Download Lecture 10 - Concord University

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