天文学英文版PPT资料 (18)

UniverseEighthEditionRogerA.Freedman•WilliamJ.KaufmannIIICHAPTER23OurGalaxyReadChapters23,24,26and27Chapter23quizdueWednesday12/1Exam4scheduledforMonday12/13,5-7PMHWOursolarsystemislocatedA.nearaglobularclusterinthehaloofourGalaxy.B.nearaglobularclusterinaspiralarmofourGalaxy.C.inaspiralarmofourGalaxy.D.inthenucleusofourGalaxy.E.inthediskofourGalaxy,butnotwithinaspiralarm.Q23.5OursolarsystemislocatedA.nearaglobularclusterinthehaloofourGalaxy.B.nearaglobularclusterinaspiralarmofourGalaxy.C.inaspiralarmofourGalaxy.D.inthenucleusofourGalaxy.E.inthediskofourGalaxy,butnotwithinaspiralarm.A23.5MostastronomersthinkthatthereisdarkmatterinourGalaxybecauseA.starsintheouteredgesofourGalaxymovefasterthanexpected.B.starsintheouteredgesofourGalaxymoveslowerthanexpected.C.largeamountsofmattercanbeseenatinfraredwavelengths.D.largeamountsofmattercanbeseenatradiowavelengths.E.largeamountsofmattercanbeseenatx-raywavelengths.Q23.9MostastronomersthinkthatthereisdarkmatterinourGalaxybecauseA.starsintheouteredgesofourGalaxymovefasterthanexpected.B.starsintheouteredgesofourGalaxymoveslowerthanexpected.C.largeamountsofmattercanbeseenatinfraredwavelengths.D.largeamountsofmattercanbeseenatradiowavelengths.E.largeamountsofmattercanbeseenatx-raywavelengths.A23.9KeyIdeasTheShapeandSizeoftheGalaxy:OurGalaxyhasadiskabout50kpc(160,000ly)indiameterandabout600pc(2000ly)thick,withahighconcentrationofinterstellardustandgasinthedisk.Thegalacticcenterissurroundedbyalargedistributionofstarscalledthecentralbulge.Thisbulgeisnotperfectlysymmetrical,butmayhaveabarorpeanutshape.ThediskoftheGalaxyissurroundedbyasphericaldistributionofglobularclustersandoldstars,calledthegalactichalo.Thereareabout200billion(21011)starsintheGalaxy’sdisk,centralbulge,andhalo.KeyIdeasTheSun’sLocationintheGalaxy:OurSunlieswithinthegalacticdisk,some8000pc(26,000ly)fromthecenteroftheGalaxy.Interstellardustobscuresourviewatvisiblewavelengthsalonglinesofsightthatlieintheplaneofthegalacticdisk.Asaresult,theSun’slocationintheGalaxywasunknownformanyyears.ThisdilemmawasresolvedbyobservingpartsoftheGalaxyoutsidethedisk.TheSunorbitsaroundthecenteroftheGalaxyataspeedofabout790,000km/h.Ittakesabout220millionyearstocompleteoneorbit.KeyIdeasTheRotationoftheGalaxyandDarkMatter:FromstudiesoftherotationoftheGalaxy,astronomersestimatethatthetotalmassoftheGalaxyisabout1012M.Onlyabout10%ofthismassisintheformofvisiblestars,gas,anddust.Theremaining90%isinsomenonvisibleform,calleddarkmatter,thatextendsbeyondtheedgeoftheluminousmaterialintheGalaxy.OurGalaxy’sdarkmattermaybeacombinationofMACHOs(dim,star-sizedobjects),massiveneutrinos,andWIMPs(relativelymassivesubatomicparticles).KeyIdeasTheGalaxy’sSpiralStructure:OBassociations,HIIregions,andmolecularcloudsinthegalacticdiskoutlinehugespiralarms.Spiralarmscanbetracedfromthepositionsofcloudsofatomichydrogen.Thesecanbedetectedthroughoutthegalacticdiskbythe21-cmradiowavesemittedbythespin-fliptransitioninhydrogen.Theseemissionseasilypenetratetheinterveninginterstellardust.KeyIdeasTheoriesofSpiralStructure:Therearetwoleadingtheoriesofspiralstructureingalaxies.Accordingtothedensity-wavetheory,spiralarmsarecreatedbydensitywavesthatsweeparoundtheGalaxy.Thegravitationalfieldofthisspiralpatterncompressestheinterstellarcloudsthroughwhichitpasses,therebytriggeringtheformationoftheOBassociationsandHIIregionsthatilluminatethespiralarms.Accordingtothetheoryofself-propagatingstarformation,spiralarmsarecausedbythebirthofstarsoveranextendedregioninagalaxy.Differentialrotationofthegalaxystretchesthestar-formingregionintoanelongatedarchofstarsandnebulae.KeyIdeasTheGalacticNucleus:TheinnermostpartoftheGalaxy,orgalacticnucleus,hasbeenstudiedthroughitsradio,infrared,andX-rayemissions(whichareabletopassthroughinterstellardust).AstrongradiosourcecalledSagittariusA*islocatedatthegalacticcenter.Thismarksthepositionofasupermassiveblackholewithamassofabout3.7106M.