GRE考试阅读练习精选:Supernova
2021-12-17点击量:206
Astronomersobserveasupernovathatisdimmerthanexpectedbecauseofdustscreening,Ifasupernova(theexplosionofamassivestar)triggeredstarformationfromdensecloudsofgasanddust,andifthemostmassivestartobeformedfromthecloudevolvedintoasupernovaandtriggeredanewroundofstarformation,andsoon,thenachainofstar-formingregionswouldresult.Ifmanysuchchainswerecreatedinadifferentiallyrotatinggalaxy,thedistributionofstarswouldresembletheobserveddistributeinaspiralgalaxy.Thislineofreasoningunderliesanexcitingnewtheoryofspiral-galaxystructure.Acomputersimulationbasedonthistheoryhasreproducedtheappearanceofmanyspiralgalaxieswithoutassuminganunderlyingdensitywave,thehallmarkofthemostwidelyacceptedtheoryofthelarge-scalestructureofspiralgalaxies.Thattheorymaintainsthatadensitywaveofspiralformsweepsthroughthecentralplaneofagalaxy,compressingcloudsofgasanddust,whichcollapseintostarsthatformaspiralpattern.1.Theprimarypurposeofthepassageisto(A)describewhatresultswhenasupernovatriggersthecreationofchainsofstar-formingregions(B)proposeamodificationinthemostwidelyacceptedtheoryofspiral-galaxystructure(C)compareandcontrasttherolesofcloudsofgasanddustintwotheoriesofspiral-galaxystructure(D)describeanewtheoryofspiral-galaxystructureandcontrastitwiththemostwidelyacceptedtheory(E)describeanewtheoryofspiral-galaxystructureanddiscussareasonwhyitisinferiortothemostwidelyacceptedtheory2.Thepassageimpliesthat,accordingtothenewtheoryofspiral-galaxystructure,aspiralgalaxycanbecreatedbysupernovaswhenthesupernovasare(A)producinganunderlyingdensitywave(B)affectedbyadensitywaveofspiralform(C)distributedinaspiralpattern(D)locatedinthecentralplaneofagalaxy(E)locatedinadifferentiallyrotatinggalaxy3.Whichofthefollowing,iftrue,wouldmostdiscreditthenewtheoryasdescribedinthepassage?(A)Theexactmechanismbywhichastarbecomesasupernovaisnotyetcompletelyknownandmayevendifferfordifferentstars.(B)Chainsofstar-formingregionslikethosepostulatedinthenewtheoryhavebeenobservedinthevicinityofdensecloudsofgasanddust.(C)Themostmassivestarsformedfromsupernovaexplosionsareunlikelytoevolveintosupernovas.(D)Computersimulationsofsupernovasprovideapoorpictureofwhatoccursjustbeforeasupernovaexplosion.(E)Adensitywavecannotcompresscloudsofgasanddusttoadensityhighenoughtocreateastar.答案:DECGRE培训:...