详解频道parsecpc秒差距

Chapter1TheuniverseatdifferentPoetssaysciencetakesawayfromthebeautyofthestars-mereglobsofgasatoms.Nothingismere.Itooseethestarsonadesertnight,andfeelthem.ButdoIseelessormore?Thevastnessoftheheavensstretchesmyimagination-stuckonthiscarousel,mylittleeyecancatchone-million-year-oldlight…Orseethem[thestars]withthegreatereyeofPalomar,rushingallapartfromsomecommonstartingpointwhentheywereperhapsalltogether.Whatisthepattern,orthemeaning,orthewhy?Itdoesnotdoharmtothemysterytoknowalittleaboutit.Forfarmoremarvellousisthetruththananyartistsofthepastimagined!Whydothepoetsofthepresentnotspeakofit?RichardUnitsofastronomicalScientificnotations:Becauseofthelargerangeofscaleswewillencounter,wewillusepowerstoexpressverylargeorsmallnumbers. 105=100,000, 103=1,000, 10-3=0.001, 300,000=3105, 4,500=4.5103, 0.000062=6.210-5.MetricPrefixes:Theseprefixesspecifypowersinaconvenientway. kilo(k)=103=1,000, mega(M)=106=1,000,000. 1kilometre=1km=1,000 1megaton=1Mton=1,000,000Astronomicalunit(AU):Theaveragedistancebetweentheearthandthe1AU1.51011Lightyear(ly;光年):Onelightyearisequaltothedistancetravelledbylightinvacuuminoneyear.Invacuum,lighttravelsataspeedof3108m/s.1ly6.3104AU9.51015Parsecpc;秒差距):Anothercommonastronomicalunitfor1pc3.3AquicktouroftheTounderstandtheuniverse,wemustunderstandtherelativescalesofplanets,stars,galaxiesandtheuniverseasawhole.Weshalljourneyfromacampusscenetothelimitsofthecosmosin13steps.Ineachstepweshallwidenourviewbyafactorof100.Thatis,eachsuccessivepicturewillshowaregionoftheuniversethatis100timeswiderthantheprecedingpicture.Fig.Fig.1-1Thissceneshowsaregionabout16macross.Itisoccupiedbyahumanbeing,asidewalkandafewtrees.Only13stepsseparatethisscenefromtheuniverseasaStepStepWestartwithobjectswhosesizeswecanunderstand.Thishelpsustocomprehendastronomicaldistancesrelativetoourselves.Fig.Fig.1-2Wenowincreaseourfieldofviewbyafactorof100.Individualpeople,trees,andsidewalksvanish,butnowwecanseeacollegecampusandthesurroundingstreetsandStepStepThesedimensionsarealsofamiliar.Wehavepersonalexperiencewithsuchdimensions,andwecanrelatethemtothescaleofourbodies.Fig.1-3Fig.1-3Fieldofviewenlarged100timesfromthepreviousimage.Ourfieldofviewnowspans160km.Theareaoftheprecedingphotographisshownbythesmallsquare(arrow).Thecollegecampusisinvisible;citiesarevisibleasdarkblotches,andfarmlandsarevisibleastinyrectangularshapes.StepStepThisisaninfraredphotographtakenfromanartificialsatellite.Atthisscaleweseenaturalfeaturesoftheearth’ssurfacebutnotmuchsignsofhumanactivities.Fig.Fig.1-4Fieldofviewenlarged100timesfromthepreviousimage(NASA).Thisstepinourjourneyshowsourentireplanet.Theearthis12,756kmindiameterandrotatesonitsaxisonceaday.StepStepThephotographshowsmostofthedaylightside(illuminatedbysunlight)oftheearth.Therotationoftheearthcarriesuseastwardacrossthedaylightside,and,asweturnawayfromthesun,nightcomes.Fig.Fig.1-6Themoonisourclosestneighbour.Itrevolvesaroundtheearthoncein27days.Fig.1-5Againweenlargeourfieldofviewbyafactorof100,andweseearegionof1,600,000kmwide.Theearthisthesmallwhitedotinthecentre,andthemoon,onlyone-fourthitsdiameter,isanevensmallerdotalongitsorbit380,000kmfromtheearth.StepStepAtthisstepwemeetthemoon,anairless,rockysatelliteoftheearth.Itshowsphases(相)asitorbitsaroundtheearth(Chapter8).Fig.Fig.1-8Thesunisastarwhichgeneratesenergybynuclearreactionsatitscentre.Fig.1-7Whenweonceagainenlargeourfieldofviewbyafactorof100,theearthandthemoonlieinthesmallredboxatlowerleft.Weseethesunandtwootherplanets-MercuryandVenus.AnAUisdefinedastheaveragedistancebetweentheearthandthesun.StepStepThesunisastar-itgivesoutlightandheat.Theplanets,includingourearth,orbitaroundthesunandshinebyreflectingsunlight.Thesunandtheplanetsareseparatedbyverylargedistancescomparedtotheirsizes.Imaginethatwereducethesuntothesizeofasmallplum,thentheearthwouldbeagrainofsalt4mfromit.StepStepFig.1-9Afteronlysixstepsofenlargingourfieldofviewbyafactorof100ateachstep,wenowseetheentiresolarsystem.Thesolarsystemspansabout60AU,i.e.,Fig.1-9Afteronlysixstepsofenlargingourfieldofviewbyafactorof100ateachstep,wenowseetheentiresolarsystem.Thesolarsystemspansabout60AU,i.e.,60timestheaveragedistancebetweentheearthandthesun.Fig.1-10Therelativesizesofthesunandtheeightplanets.planets矮行星are“minorplanets”.Smallsolar-systemBodies(太陽系小星體)includeAsteroids(小行星)andComets彗星thelatteraresmallicydebrismovingaroundthesuninveryelongatedorbits(Chapter9).Fig.1-11Fig.1-11Whenweagainenlargeourfieldofviewbyafactorof100,oursolarsystemvanishes.Thesunisvisibleasapointoflight,butalltheplanetsandtheirorbitsarenowcrowdedintothesmallsquareatthecentre.StepStepThedistancesamongstarsarehuge.Inthisview,wecannotseeanystarsexceptthesun.ThestarsaretypicallyseparatedbydistancesabouttentimeslargerthanthisFig.Fig.1-12Ourfieldofviewhasnowexpandedouttoadiameterofabitover1millionAU.Thesolarsystemislocatedatthecentre,andweseeafewoftheneareststars.StepStepTheneareststarProximaCentauri;毗鄰星isabout270,000AUfromthesun,thatis,270,000timesthedistancebetweenthesunandtheearth.Inthisscale,weshoulduselightyear(ly)asaunitofdistance.Lightfromtheneareststartakesabout4.2yearstoreachtheearth,andthereforewesaythatitsdistanceis4.2ly.Fig.1-13Fig.1-13Asweexpandourfieldofviewbyanotherfactorof100,wefindthatthesunanditsneighbouringstarsvanishintothebackgroundofthousandsofstars.Thefieldofviewisnow1700lyindiameter.StepStepWenowseethousandsofstarsandtheirtendencytoformclusters.Likeoursun,theyallshinebythenuclearenergygeneratedintheircores,andprobablymanyofthemhaveplanetarysystemssimilartooursolarsystem.Fig.Fig.1-14Ifweexpandourfieldofviewbyanotherfactorof100,wecanseeourownMilkyWayGalaxy.Oursunandtheneighbouringstarsofthepreviousfigurewouldbelostamongthe100billion(1011)starsofthegalaxy.Ourgalaxyspansabout100,000ly.StepStepThediskofMilkyWayGalaxyhasaspiralstructurefilledwithgas,dustandstars.Starsundergoacompletelifecyclefrombirthtodeath,evolvingintodifferentphasesatdifferentages-includingprotostars(原恆星),main-Fig.1-15TheapproximatepositionofthesunintheMilkyWayGalaxy.sequencestars(主序星)Fig.1-15TheapproximatepositionofthesunintheMilkyWayGalaxy.Fig.Fig.1-16Asweexpandourfieldofviewbyanotherfactorof100,ourMilkyWayGalaxy(arrow)becomesatinydotsurroundedbyotherdots,eachrepresentingagalaxy.Thisdiagramincludesaregion17millionlyindiameter.StepStepGalaxiesarenotscatteredrandomlybuttendtoformgalaxyclusters星系團)LocalGroup(本星系團)isasmallgalaxyclusterconsistsofroughlytwodozensgalaxies(includingFig.1-17LocalGroupFig.1-17LocalGroupofFig.1-18Fig.1-18Werewetoagainexpandourfieldofview,wewouldseethatourLocalGroupofgalaxiesispartofalargesupercluster,aclusterofclusters.Othergalaxiesarenotscatteredatrandomthroughouttheuniversebutlieinclusterswithinlargersuperclusters.StepStepAtthisscale,clustersofgalaxiesgrouptoformsuperclusters(超星系團),whichareinturnlinkedtoformfilamentsoutliningvoidsthatseemnearlyempty.Fig.1-19FeaturesliketheGreatWall(長城)andtheVoid大空隙areamongFig.1-19FeaturesliketheGreatWall(長城)andtheVoid大空隙