Readingcomprehension.
Theoldbeliefthattheuniverseneverchangesisquitewrong.Evenbeforetheinventionofthetelescope
astronomersnoticedthatbrightstarssuddenlyappearedintheskyandthenlaterdisappeared.Thesestars
werecalled"novae"becausetheywerethoughttobenew.Infactwenowknowthattheyarereallyoldstars
whichareslowlydying.
Novaeareoldstarswhichareslowlydying.Astheydosotheyletouthugecloudsofmaterialsometimes
aslargeastheearthandtheseexplodeintospaceataspeedofabout8000000kilometersperhour.When
thishappensthehotterpartsofthestarbecomevisibleandthisiswhynovaearesobright.Althoughthe
explosionsaresohugeonahumanscaletheyonlyconsumeasmallpartofthedyingstar'senergy.Thedeath
isaslowoneandthestarmaycontinuetoexplodeforthousandsofyears.Indeedthereareevensomestars
whichexplodeonceeverytwoweeks.
Thereareotheroldstarswhichdonotdieslowlybutarecompletelydestroyedbyonegreatexplosion.
Theseareknownas"supernovae".Theexplosionofasupernovaeisequivalenttoaboutamillionmillion
millionmillionhydrogenbombsgoingoffatthesametime.Justbeforetheexplosionthestar'sdensitybecomes
verygreatanditspinsataveryhighspeed.Amatchboxofmaterialstakenfromthestaratthattimewould
weighabout1000tonsandthestarwouldbeturningatabout16000000kilometersperhour.Theexplosion
itselfoccurssuddenlyintheintervalofaminutebutthesupernovaecontinuestoshinelongaftertheevent.
OnesupernovaewhichChineseastronomersobservedin1054canstillbeseenbyustoday.Ithasbeenshining
foratleastninehundredyears.
1.Novaewereoriginallythoughttobe_____.
A.starsthatdisappearsuddenly
B.starsthatexplode
C.newstars
D.dyingstars
2.Supernovaearedifferentfromnovaebecause_____.
A.theyaredyingstars
B.theydiegradually
C.theycanbeseenfromtheearth
D.theyexplodesuddenly
3."Astheydoso"(line1Paragraph2)means_____.