天文学英文版PPT资料 (17)

UniverseEighthEditionRogerA.Freedman•WilliamJ.KaufmannIIICHAPTER18TheBirthofStarsChapter18onlinequizdueThursdaynight11/11HWMain-sequencestarshavemassesgreaterthanabout0.08solarmasses.Thereasonforthisisthatgascloudssmallerthan0.08solarmassesA.donotdevelopthenecessaryhightemperatureandpressurerequiredfornuclearfusionwhentheycollapse.B.aretoosmalltobegintocollapse.C.generateenoughenergythattheyfragmentintosmallermassobjectswhentheycollapse.D.becomewhitedwarfstars.E.donotexistasfarasweknow.Q18.9Main-sequencestarshavemassesgreaterthanabout0.08solarmasses.Thereasonforthisisthatgascloudssmallerthan0.08solarmassesA.donotdevelopthenecessaryhightemperatureandpressurerequiredfornuclearfusionwhentheycollapse.B.aretoosmalltobegintocollapse.C.generateenoughenergythattheyfragmentintosmallermassobjectswhentheycollapse.D.becomewhitedwarfstars.E.donotexistasfarasweknow.A18.9KeyIdeasStellarEvolution:Becausestarsshinebythermonuclearreactions,theyhaveafinitelifespan.Thetheoryofstellarevolutiondescribeshowstarsformandchangeduringthatlifespan.TheInterstellarMedium:Interstellargasanddust,whichmakeuptheinterstellarmedium,areconcentratedinthediskoftheGalaxy.Cloudswithintheinterstellarmediumarecallednebulae.KeyIdeasDarknebulaearesodensethattheyareopaque.Theyappearasdarkblotsagainstabackgroundofdistantstars.Emissionnebulae,orHIIregions,areglowing,ionizedcloudsofgas.Emissionnebulaearepoweredbyultravioletlightthattheyabsorbfromnearbyhotstars.Reflectionnebulaeareproducedwhenstarlightisreflectedfromdustgrainsintheinterstellarmedium,producingacharacteristicbluishglow.KeyIdeasProtostars:Starformationbeginsindense,coldnebulae,wheregravitationalattractioncausesaclumpofmaterialtocondenseintoaprotostar.Asaprotostargrowsbythegravitationalaccretionofgases,Kelvin-Helmholtzcontractioncausesittoheatandbeginglowing.Itsrelativelylowtemperatureandhighluminosityplaceitintheupper-rightregiononanH-Rdiagram.FurtherevolutionofaprotostarcausesittomovetowardthemainsequenceontheH-Rdiagram.Whenitscoretemperaturesbecomehighenoughtoignitesteadyhydrogenburning,itbecomesamain-sequencestar.Themoremassivetheprotostar,themorerapidlyitevolves.KeyIdeasMassLossbyProtostars:Inthefinalstagesofpre–main-sequencecontraction,whenthermonuclearreactionsareabouttobegininitscore,aprotostarmayejectlargeamountsofgasintospace.Low-massstarsthatvigorouslyejectgasarecalledTTauristars.Acircumstellaraccretiondiskprovidesmaterialthatayoungstarejectsasjets.ClumpsofglowinggascalledHerbig-Haroobjectsaresometimesfoundalongthesejetsandattheirends.KeyIdeasStarClusters:Newbornstarsmayformanopenorgalacticcluster.Starsareheldtogetherinsuchaclusterbygravity.Occasionallyastarmovingmorerapidlythanaveragewillescape,or“evaporate,”fromsuchacluster.Astellarassociationisagroupofnewbornstarsthataremovingapartsorapidlythattheirgravitationalattractionforoneanothercannotpullthemintoorbitaboutoneanother.KeyIdeasOandBStarsandTheirRelationtoHIIRegions:ThemostmassiveprotostarstoformoutofadarknebularapidlybecomemainsequenceOandBstars.Theyemitstrongultravioletradiationthationizeshydrogeninthesurroundingcloud,thuscreatingthereddishemissionnebulaecalledHIIregions.UltravioletradiationandstellarwindsfromtheOandBstarsatthecoreofanHIIregioncreateshockwavesthatmoveoutwardthroughthegascloud,compressingthegasandtriggeringtheformationofmoreprotostars.KeyIdeasGiantMolecularClouds:ThespiralarmsofourGalaxyarelacedwithgiantmolecularclouds,immensenebulaesocoldthattheirconstituentatomscanformintomolecules.Star-formingregionsappearwhenagiantmolecularcloudiscompressed.Thiscanbecausedbythecloud’spassagethroughoneofthespiralarmsofourGalaxy,byasupernovaexplosion,orbyothermechanisms.