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Whatisablackhole?Ablackholeisaregionofspacetimefromwhichnothingcanescape,evenlight.Toseewhythishappens,imaginethrowingatennisballintotheair.Th

eharderyouthrowthetennisball,thefasteritistravellingwhenitleavesyourhandandthehighertheballwillgobeforeturningback.Ifyouthrowithardenoughitwillnever

return,thegravitationalattractionwillnotbeabletopullitbackdown.Thevelocitytheballmusthavetoescapeisknownastheescapevelocityandfortheearthisab

out7milesasecond.Asabodyiscrushedintoasmallerandsmallervolume,thegravitationalattractionincreases,andhencetheescapevel

ocitygetsbigger.Thingshavetobethrownharderandhardertoescape.Eventuallyapointisreachedwhenevenlight,whic

htravelsat186thousandmilesasecond,isnottravellingfastenoughtoescape.Atthispoint,nothingcangetoutasnoth

ingcantravelfasterthanlight.Thisisablackhole.Dotheyreallyexist?Itisimpossibletoseeablackholedirectlybecausenolightcanescapefromthem;theyareblack.Butt

herearegoodreasonstothinktheyexist.Whenalargestarhasburntallitsfuelitexplodesintoasupernova.Thestuffthatisleftcollapsesdowntoanextremelydenseobjectk

nownasaneutronstar.Weknowthattheseobjectsexistbecauseseveralhavebeenfoundusingradiotelescopes.Iftheneutronstaristoolarge,thegravitationalforce

soverwhelmthepressuregradientsandcollapsecannotbehalted.Theneutronstarcontinuestoshrinkuntilitfinallybeco

mesablackhole.Thismasslimitisonlyacoupleofsolarmasses,thatisabouttwicethemassofoursun,andsoweshouldexpectatleasta

fewneutronstarstohavethismass.(Oursunisnotparticularlylarge;infactitisquitesmall.)Asupernovaoccursinourga

laxyonceevery300years,andinneighbouringgalaxiesabout500neutronstarshavebeenidentified.Thereforewearequiteconfidentthatthereshouldalsobesomeblackhol

es.