A Short History of Nearly Everything-第9章

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dme　depicted　a　swarm　of　stars　with　a　trifling　flare　that　i　had　to　put　close　to　my　face　to　see。

this；　evans　told　me；　was　a　star　in　a　constellation　called　fornax　from　a　galaxy　known　toastronomy　as　ngc1365。　（ngc　stands　for　new　general　catalogue；　where　these　things　arerecorded。　once　it　was　a　heavy　book　on　someone’s　desk　in　dublin；　today；　needless　to　say；　it’sa　database。）　for　sixty　million　silent　years；　the　light　from　the　star’s　spectacular　demise　traveledunceasingly　through　space　until　one　night　in　august　of　2001　it　arrived　at　earth　in　the　form　ofa　puff　of　radiance；　the　tiniest　brightening；　in　the　night　sky。　it　was　of　course　robert　evans　onhis　eucalypt…scented　hillside　who　spotted　it。

“there’s　something　satisfying；　i　think；”　evans　said；　“about　the　idea　of　light　traveling　formillions　of　years　through　space　and　just　at　the　right　moment　as　it　reaches　earth　someonelooks　at　the　right　bit　of　sky　and　sees　it。　it　just　seems　right　that　an　event　of　that　magnitudeshould　be　witnessed。”

supernovae　do　much　more　than　simply　impart　a　sense　of　wonder。　they　e　in　severaltypes　（one　of　them　discovered　by　evans）　and　of　these　one　in　particular；　known　as　a　iasupernova；　is　important　to　astronomy　because　it　always　explodes　in　the　same　way；　with　thesame　critical　mass。　for　this　reason　it　can　be　used　as　a　standard　candle　to　measure　theexpansion　rate　of　the　universe。

in　1987　saul　perlmutter　at　the　lawrence　berkeley　lab　in　california；　needing　more　iasupernovae　than　visual　sightings　were　providing；　set　out　to　find　a　more　systematic　method　ofsearching　for　them。　perlmutter　devised　a　nifty　system　using　sophisticated　puters　andcharge…coupled　devices—in　essence；　really　good　digital　cameras。　it　automated　supernovahunting。　telescopes　could　now　take　thousands　of　pictures　and　let　a　puter　detect　thetelltale　bright　spots　that　marked　a　supernova　explosion。　in　five　years；　with　the　new　technique；perlmutter　and　his　colleagues　at　berkeley　found　forty…two　supernovae。　now　even　amateursare　finding　supernovae　with　charge…coupled　devices。　“with　ccds　you　can　aim　a　telescope　atthe　sky　and　go　watch　television；”　evans　said　with　a　touch　of　dismay。　“it　took　all　the　romanceout　of　it。”

i　asked　him　if　he　was　tempted　to　adopt　the　new　technology。　“oh；　no；”　he　said；　“i　enjoy　myway　too　much。　besides”—he　gave　a　nod　at　the　photo　of　his　latest　supernova　and　smiled—“ican　still　beat　them　sometimes。”

the　question　that　naturally　occurs　is　“what　would　it　be　like　if　a　star　exploded　nearby？”　ournearest　stellar　neighbor；　as　we　have　seen；　is　alpha　centauri；　4。3　light…years　away。　i　hadimagined　that　if　there　were　an　explosion　there　we　would　have　4。3　years　to　watch　the　light　ofthis　magnificent　event　spreading　across　the　sky；　as　if　tipped　from　a　giant　can。　what　would　itbe　like　if　we　had　four　years　and　four　months　to　watch　an　inescapable　doom　advancing　towardus；　knowing　that　when　it　finally　arrived　it　would　blow　the　skin　right　off　our　bones？　wouldpeople　still　go　to　work？　would　farmers　plant　crops？　would　anyone　deliver　them　to　the　stores？

weeks　later；　back　in　the　town　in　new　hampshire　where　i　live；　i　put　these　questions　to　johnthorstensen；　an　astronomer　at　dartmouth　college。　“oh　no；”　he　said；　laughing。　“the　news　ofsuch　an　event　travels　out　at　the　speed　of　light；　but　so　does　the　destructiveness；　so　you’d　learnabout　it　and　die　from　it　in　the　same　instant。　but　don’t　worry　because　it’s　not　going　to　happen。”

for　the　blast　of　a　supernova　explosion　to　kill　you；　he　explained；　you　would　have　to　be“ridiculously　close”—probably　within　ten　light…years　or　so。　“the　danger　would　be　varioustypes　of　radiation—cosmic　rays　and　so　on。”　these　would　produce　fabulous　auroras；shimmering　curtains　of　spooky　light　that　would　fill　the　whole　sky。　this　would　not　be　a　goodthing。　anything　potent　enough　to　put　on　such　a　show　could　well　blow　away　themagnetosphere；　the　magnetic　zone　high　above　the　earth　that　normally　protects　us　fromultraviolet　rays　and　other　cosmic　assaults。　without　the　magnetosphere　anyone　unfortunateenough　to　step　into　sunlight　would　pretty　quickly　take　on　the　appearance　of；　let　us　say；　anovercooked　pizza。

the　reason　we　can　be　reasonably　confident　that　such　an　event　won’t　happen　in　our　cornerof　the　galaxy；　thorstensen　said；　is　that　it　takes　a　particular　kind　of　star　to　make　a　supernova　inthe　first　place。　a　candidate　star　must　be　ten　to　twenty　times　as　massive　as　our　own　sun　and“we　don’t　have　anything　of　the　requisite　size　that’s　that　close。　the　universe　is　a　mercifully　bigplace。”　the　nearest　likely　candidate　he　added；　is　betelgeuse；　whose　various　sputterings　havefor　years　suggested　that　something　interestingly　unstable　is　going　on　there。　but　betelgeuse　isfifty　thousand　light…years　away。

only　half　a　dozen　times　in　recorded　history　have　supernovae　been　close　enough　to　bevisible　to　the　naked　eye。　one　was　a　blast　in　1054　that　created　the　crab　nebula。　another；　in1604；　made　a　star　bright　enough　to　be　seen　during　the　day　for　over　three　weeks。　the　mostrecent　was　in　1987；　when　a　supernova　flared　in　a　zone　of　the　cosmos　known　as　the　largemagellanic　cloud；　but　that　was　only　barely　visible　and　only　in　the　southern　hemisphere—andit　was　a　fortably　safe　169；000　light…years　away。

supernovae　are　significant　to　us　in　one　other　decidedly　central　way。　without　them　wewouldn’t　be　here。　you　will　recall　the　cosmological　conundrum　with　which　we　ended　the　firstchapter—that　the　big　bang　created　lots　of　light　gases　but　no　heavy　elements。　those　camelater；　but　for　a　very　long　time　nobody　could　figure　out　　how　they　came　later。　the　problem　wasthat　you　needed　something　really　hot—hotter　even　than　the　middle　of　the　hottest　stars—toforge　carbon　and　iron　and　the　other　elements　without　which　we　would　be　distressinglyimmaterial。　supernovae　provided　the　explanation；　and　it　was　an　english　cosmologist　almostas　singular　in　manner　as　fritz　zwicky　who　figured　it　out。

he　was　a　yorkshireman　named　fred　hoyle。　hoyle；　who　died　in　2001；　was　described　in　anobituary　in　nature　as　a　“cosmologist　and　controversialist”　and　both　of　those　he　most　certainlywas。　he　was；　according　to　nature　’s　obituary；　“embroiled　in　controversy　for　most　of　his　life”

and　“put　his　name　to　much　rubbish。”　he　claimed；　for　instance；　and　without　evidence；　that　thenatural　history　museum’s　treasured　fossil　of　an　archaeopteryx　was　a　forgery　along　the　linesof　the　piltdown　hoax；　causing　much　exasperation　to　the　museum’s　paleontologists；　who　had　tospend　days　fielding　phone　calls　from　journalists　from　all　over　the　world。　he　also　believed　thatearth　was　not　only　seeded　by　life　from　space　but　also　by　many　of　its　diseases；　such　asinfluenza　and　bubonic　plague；　and　suggested　at　one　point　that　humans　evolved　projectingnoses　with　the　nostrils　underneath　as　a　way　of　keeping　cosmic　pathogens　from　falling　intothem。

it　was　he　who　coined　the　term　“big　bang；”　in　a　moment　of　facetiousness；　for　a　radiobroadcast　in　1952。　he　pointed　out　that　nothing　in　our　understanding　of　physics　could　accountfor　why　everything；　gathered　to　a　point；　would　suddenly　and　dramatically　begin　to　expand。

hoyle　favored　a　steady…state　theory　in　which　the　universe　was　constantly　expanding　andcontinually　creating　new　matter　as　it　went。　hoyle　also　realized　that　if　stars　imploded　theywould　liberate　huge　amounts　of　heat—100　million　degrees　or　more；　enough　to　begin　togenerate　the　heavier　elements　in　a　process　known　as　nucleosynthesis。　in　1957；　working　withothers；　hoyle　showed　how　the　heavier　elements　were　formed　in　supernova　explosions。　forthis　work；　w。　a。　fowler；　one　of　his　collaborators；　received　a　nobel　prize。　hoyle；　shamefully；did　not。

according　to　hoyle’s　theory；　an　exploding　star　would　generate　enough　heat　to　create　all　thenew　elements　and　spray　them　into　the　cosmos　where　they　would　form　gaseous　clouds—theinterstellar　medium　as　it　is　known—that　could　eventually　coalesce　into　new　solar　systems。

with　the　new　theories　it　became　possible　at　last　to　construct　plausible　scenarios　for　how　wegot　here。　what　we　now　think　we　know　is　this：

about　4。6　billion　years　ago；　a　great　swirl　of　gas　and　dust　some　15　billion　miles　acrossaccumulated　in　space　where　we　are　now　and　began　to　aggregate。　virtually　all　of　it—99。9percent　of　the　mass　of　the　solar　system—went　to　make　the　sun。　out　of　the　floating　materialthat　was　left　over；　two　microscopic　grains　floated　close　enough　together　to　be　joined　byelectrostatic　forces。　this　was　the　moment　of　conception　for　our　planet。　all　over　the　inchoatesolar　system；　the　same　was　happening。　colliding　dust　grains　formed　larger　and　larger　clumps。

eventually　the　clumps　grew　large　enough　to　be　called　planetesimals。　as　these　endlesslybumped　and　collided；　they　fractured　or　split　or　rebined　in　endless　random　permutations；but　in　every　encounter　there　was　a　winner；　and　some　of　the　winners　grew　big　enough　todominate　the　orbit　around　which　they　traveled。

it　all　happened　remarkably　quickly。　to　grow　from　a　tiny　cluster　of　grains　to　a　baby　planetsome　hundreds　of　miles　across　is　thought　to　have　taken　only　a　few　tens　of　thousands　of　years。

in　just　200　million　years；　possibly　less；　the　earth　was　essentially　formed；　though　still　moltenand　subject　to　constant　bombardment　from　all　the　debris　that　remained　floating　about。

at　this　point；　about　4。5　billion