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anovercooked pizza。
the reason we can be reasonably confident that such an event won鈥檛 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鈥渨e don鈥檛 have anything of the requisite size that鈥檚 that close。 the universe is a mercifully bigplace。鈥潯he 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鈥攁ndit was a fortably safe 169；000 light…years away。
supernovae are significant to us in one other decidedly central way。 without them wewouldn鈥檛 be here。 you will recall the cosmological conundrum with which we ended the firstchapter鈥攖hat 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鈥攈otter even than the middle of the hottest stars鈥攖oforge 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 鈥渃osmologist and controversialist鈥潯nd both of those he most certainlywas。 he was； according to nature 鈥檚 obituary； 鈥渆mbroiled in controversy for most of his life鈥�
and 鈥減ut his name to much rubbish。鈥潯e claimed； for instance； and without evidence； that thenatural history museum鈥檚 treasured fossil of an archaeopteryx was a forgery along the linesof the piltdown hoax； causing much exasperation to the museum鈥檚 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 鈥渂ig bang；鈥潯n 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鈥�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鈥檚 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鈥攖heinterstellar medium as it is known鈥攖hat 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鈥�99。9percent of the mass of the solar system鈥攚ent 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 years ago； an object the size of mars crashed into earth；blowing out enough material to form a panion sphere； the moon。 within weeks； it isthought； the flung material had reassembled itself into a single clump； and within a year it had formed into the spherical rock that panions us yet。 most of the lunar material； it isthought； came from the earth鈥檚 crust； not its core； which is why the moon has so little ironwhile we have a lot。 the theory； incidentally； is almost always presented as a recent one； butin fact it was first proposed in the 1940s by reginald daly of harvard。 the only recent thingabout it is people paying any attention to it。
when earth was only about a third of its eventual size； it was probably already beginning toform an atmosphere； mostly of carbon dioxide； nitrogen； methane； and sulfur。 hardly the sortof stuff that we would associate with life； and yet from this noxious stew life formed。 carbondioxide is a powerful greenhouse gas。 this was a good thing because the sun wassignificantly dimmer back then。 had we not had the benefit of a greenhouse effect； the earthmight well have frozen over permanently； and life might never have gotten a toehold。 butsomehow life did。
for the next 500 million years the young earth continued to be pelted relentlessly byets； meteorites； and other galactic debris； which brought water to fill the oceans and theponents necessary for the successful formation of life。 it was a singularly hostileenvironment and yet somehow life got going。 some tiny bag of chemicals twitched andbecame animate。 we were on our way。
four billion years later people began to wonder how it had all happened。 and it is there thatour story next takes us。
part ii the size of the earthnature and nature鈥檚 laws lay hid innight；god said； let newton be！ and allwas light。
…alexander pope
4 THE MEASURE OF THINGS
if you had to select the least convivial scientific field trip of all time； you could certainlydo worse than the french royal academy of sciences鈥櫋eruvi