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ID:37114098
大小:221.57 KB
页数:49页
时间:2019-05-17
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1、READING&WRITINGCOURSEextensivereadingmaterialtrigger;compress;reptiles;pattern;sweep;intense;principal;unique;spiral;elliptical;irregular;simplistic;apparent;proportionally;steadily;Galaxiesarethemajorbuildingblocksoftheuniverse.Agalaxyisgiantfamilyofmanymillionsofstars,anditisheld
2、togetherbyitsowngravitationalfield.Mostofthematerialuniverseisorganizedintogalaxiesofstarstogetherwithgasanddust.Therearethreemaintypesofgalaxy:spiral,elliptical,andirregular.TheMilkyWayisaspiralgalaxy,aflattishdiscofstarswithtwospiralarmsemergingfromitscentralnucleus.Aboutone-quar
3、terofallgalaxieshavethisshape.Spiralgalaxiesarewellsuppliedwiththeinterstellargasinwhichnewstarsform:astherotatingspiralpatternsweepsaroundthegalaxyitcompressesgasanddust,triggeringtheformationofbrightyoungstarsandinitsarms.Theellipticalgalaxieshaveasymmetricalellipticalorspheroida
4、lshapewithnoobviousstructure.Mostoftheirmemberstarsareveryoldandsinceellipticalsaredevoidofinterstellargas,nonewstarsareforminginthem.Thebiggestandbrightestgalaxiesintheuniverseareellipticalswithmassesofabout1013timesthatoftheSun,thesegiantsmayfrequentlybesourcesofstrongradioemissi
5、on,inwhichcasetheyarecalledradiogalaxies.Abouttwo-thirdsofallgalaxiesareelliptical.Irregulargalaxiescompriseaboutone-tenthofallgalaxiesandtheycomeinmanysubclasses.MeasurementinspaceisquitedifferentfrommeasurementonEarth.Someterrestrialdistancescanbeexpressedasintervalsoftime,thetim
6、etoflyfromonecontinenttoanotherorthetimeittakestodrivetowork,forexample.Bycomparisonwiththesefamiliaryardsticks,thedistancestothegalaxiesareincomprehensiblylarge,buttheytooaremademoremanageablebyusingatimecalibration,inthiscasethedistancethatlighttravelsinoneyear.Onsuchascalethenea
7、restgiantspiralgalaxy,theAndromedagalaxy,istwomillionlightyearsaway.Themostdistantluminousobjectsseenbytelescopesareprobablytenthousandmillionlightyearsaway.TheirlightwasalreadyhalfwayherebeforetheEarthevenformed.ThelightfromthenearbyVirgogalaxysetoutwhenreptilesstilldominatedthean
8、imalworld.39.Theword"major
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