厦门大学2005年英语语言文学考研试题(五)

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1、Passage6　　Untilrecentlyastronomershavebeenpuzzledbythefateofredgiantandsupergiantstars.Whenthecoreofagiantstarwhosemasssurpasses1.4timesthepresentmassofourSun(M⊙)exhaustsitsnuclearfuel,itisunabletosupportitsownweightandcollapsesintoatinyneutronstar.Thegra

2、vitationalenergyreleasedduringthisimplosionofthecoreblowsofftheremainderofthestarinagiganticexplosion,orasupernova.Sincearound50percentofallstarsarebelievedtobegintheirliveswithmassesgreaterthan1.4M⊙,wemightexpectthatoneoutofeverytwostarswoulddieasasupern

3、ova.Butinfact,onlyonestarinthirtydiessuchaviolentdeath.Therestexpiremuchmorepeacefullyasplanetarynebulas.Apparentlymostmassivestarsmanagetolosesufficientmaterialthattheirmassesdropbelowthecriticalvalueof1.4M⊙beforetheyexhausttheirnuclearfuel.　　Evidencesup

4、portingthisviewcomesfromobservationsofIRC+10216,apulsatinggiantstarlocated700light-yearsawayfromEarth.Ahugerateofmassloss(1M⊙every10,000years)hasbeendeducedfrominfraredobservationsofammonia(NH3)moleculeslocatedinthecircumstellarcloudaroundIRC+10216.Recent

5、microwaveobservationsofcarbonmonoxide(CO)moleculesindicateasimilarrateofmasslossanddemonstratethattheescapingmaterialextendsout-wardfromthestarforadistanceofatleastonelight-year.BecauseweknowthesizeofthecloudaroundIRC+10216andcanuseourobservationsofeither

6、NH3orCOtomeasuretheoutflowvelocity,wecancalculateanageforthecircumstellarcloud.IRC+10216hasapparentlyexpelled,intheformofmoleculesanddustgrains,amassequaltothatofourentireSunwithinthepasttenthousandyears.Thisimpliesthatsomestarscanshedhugeamountsofmatterv

7、eryquicklyandthusmayneverexpireassupernovas.Theoreticalmodelsaswellasstatisticsonsupernovasandplanetarynebulassuggestthatstarsthatbegintheirliveswithmassesaround6M⊙shedsufficientmaterialtodropbelowthecriticalvalueof1.4M⊙。IRC+10216,forexample,shoulddothisi

8、namere50,000yearsfromitsbirth,onlyaninstantinthelifeofastar.　　ButwhatplacedoesIRC+10216haveinstellarevolution?AstronomerssuggestthatstarslikeIRC+10216areactually"protoplanetarynebulas"-oldgiantstarswhosedensecoresha