A2Physics物理出國(guó)英語(yǔ)AstrophysicsMS

1.Astarisinhydrostaticequilibrium.Explainwhatismeantbyhydrostaticequilibrium.

Hydrostaticequilibriuminastarexistswhentheinwardactinggravitationalforceisbalancedbytheoutwardactingpressureforce.Thegravitationalforcetriestocompressthestar,pullingallthemasstowardsthecenter.Thepressureforce,whichisaresultofthehightemperatureanddensityinthestar'sinterior,actsintheoppositedirection,pushingthematteroutwards.Whenthesetwoforcesareequal,thestarmaintainsastablesizeandshape.Ifthegravitationalforcewerestronger,thestarwouldstarttocontract;ifthepressureforcewerestronger,thestarwouldexpand.

2.Statethesourceoftheenergythatmaintainsthehightemperatureinthecoreofamainsequencestar.

Thesourceoftheenergythatmaintainsthehightemperatureinthecoreofamainsequencestarisnuclearfusion.InthecoreofamainsequencestarlikeourSun,hydrogennuclei(protons)fusetogethertoformheliumnucleithroughaseriesofreactionsknownastheprotonprotonchain.Duringthesefusionreactions,asmallamountofmassisconvertedintoenergyaccordingtoEinstein'smassenergyequivalenceprinciple(E=mc^{2}).Thisreleasedenergyheatsupthecoreofthestar,maintainingthehightemperaturerequiredforfurtherfusionreactions.

3.Explainwhyaredgiantstarisred.

Aredgiantstarisredbecauseofitsrelativelylowsurfacetemperature.ThecolorofastarisrelatedtoitssurfacetemperaturethroughWien'sdisplacementlaw(lambda_{max}=frac{T}),where(lambda_{max})isthewavelengthatwhichthestaremitsthemaximumamountofradiation,(b)isWien'sconstant((b=2.898times10^{3}mcdotK)),and(T)isthesurfacetemperatureofthestar.Asastarevolvesintoaredgiant,itsouterlayersexpandandcool.Alowersurfacetemperaturemeansthatthepeakwavelengthofitsemittedradiationshiftstowardsthelongerwavelengthendoftheelectromagneticspectrum.Redlighthasalongerwavelengthcomparedtoothervisiblecolors,sothestarappearsred.

4.DescribetheprocessbywhichastarliketheSunevolvesfromthemainsequencetoawhitedwarf.

Asamainsequencestar,theSunfuseshydrogenintoheliuminitscore.Whenthehydrogeninthecoreisexhausted,thecorecontractsundergravity.Thiscontractionreleasesgravitationalpotentialenergy,whichheatsupashellofhydrogenaroundthecore.Thehydrogeninthisshellthenstartstofuse,causingtheouterlayersofthestartoexpandandcool,turningthestarintoaredgiant.

Intheredgiantphase,thecorecontinuestocontractuntilitishotenoughforheliumfusiontooccur.Heliumfusesintocarbonthroughthetriplealphaprocess.Oncetheheliuminthecoreisexhausted,ifthestarisnotmassiveenoughtoinitiatefurtherfusionreactions,thecorecollapsesagain.

Theouterlayersoftheredgiantareejectedintospace,formingaplanetarynebula.Theremainingcore,whichismainlycomposedofcarbonandoxygen,isverydense.Itnolongerhasasourceofnuclearenergy,butitisextremelyhot.Overtime,thiscoreradiatesawayitsheatandcoolsdown,eventuallybecomingawhitedwarf.Awhitedwarfissupportedagainstfurthercollapsebyelectrondegeneracypressure.

5.Explaintheconceptofablackhole'seventhorizon.

Theeventhorizonofablackholeisaboundaryinspacetimearoundablackholebeyondwhichnothing,notevenlight,canescape.ItisdefinedbytheSchwarzschildradius(R_s=frac{2GM}{c^{2}}),where(G)isthegravitationalconstant,(M)isthemassoftheblackhole,and(c)isthespeedoflight.

Onceanobjectcrossestheeventhorizon,thegravitationalpulloftheblackholeissostrongthattheescapevelocityexceedsthespeedoflight.Sincenothingcantravelfasterthanlight,anyobjectthathaspassedtheeventhorizonistrappedinsidetheblackhole.Theeventhorizonactsasaonewaymembrane;matterandradiationcanentertheblackholethroughit,butnothingcancomeout.

6.StatetwodifferencesbetweenatypeIandatypeIIsupernova.

1.Stellarmassandprogenitor:AtypeIsupernovaoccursinabinarystarsystemwhereawhitedwarfaccretesmatterfromitscompanionstar.IfthemassofthewhitedwarfexceedstheChandrasekharlimit((1.4M_{odot})),arunawaynuclearfusionreactionoccurs,leadingtoasupernovaexplosion.AtypeIIsupernova,ontheotherhand,occursattheendofthelifeofamassivestar(morethanabout8(M_{odot}))whenthecoreofthestarcollapsesafterrunningoutofnuclearfuel.

2.Spectrallines:TypeIsupernovaedonotshowhydrogenspectrallinesintheirspectra.Thisisbecausetheprogenitorwhitedwarfhasalreadyexhausteditshydrogenduringitsevolution.TypeIIsupernovaeshowprominenthydrogenspectrallinesbecausetheouterlayersofthemassiveprogenitorstarstillcontainasignificantamountofhydrogen.

7.Explainhowthemassofastaraffectsitslifespanonthemainsequence.

Themassofastarisacrucialfactorindeterminingitsmainsequencelifespan.Theluminosity(L)ofastarisrelatedtoitsmass(M)bythemassluminosityrelation(LproptoM^{3.5})(approximateformainsequencestars).Theamountoffuel(hydrogen)availableforfusioninastarisproportionaltoitsmass(M).

Thelifespan(t)ofastaronthemainsequenceisgivenbytheratiooftheavailablefueltotherateatwhichitisconsumed.Sincetherateofenergyproduction(luminosity)istherateatwhichfuelisconsumed,(t=frac{M}{L}).Substituting(LproptoM^{3.5})intotheequation,weget(tproptoM^{2.5}).

Thismeansthatmoremassivestarshaveashortermainsequencelifespan.Amassivestarhasmorefuel,butitburnsthroughitatamuchfasterrateduetoitshighluminosity.Forexample,averymassivestarmayhaveamainsequencelifespanofonlyafewmillionyears,whilealowmassstarlikeareddwarfcanhaveamainsequencelifespanoftrillionsofyears.

8.WhatisthesignificanceoftheChandrasekharlimitinstellarevolution?

TheChandrasekharlimit,approximately(1.4M_{odot}),isofgreatsignificanceinstellarevolution.Itisthemaximummassthatawhitedwarfcanhaveandstillbesupportedbyelectrondegeneracypressure.

IfawhitedwarfinabinarysystemaccretesmatterfromitscompanionstaranditsmassexceedstheChandrasekharlimit,electrondegeneracypressureisnolongerabletocounteractthegravitationalforce.Thisleadstoacatastrophiccollapseofthewhitedwarf,triggeringatypeIsupernovaexplosion.

Inthecontextoftheendstageofstellarevolution,theChandrasekharlimitdetermineswhetherastarwillendupasawhitedwarforundergoamoreviolentendstagesuchasasupernova.StarswithamasslessthantheChandrasekharlimitaftersheddingtheirouterlayerscanformstablewhitedwarfs,whilethosethatexceedthislimitthroughmassaccretionorothermeanswillexperienceamuchmoreenergeticevent.

9.Describethecharacteristicsofaneutronstar.

AneutronstarisanextremelydensecelestialobjectthatformsafteratypeIIsupernova.

Highdensity:Neutronstarsareincrediblydense.Theirmassistypicallybetween1.4and3(M_{odot}),buttheyhavearadiusofonlyabout1020km.Thedensityofaneutronstariscomparabletothedensityofanatomicnucleus.

Rapidrotation:Manyneutronstarsrotateveryrapidly.Thisisduetotheconservationofangularmomentumduringthecollapseofthemassivestar'score.Asthecorecollapsesfromamuchlargersizetoaverysmallradius,itsrotationspeedincreasessignificantly.Someneutronstars,knownaspulsars,canrotatehundredsoftimespersecond.

Strongmagneticfield:Neutronstarshaveextremelystrongmagneticfields,whichcanbeupto(10^{12})timesstrongerthantheEarth'smagneticfield.Themagneticfieldplaysacrucialroleintheemissionofradiationfrompulsars.

Highsurfacetemperature:Thesurfacetemperatureofaneutronstarisveryhigh,ontheorderofmillionsofKelvin,duetotheenergyreleasedduringthecollapseandthesubsequentcoolingprocess.

10.Explainhowapulsaremitsregularpulsesofradiation.

Apulsarisarotatingneutronstarwithastrongmagneticfield.Themagneticaxisoftheneutronstarisusuallymisalignedwithitsrotationaxis.

Chargedparticles,suchaselectronsandprotons,areacceleratedalongthemagneticfieldlinesnearthemagneticpolesoftheneutronstar.Theseacceleratedchargedparticlesemitradiationinanarrowbeam,similartoalighthousebeam.

Astheneutronstarrotates,thebeamofradiationsweepsacrossspace.IftheEarthliesinthepathofthisrotatingbeam,weobserveregularpulsesofradiation.Thetimeintervalbetweenthepulsesisequaltotherotationperiodoftheneutronstar.So,therotationoftheneutronstarcombinedwiththeemissionofafocusedradiationbeamisresponsiblefortheregularpulsesthatwedetectfrompulsars.

11.StatetherelationshipbetweentheperiodofaCepheidvariablestaranditsluminosity.

TheperiodluminosityrelationshipforCepheidvariablestarsstatesthatthereisadirectcorrelationbetweentheperiodofvariationofaCepheidvariablestaranditsintrinsicluminosity.LongerperiodCepheidvariableshavehigherluminosities.

Mathematically,if(P)istheperiodoftheCepheidvariable(thetimeittakesforthestar'sbrightnesstogothroughonecompletecycleofvariation)and(L)isitsluminosity,therelationshipcanbeapproximatedbyapowerlawfunction.BymeasuringtheperiodofaCepheidvariablestar,astronomerscandetermineitsluminosity.Then,bycomparingtheobservedbrightness(apparentmagnitude)ofthestarwithitsknownluminosity(absolutemagnitude),thedistancetothestarcanbecalculatedusingthedistancemagnitudeformula.

12.ExplainhowCepheidvariablestarsareusedtomeasureastronomicaldistances.

First,astronomersobserveaCepheidvariablestarandmeasureitsperiodofbrightnessvariation.Usingtheperiodluminosityrelationship,theycandeterminethestar'sintrinsicluminosity(absolutemagnitude(M)).

Next,theymeasuretheapparentmagnitude(m)oftheCepheidvariablestar.Thedistance(d)(inparsecs)tothestarcanbecalculatedusingthedistancemagnitudeformula(mM=5log_{10}(d)5).

Byrearrangingtheformula,weget(d=10^{frac{(mM+5)}{5}}).SincetheluminosityoftheCepheidvariablecanbeaccuratelydeterminedfromitsperiod,andtheapparentmagnitudecanbemeasureddirectly,thismethodprovidesareliablewaytomeasuredistancestostarswithinourgalaxyandtonearbygalaxies.Cepheidvariablesareoftenreferredtoas"standardcandles"becausetheirknownluminositiesallowfordistancemeasurementsinastronomy.

13.DescribetheBigBangtheory.

TheBigBangtheoryistheprevailingcosmologicalmodelfortheoriginandevolutionoftheuniverse.Itstatesthattheuniversebeganasanextremelyhotanddensepoint,oftenreferredtoasasingularity,approximately13.8billionyearsago.

AtthemomentoftheBigBang,allmatter,energy,space,andtimewerecreated.Intheinitialmoments,theuniversewasfilledwithahot,denseplasmaofelementaryparticlessuchasquarks,leptons,andphotons.Astheuniverseexpanded,itcooleddown.

Afterabout100seconds,protonsandneutronsbegantocombinetoformthelightestatomicnuclei,suchashydrogen,helium,andasmallamountoflithiumthroughaprocesscalledBigBangnucleosynthesis.

Astheuniversecontinuedtoexpandandcoolfurther,electronscombinedwithatomicnucleitoformneutralatoms.Thisallowedphotonstotravelfreelythroughspace,andtheuniversebecametransparent.Theleftoverradiationfromthiseraisknownasthecosmicmicrowavebackground(CMB),whichisobservedasanearlyuniformradiationfieldwithatemperatureofabout2.725Ktoday.

Overtime,matterbegantoclumptogetherundertheinfluenceofgravity,formingstars,galaxies,andlargescalestructuresintheuniverse.

14.ExplaintheevidencefortheBigBangtheory.

Cosmicmicrowavebackground(CMB):TheCMBistheafterglowoftheBigBang.Itisanearlyuniformradiationfieldthatfillstheentireuniverse,withablackbodyspectrumcorrespondingtoatemperatureofabout2.725K.ThediscoveryoftheCMBin1965wasamajorpieceofevidencefortheBigBangtheory.TheuniformityandthespecifictemperatureoftheCMBareconsistentwiththepredictionsoftheBigBangmodel,whichstatesthattheearlyuniversewasahot,denseplasmathatcooledasitexpanded.

Hubble'slaw:Hubble'slawstatesthattherecessionalvelocity(v)ofagalaxyisproportionaltoitsdistance(d)fromus,(v=H_0d),where(H_0)istheHubbleconstant.Thisimpliesthattheuniverseisexpanding.Iftheuniverseisexpanding,theninthepast,allmatterintheuniversemusthavebeenclosertogether.Extrapolatingthisexpansionbackwardsintimeleadstotheconclusionthattheuniversebeganfromasinglepoint,whichistheessenceoftheBigBangtheory.

Abundanceoflightelements:Theobservedabundancesoflightelementssuchashydrogen,helium,andlithiumintheuniverseareingoodagreementwiththepredictionsofBigBangnucleosynthesis.Intheearlyuniverse,duringthefirstfewminutesaftertheBigBang,theconditionswererightfortheformationoftheselightelementsthroughnuclearreactions.ThecalculatedabundancesbasedontheBigBangmodelmatchtheabundancesmeasuredinstars,galaxies,andtheinterstellarmedium.

15.Whatisthesignificanceofthecosmicmicrowavebackgroundradiation?

Thecosmicmicrowavebackground(CMB)radiationisofgreatsignificanceincosmology.

ConfirmationoftheBigBangtheory:ItprovidesstrongevidencefortheBigBangtheory.ThefactthattheCMBhasablackbodyspectrumwithatemperatureofabout2.725Kisconsistentwiththepredictionthattheearlyuniversewasahot,denseplasmathatcooledasitexpanded.TheuniformityoftheCMBacrosstheskyalsosupportstheideathattheuniversewasonceinahighlyhomogeneousandisotropicstate.

Probeoftheearlyuniverse:TheCMBallowsustostudytheconditionsoftheuniversewhenitwasonlyabout380,000yearsold.TinyfluctuationsinthetemperatureoftheCMB,ontheorderofonepartin100,000,arethoughttobetheseedsfromwhichgalaxiesandlargescalestructuresintheuniverseformed.Byanalyzingthesefluctuations,astronomerscanlearnaboutthedensityvariationsandtheinitialconditionsoftheuniverse.

Testingcosmologicalmodels:ThepropertiesoftheCMB,suchasitspowerspectrum,areusedtotestdifferentcosmologicalmodels.DeviationsfromtheexpectedbehavioroftheCMBcanindicatenewphysicsormodificationstothestandardcosmologicalmodel.

16.ExplainhowHubble'slawisusedtoestimatetheageoftheuniverse.

Hubble'slawisgivenby(v=H_0d),where(v)istherecessionalvelocityofagalaxy,(d)isitsdistancefromus,and(H_0)istheHubbleconstant.

Toestimatetheageoftheuniverse,weassumethattheexpansionoftheuniversehasbeenoccurringataconstantrate(asimplifyingassumption).Ifweconsideragalaxymovingawayfromusatavelocity(v)andatadistance(d),thetime(t)ithastakenforthegalaxytoreachitscurrentpositionfromaninitialpoint(whenallmatterwastogether)canbecalculatedusingtheformula(t=fracxj1zxtr{v}).

FromHubble'slaw(v=H_0d),wecansubstitute(v)inthetimeformula.So,(t=fracr1zzj1t{H_0d}=frac{1}{H_0}).ThereciprocaloftheHubbleconstant(frac{1}{H_0})givesanestimateoftheageoftheuniverse.

If(H_0)isexpressedinunitsofkm/s/Mpc(kilometerspersecondpermegaparsec),afterconvertingtheunitsappropriately,wecancalculatetheageoftheuniverse.Forexample,if(H_0=70km/s/Mpc),theestimatedageoftheuniverseisabout14billionyears.

17.Describetheconceptofdarkmatter.

Darkmatterisaformofmatterthatdoesnotemit,absorb,orreflectlightorotherformsofelectromagneticradiation,makingitinvisibletotraditionalastronomicalobservations.

However,itspresencecanbeinferredfromitsgravitationaleffectsonvisiblematter.Forexample,ingalaxies,theobservedrotationcurvesofstarsandgasintheouterregionsofgalaxiesdonotmatchthepredictionsbasedonthevisiblemassalone.AccordingtoNewtoniangravity,theorbitalspeedofstarsshoulddecreasewithincreasingdistancefromthegalacticcenter.Butobservationsshowthattheorbitalspeedsremainrelativelyconstantintheouterregionsofgalaxies.Thisimpliesthatthereisadditionalmasspresentinthegalaxiesthatwecannotsee,whichisattributedtodarkmatter.

Darkmatteristhoughttomakeupabout27%ofthetotalmassenergycontentoftheuniverse.Itisbelievedtoplayacrucialroleintheformationandevolutionofgalaxiesandlargescalestructuresintheuniverse,asitsgravitationalpullhelpstogatherordinarymatterandformthestructuresthatweobservetoday.

18.Explaintheevidencefortheexistenceofdarkmatter.

Galacticrotationcurves:Asmentionedearlier,therotationcurvesofgalaxiesshowthattheorbitalspeedsofstarsandgasintheouterregionsofgalaxiesremainconstantorevenincreaseslightlywithdistancefromthegalacticcenter.Thisisnotconsistentwiththedistributionofvisiblematterinthegalaxies.Theadditionalmassrequiredtoexplaintheserotationcurvesisthoughttobedarkmatter.

Gravitationallensing:Gravitationallensingoccurswhenthegravitationalfieldofamassiveobjectbendsthepathoflightfromamoredistantobject.Observationsofgravitationallensingingalaxyclustersshowthattheamountofmassrequiredtoproducetheobservedlensingeffectsismuchgreaterthanthemassofthevisiblematterintheclusters.Thisexcessmassislikelyduetodarkmatter.

Cosmicmicrowavebackground(CMB):ThesmalltemperaturefluctuationsintheCMBareaffectedbythepresenceofdarkmatter.Thepatternsofthesefluctuationscanbeanalyzedtoinfertheamountanddistributionofdarkmatterintheearlyuniverse.TheagreementbetweentheobservedCMBfluctuationsandthepredictionsofcosmologicalmodelsthatincludedarkmatterprovidesfurtherevidenceforitsexistence.

19.Whatisdarkenergy?

Darkenergyisamysteriousformofenergythatisthoughttopermeateallofspace.Itisresponsiblefortheobservedacceleratedexpansionoftheuniverse.

Inthe1990s,observationsofdistantsupernovaeshowedthattheexpansionoftheuniverseisnotslowingdownasexpectedduetothegravitationalattractionofmatter.Instead,theexpansionisaccelerating.Toexplainthisacceleration,scientistsproposedtheexistenceofdarkenergy.

Darkenergyisestimatedtomakeupabout68%ofthetotalmassenergycontentoftheuniverse.Ithasanegativepressure,whichcounteractsthegravitationalattractionofmatterandcausestheuniversetoexpandataneverincreasingrate.Thenatureofdarkenergyisstillunknown,anditisoneofthebiggestunsolvedproblemsinmoderncosmology.

20.Explaintheevidencefortheacceleratedexpansionoftheuniverse.

TheevidencefortheacceleratedexpansionoftheuniversecomesmainlyfromobservationsofdistanttypeIasupernovae.

TypeIasupernovaeareconsideredtobe"standardcandles"becausetheyhavearelativelyconsistentpeakluminosity.Bymeasuringtheapparentbrightnessofthesesupernovaeandcomparingitwiththeirknownluminosity,astronomerscandeterminetheirdistances.

Atthesametime,theredshiftofthelightfromthesesupernovaeismeasured.Redshiftisrelatedtotherecessionalvelocityoftheobjectand,byextension,totheexpansionoftheuniverse.

ObservationsofdistanttypeIasupernovaerevealedthattheyarefainterthanexpectedbasedonauniversethatisexpandingataconstantrateordecelerating.Thismeansthatthesesupernovaearefartherawaythantheywouldbeinanonacceleratinguniverse.Inotherwords,theexpansionoftheuniversehasbeenspeedingupovertime,whichledtothediscoveryoftheacceleratedexpansionandtheproposalofdarkenergytoexplainthisphenomenon.

21.Describethedifferencebetweenapparentmagnitudeandabsolutemagnitude.

Apparentmagnitude((m))isameasureofthebrightnessofastarasitappearstoanobserveronEarth.Itdependsonboththeintrinsicluminosityofthestaranditsdistancefromus.AlowerapparentmagnitudevalueindicatesabrighterstarasseenfromEarth.Forexample,astarwithanapparentmagnitudeof1isbrighterthanastarwithanapparentmagnitudeof6.

Absolutemagnitude((M))isameasureoftheintrinsicluminosityofastar.Itisdefinedastheapparentmagnitudethatastarwouldhaveifitwereplacedatastandarddistanceof10parsecsfromtheobserver.Bycomparingtheabsolutemagnitudesofdifferentstars,wecandirectlycomparetheirtrueluminosities.

Therelationshipbetweenapparentmagnitude,absolutemagnitude,anddistance((d)inparsecs)isgivenbytheformula(mM=5log_{10}(d)5).

22.ExplainhowtheHertzsprungRussell(HR)diagramisconstructed.

TheHertzsprungRusselldiagramisaplotthatshowstherelationshipbetweentheluminosity(orabsolutemagnitude)andthesurfacetemperature(orspectraltype)ofstars.

ToconstructanHRdiagram,astronomersfirstmeasurethepropertiesofalargenumberofstars.Thesurfacetemperatureofastarcanbedeterminedfromitsspectraltype.StarsareclassifiedintospectraltypesO,B,A,F,G,K,M,withOtypestarsbeingthehottestandMtypestarsbeingthecoolest.

Theluminosityofastarcanbecalculatedindifferentways.Onemethodistomeasuretheapparentmagnitudeofthestarandthenusethedistancetothestar(determinedthroughmethodssuchasparallaxortheuseofstandardcandles)tocalculatetheabsolutemagnitude,whichisrelatedtotheluminosity.

Oncethesurfacetemperatureandluminosity(orabsolutemagnitude)ofalargenumberofstarsareknown,eachstarisplottedonagraphwiththesurfacetemperature(orspectraltype)onthehorizontalaxis(usuallydecreasingfromlefttoright)andtheluminosity(orabsolutemagnitude)ontheverticalaxis(withhigherluminositiesatthetop).

23.IdentifythemainregionsonanHRdiagramanddescribethetypesofstarsfoundineachregion.

Mainsequence:Themainsequenceisadiagonalbandthatrunsfromthetopleft(hot,luminousstars)tothebottomright(cool,lessluminousstars)oftheHRdiagram.Starsonthemainsequenceareinthestageoftheirliveswheretheyarefusinghydrogenintoheliumintheircores.Moststars,includingourSun,spendthemajorityoftheirlivesonthemainsequence.Themassofamainsequencestardeterminesitspositiononthemainsequence,withmoremassivestarsbeinghotterandmoreluminous.

Redgiantsandsupergiants:ThesearelocatedinthetoprightregionoftheHRdiagram.Redgiantsandsupergiantsarelarge,cool,andveryluminousstars.Theyareinalaterstageofstellarevolution,havingexhaustedthehydrogenintheircoresandstartedtoexpandandcool.Supergiantsareevenmoremassiveandluminousthanredgiants.

Whitedwarfs:WhitedwarfsarefoundinthebottomleftregionoftheHRdiagram.Theyaresmall,hot,andrelativelyfaintstars.Whitedwarfsaretheremnantsoflowtomediummassstarsthathaveexhaustedtheirnuclearfuelandshedtheirouterlayers.Theyaresupportedbyelectrondegeneracypressure.

24.ExplainhowthepositionofastarontheHRdiagramchangesasitevolves.

Astarbeginsitslifeonthemainsequence.Thepositionofastaronthemainsequenceisdeterminedbyitsmass.Moremassivestarsarelocatedatthetopleftofthemainsequence(hotandluminous),whilelessmassivestarsareatthebottomright(coolandlessluminous).

Asastarexhauststhehydrogeninitscore,itleavesthemainsequence.Thecorecontracts,andtheouterlayersexpandandcool.ThiscausesthestartomoveupwardsandtotherightontheHRdiagram,becomingaredgiantorasupergiantdependingonitsmass.

Ifthestarisalowtomediummassstar,aftertheredgiantphase,itejectsitsouterlayerstoformaplanetarynebula.Theremainingcorebecomesawhitedwarf,whichmovesdownwardsandtotheleftontheHRdiagramasitcoolsovertime.

Foramassivestar,aftertheredsupergiantphase,itmayenditslifeinasupernovaexplosion.Theremnantcouldbeaneutronstarorablackhole,whichdonothaveadirectrepresentationonthetraditionalHRdiagramastheydonotemitlightinthesamewayasnormalstars.

25.Describetheprocessofnuclearfusioninthecoreofamassivestar.

Inthecoreofamassivestar,nuclearfusionoccursinaseriesofstages.

Hydrogenfusion:Likemainsequencestars,massivestarsfirstfusehydrogenintoheliumthroughtheprotonprotonchainortheCNOcycle(carbonnitrogenoxygencycle).TheCNOcycleismoreimportantinmoremassivestarsbecauseitrequireshighertemperatures.IntheCNOcycle,carbon,nitrogen,andoxygenactascatalyststofacilitatethefusionofhydrogenintohelium.

Heliumfusion:Oncethehydrogeninthecoreisexhausted,thecorecontractsandheatsupuntilheliumfusioncanoccur.Heliumnuclei(alphaparticles)fusetogetherthroughthetriplealphaprocesstoformcarbon.Insomecases,furtherreactionscanoccurtoproduceoxygenfromcarbonandhelium.

Heavierelementfusion:Inverymassivestars,afterheliumisexhausted,thecorecancontinuetocontractandheatupenoughforthefusionofheavierelements.Carboncanfusewithothercarbonnucleitoformelementssuchasneon,magnesium,andsodium.Neoncanthenfusetoformoxygenandmagnesium,andsoon.Thisprocesscontinuesuptotheformationofironinthecore.Fusionreactionsbeyondirondonotreleaseenergybutinsteadrequireenergyinputbecausethebindingenergypernucleonismaximumforiron.

26.Explainwhythefusionofelementsbeyondirondoesnotreleaseenergy.

Thebindingenergypernucleonisakeyconceptinunderstandingnuclearfusionreactions.Thebindingenergypernucleonistheenergyrequiredtoseparateanucleusintoitsindividualnucleons(protonsandneutrons),dividedbythenumberofnucleonsinthenucleus