Starswith4<M<8Msun• AYer1000years:– InertO‐Ne‐Mgcorecontracts&heatsup
– C,He,&Hburningshells• Thermalpulsesdestabilizetheenvelope:– Ejecttheenvelopeinamassivestellarwind.
– LeaveO‐Ne‐Mgwhitedwarfcorebehind.
HighMassStars:M>8Msun
• AttheonsetofCarbonBurning:– Evolu&onissofastthattheenvelopecannolongerrespond.
– Shouldseelijleoutwardsignoftheinwardturmoiltocome.
• Excep&on:Strongstellarwindscanerodetheenvelope,changingtheoutwardappearanceofthestar.
NeonBurning
• O‐Ne‐Mgcorecontracts&heatsupun&l:– Tcore~1.5BillionK– density~107g/cc
• IgniteNeonburning:– reac&onnetworkmakesO,Mg,&others– Hugeneutrinolosses:>L*!– BuildsaheavyO‐Mgcore
• LastsforafewyearsbeforeNerunsout.
OxygenBurning
• Nerunsout,corecontracts&heatsupun&l:– Tcore~2.1BillionK– density~fewx107g/cc
• IgniteOxygenburning:– reac&onnetworkmakingSi,S,P,&others– Hugeneutrinolosses:>100,000L*!– BuildsaheavySicore.
• Lastsfor~1yearbeforeOrunsout.
SiliconBurning
• Orunsout,Sicorecontracts&heatsupun&l:– Tcore~3.5BillionK– density~108g/cc
• IgniteSiliconburning:– Simeltsintoaseaof4He,p,&n– FuseswithrestintoNickel(Ni)&Iron(Fe)– BuildsaheavyNi/Fecore.
• Lastsfor~1day...
TheNuclearImpasse
• Fusionoflightelementsreleasesnuclearbindingenergy.
• Iron(Fe)isthemost&ghtlyboundnucleus:– FusionofnucleilighterthanFereleaseenergy.– FusionofnucleiheavierthanFeabsorbenergy.
• OnceanFecoreforms,therearenonewfusionreac&onsleYforthestartotap.
EndofSiliconBurningPhase:
Inert Fe-Ni Core
Si Burning Shell
O Burning Shell
Ne Burning Shell
C Burning Shell
He Burning Shell
H Burning Shell
Envelope: ~ 5 AU
Core Radius: ~1 Rearth
EndoftheRoad
• AttheendoftheSiliconBurningDay:– StarbuildsupaninertFecore– Seriesofnestednuclearburningshells
• Finally,theFecoreexceeds1.2-2Msun:– Fecorebeginstocontract&heatup.– Thiscollapseisfinal&catastrophic
LastDaysofaMassiveStar
• Burnasuccessionofnuclearfuels:– Hydrogenburning:10Myr– Heliumburning:1Myr– Carbonburning:1000years– Neonburning:~10years– Oxygenburning:~1year– Siliconburning:~1day
• BuildupaninertIroncoreinthecenter.
InsideaMassiveStarontheBrink:
Inert Fe-Ni Core
Si Burning Shell
O Burning Shell
Ne Burning Shell
He Burning Shell
C Burning Shell
H Burning Shell
Envelope: ~ 5 AU
Core Radius: ~1 Rearth
IronCoreCollapse
• IroncorewithM~1.2‐2Msun– Collapses&beginstoheatup– ReachesT>10BillionK&density~1010g/cc
• Twoenergyconsumingprocesseskickin:1)NucleiphotodisintegrateintoHe,p&n
2)protons&electronscombinetoformneutrons&neutrinos.Neutrinosescape.
• Bothrobenergy,hasteningthecore’scollapse
CatastrophicCollapse
• StartofIronCorecollapse:– Radius~6000km(~Rearth)
– Density~108g/cc• Within1second:– Radius~50km– Density~1014g/cc– CollapseSpeed~0.25c!
CoreBounce
• Densityofcollapsingcorehits~2.4x1014g/cc=densityofatomicnuclei!
• Strongnuclearforcecomesintoplay!
• Inner0.7Msunofthecore:– comestoascreechinghalt– overshoots&springsbackalijle(“bounces”)
• Infallinggashitsthebouncingcorehead‐on!
Post‐BounceShockwave
• Shockwavespreadsoutfromcorebounce:– Kine&cEnergyis~1051ergs!– StallsoutaYeronly25‐40millisecbecauseofatrafficjambetweeninfalling&ouplowinggas.
• Meanwhile,neutrinospouroutofthecore:– trappedbythedensesurroundinggas– leadstorapidhea&ngofthegas– inturnleadstoviolentconvec&on
New,ImprovedShockwave
• Violentconvec&onbreaksthetrafficjam
• Shockwaveisregeneratedin~300millisec.
• Smashesoutthroughthestar:– Breakoutspeed~0.1c!– Explosivenuclearfusioninwakeofblastproducesmoreheavyelements
– Heatsupandacceleratesenvelopegas• Inafewhours,shockbreaksoutofthesurface
Supernova!
• Atshockbreakout:– Starbrightensto~10BillionLsuninminutes.
– Canoutshineanen®alaxyofstars!• Outerenvelopeblastedoff:– acceleratedtoafew×10,000km/sec– gasexpands&coolsoff
• Supernovafadesoutoverafewmonths.
HistoricalSupernovae
• 1054AD:“GuestStar”inTaurusobservedbyChineseastronomers(Songdynasty).– Visibleindaylightfor23days.
• 1572:TychoBrahe’sSupernova• 1604:JohannesKepler’sSupernova• 6000‐8000BC:Velasupernova– observedbytheSumerians;appearsinlegendsaboutthegodEa.
CrabSupernova
Supernova1987a
• Nearestvisiblesupernovasince1604.• January1987:– 15MsunBlueSupergiantStarSK‐69o202ExplodedintheLargeMagellanicCloud.
– Sawapulseofneutrinos,thentheblast.– Con&nuedtofollowitforthelastdecade.
• Wealthofinforma&ononsupernovaphysics.
Nucleosynthesis
• StartwithHydrogen&Helium:– FuseHydrogenintoelementsuptoIron/Nickel
– Theseaccumulateinthecorelayersofstars.
• SupernovaExplosion:– “explosive”nuclearfusionbuildsmorelightelementsuptoIron&Nickel.
– fast&slowneutronreac&onsbuildIron&Nickelintoheavyelementsupto254Cf
TopTenMostAbundantElements• 10)Sulfur• 9)Magnesium• 8)Iron• 7)Silicon• 6)Nitrogen• 5)Neon• 4)Carbon• 3)Oxygen• 2)Helium
• 1)Hydrogen
SupernovaRemnants
• Whathappenstotheenvelope?– Enrichedwithmetalsintheexplosion
– Expandsatafew×10,000km/sec
• SupernovaBlastWave:– Plowsupthesurroundinginterstellargas– Heats&s&rsuptheinterstellarmedium
– HotenoughtoshineasionizednebulaeuptoafewthousandyearsaYertheexplosion
Stardust
• Metal‐enrichedgasmixeswithinterstellargas– Nextgenera&onofstarsincludesthesemetals.– Successivegenera&onsaremoremetalrich.
• Sun&planets(&us):– Containmanymetals(iron,silicon,etc.)– Only~5Gyrold
• TheSolarSystemformedfromgasenrichedbyapreviousgenera&onofmassivestars.
CygnusLoop:ScrapsofanoldSupernovaRemnant
Summary:
• EndoftheLifeofaMassiveStar:– BurnHthroughSiinsuccessivecores– FinallybuildamassiveIroncore.
• Ironcorecollapse&corebounce• SupernovaExplosion:– Explosiveenvelopeejec&on– Mainsourcesofheavyelements
Ques&ons:
• WheredidelementslikeU,Th,Pb,Au,Ag,etc.comefrom?
• WheredidC,O,N,etc.comefrom?
• HowdidallthatgetmixedupintheSun?
• DoSupernovaes&llexplodeintheUniverse?• WhatwouldhappenifaSnexplodedneartheEarth?