天文学英文版PPT资料 (16)

UniverseEighthEditionRogerA.Freedman•WilliamJ.KaufmannIIICHAPTER17TheNatureofStarsChapter16and17onlinequizzesdueMonday11/8bymidnightHWParallaxmeasurementsarebestmadeusingatelescopeinorbit.ThisisbecauseA.atelescopeinorbitisclosertothestars.B.largertelescopescanbeplacedinorbitandsotheresolutionissignificantlyimproved.C.thebaselineislongerandsotheparallaxangleislarger.D.chromaticaberrationfromthetelescopelensiseliminated.E.anobservatoryinspaceisunhamperedbytheEarth’satmosphere.Q17.1Parallaxmeasurementsarebestmadeusingatelescopeinorbit.ThisisbecauseA.atelescopeinorbitisclosertothestars.B.largertelescopescanbeplacedinorbitandsotheresolutionissignificantlyimproved.C.thebaselineislongerandsotheparallaxangleislarger.D.chromaticaberrationfromthetelescopelensiseliminated.E.anobservatoryinspaceisunhamperedbytheEarth’satmosphere.A17.1AtthedistanceoftheEarthfromtheSun(1AU)theintensityofsunlightis1370watts/m2.WhatistheintensityatthedistanceofSaturnfromtheSun(10AU)?A.13,700watts/m2B.1370watts/m2C.137watts/m2D.13.7watts/m2E.1.37watts/m2Q17.4AtthedistanceoftheEarthfromtheSun(1AU)theintensityofsunlightis1370watts/m2.WhatistheintensityatthedistanceofSaturnfromtheSun(10AU)?A.13,700watts/m2B.1370watts/m2C.137watts/m2D.13.7watts/m2E.1.37watts/m2A17.4WhereistheSunlocatedonthisH-Rdiagram?A.AB.BC.CD.DE.EQ17.10WhereistheSunlocatedonthisH-Rdiagram?A.AB.BC.CD.DE.EA17.10KeyIdeasMeasuringDistancestoNearbyStars:Distancestothenearerstarscanbedeterminedbyparallax,theapparentshiftofastaragainstthebackgroundstarsobservedastheEarthmovesalongitsorbit.Parallaxmeasurementsmadefromorbit,abovetheblurringeffectsoftheatmosphere,aremuchmoreaccuratethanthosemadewithEarth-basedtelescopes.Stellarparallaxescanonlybemeasuredforstarswithinafewhundredparsecs.TheInverse-SquareLaw:Astar’sluminosity(totallightoutput),apparentbrightness,anddistancefromtheEartharerelatedbytheinverse-squarelaw.Ifanytwoofthesequantitiesareknown,thethirdcanbecalculated.KeyIdeasThePopulationofStars:Starsofrelativelylowluminosityaremorecommonthanmoreluminousstars.OurownSunisaratheraveragestarofintermediateluminosity.TheMagnitudeScale:Theapparentmagnitudescaleisanalternativewaytomeasureastar’sapparentbrightness.Theabsolutemagnitudeofastaristheapparentmagnitudeitwouldhaveifviewedfromadistanceof10parsecs.Aversionoftheinverse-squarelawrelatesastar’sabsolutemagnitude,apparentmagnitude,anddistance.KeyIdeasPhotometryandColorRatios:Photometrymeasurestheapparentbrightnessofastar.Thecolorratiosofastararetheratiosofbrightnessvaluesobtainedthroughdifferentstandardfilters,suchastheU,B,andVfilters.Theseratiosareameasureofthestar’ssurfacetemperature.SpectralTypes:Starsareclassifiedintospectraltypes(subdivisionsofthespectralclassesO,B,A,F,G,K,andM),basedonthemajorpatternsofspectrallinesintheirspectra.Thespectralclassandtypeofastarisdirectlyrelatedtoitssurfacetemperature:OstarsarethehottestandMstarsarethecoolest.MostbrowndwarfsareinevencoolerspectralclassescalledLandT.Unliketruestars,browndwarfsaretoosmalltosustainthermonuclearfusion.KeyIdeasHertzsprung-RussellDiagram:TheHertzsprung-Russell(H-R)diagramisagraphplottingtheabsolutemagnitudesofstarsagainsttheirspectraltypes—or,equivalently,theirluminositiesagainstsurfacetemperatures.ThepositionsontheH-Rdiagramofmoststarsarealongthemainsequence,abandthatextendsfromhighluminosityandhighsurfacetemperaturetolowluminosityandlowsurfacetemperature.KeyIdeasOntheH-Rdiagram,giantandsupergiantstarslieabovethemainsequence,whilewhitedwarfsarebelowthemainsequence.Bycarefullyexaminingastar’sspectrallines,astronomerscandeterminewhetherthatstarisamain-sequencestar,giant,supergiant,orwhitedwarf.UsingtheH-Rdiagramandtheinversesquarelaw,thestar’sluminosityanddistancecanbefoundwithoutmeasuringitsstellarparallax.KeyIdeasBinaryStars:Binarystars,inwhichtwostarsareheldinorbitaroundeachotherbytheirmutualgravitationalattraction,aresurprisinglycommon.ThosethatcanberesolvedintotwodistinctstarimagesbyanEarth-basedtelescopearecalledvisualbinaries.Eachofthetwostarsinabinarysystemmovesinanellipticalorbitaboutthecenterofmassofthesystem.Binarystarsareimportantbecausetheyallowastronomerstodeterminethemassesofthetwostarsinabinarysystem.Themassescanbecomputedfrommeasurementsoftheorbitalperiodandorbitaldimensionsofthesystem.KeyIdeasMass-LuminosityRelationforMain-SequenceStars:Main-sequencestarsarestarsliketheSunbutwithdifferentmasses.Themass-luminosityrelationexpressesadirectcorrelationbetweenmassandluminosityformain-sequencestars.Thegreaterthemassofamain-sequencestar,thegreateritsluminosity(andalsothegreateritsradiusandsurfacetemperature).KeyIdeasSpectroscopicObservationsofBinaryStars:Somebinariescanbedetectedandanalyzed,eventhoughthesystemmaybesodistantorthetwostarssoclosetogetherthatthetwostarimagescannotberesolved.Aspectrumbinaryappearstobeasinglestarbuthasaspectrumwiththeabsorptionlinesfortwodistinctlydifferentspectraltypes.KeyIdeasAspectroscopicbinaryhasspectrallinesthatshiftbackandforthinwavelength.ThisiscausedbytheDopplereffect,astheorbitsofthestarscarrythemfirst