extensive reading-Sherry

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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