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.
The term supernova was coined in the 1930s by a memorably odd astrophysicist namedFritz Zwicky. Born in Bulgaria and raised in Switzerland, Zwicky came to the CaliforniaInstitute of Technology in the 1920s and there at once distinguished himself by his abrasivepersonality and erratic talents. He didn’t seem to be outstandingly bright, and many of hiscolleagues considered him little more than “an irritating buffoon.” A fitness buff, he wouldoften drop to the floor of the Caltech dining hall or other public areas and do one-armedpushups to demonstrate his virility to anyone who seemed inclined to doubt it. He wasnotoriously aggressive, his manner eventually becoming so intimidating that his closestcollaborator, a gentle man named Walter Baade, refused to be left alone with him. Amongother things, Zwicky accused Baade, who was German, of being a Nazi, which he was not. Onat least one occasion Zwicky threatened to kill Baade, who worked up the hill at the MountWilson Observatory, if he saw him on the Caltech campus.
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?
Evans’s is a talent so exceptional that Oliver Sacks, in An Anthropologist on Mars, devotesa passage to him in a chapter on autistic savants—quickly adding that “there is no suggestionthat he is autistic.” Evans, who has not met Sacks, laughs at the suggestion that he might beeither autistic or a savant, but he is powerless to explain quite where his talent comes from.
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.
Imagine a million really weighty cannonballs squeezed down to the size of a marble and—well, you’re still not even close. The core of a neutron star is so dense that a single spoonfulof matter from it would weigh 200 billion pounds. A spoonful! But there was more. Zwickyrealized that after the collapse of such a star there would be a huge amount of energy leftover—enough to make the biggest bang in the universe. He called these resultant explosionssupernovae. They would be—they are—the biggest events in creation.
He nodded frankly again and grinned, then asked me if I’d like to see his telescope. I hadimagined that Evans would have a proper observatory in his backyard—a scaled-downversion of a Mount Wilson or Palomar, with a sliding domed roof and a mechanized chair thatwould be a pleasure to maneuver. In fact, he led me not outside but to a crowded storeroomoff the kitchen where he keeps his books and papers and where his telescope—a whitecylinder that is about the size and shape of a household hot-water tank—rests in a homemade,swiveling plywood mount. When he wishes to observe, he carries them in two trips to a smalldeck off the kitchen. Between the overhang of the roof and the feathery tops of eucalyptustrees growing up from the slope below, he has only a letter-box view of the sky, but he says itis more than good enough for his purposes. And there, when the skies are clear and the Moonnot too bright, he finds his supernovae.
PART II THE SIZE OF THE EARTHNature and Nature’s laws lay hid innight;God said, Let Newton be! And allwas light.
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.
Looking into the past is of course the easy part. Glance at the night sky and what you see ishistory and lots of it—the stars not as they are now but as they were when their light leftthem. For all we know, the North Star, our faithful companion, might actually have burnedout last January or in 1854 or at any time since the early fourteenth century and news of it justhasn’t reached us yet. The best we can say—can ever say—is that it was still burning on thisdate 680 years ago. Stars die all the time. What Bob Evans does better than anyone else whohas ever tried is spot these moments of celestial farewell.
To understand what a feat this is, imagine a standard dining room table covered in a blacktablecloth and someone throwing a handful of salt across it. The scattered grains can bethought of as a galaxy. Now imagine fifteen hundred more tables like the first one—enough tofill a Wal-Mart parking lot, say, or to make a single line two miles long—each with a randomarray of salt across it. Now add one grain of salt to any table and let Bob Evans walk amongthem. At a glance he will spot it. That grain of salt is the supernova.
Supernovae occur when a giant star, one much bigger than our own Sun, collapses and thenspectacularly explodes, releasing in an instant the energy of a hundred billion suns, burningfor a time brighter than all the stars in its galaxy. “It’s like a trillion hydrogen bombs going offat once,” says Evans. If a supernova explosion happened within five hundred light-years of us,we would be goners, according to Evans—“it would wreck the show,” as he cheerfully puts it.
By day, Evans is a kindly and now semiretired minister in the Uniting Church in Australia,who does a bit of freelance work and researches the history of nineteenth-century religiousmovements. But by night he is, in his unassuming way, a titan of the skies. He huntssupernovae.
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.
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 distressinglyimmaterial. Supernovae provided the explanation, and it was an English cosmologist almostas singular in manner as Fritz Zwicky who figured it out.
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 computers andcharge-coupled devices—in essence, really good digital cameras. It automated supernovahunting. Telescopes could now take thousands of pictures and let a computer 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.”
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 companion sphere, the Moon. Within weeks, it isthought, the flung material had reassembled itself into a single clump, and within a year it hadformed into the spherical rock that companions us yet. Most of the lunar material, it isthought, came from the Earth’s 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.
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.
It was he who coined the term “Big Bang,” in a momentof 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.
But Zwicky was also capable of insights of the most startling brilliance. In the early 1930s,he turned his attention to a question that had long troubled astronomers: the appearance in theskyof occasional unexplained points of light, new stars. Improbably he wondered if theneutron—the subatomic particle that had just been discovered in England by JamesChadwick, and was thus both novel and rather fashionable—might be at the heart of things. Itoccurred to him that if a star collapsed to the sort of densities found in the core of atoms, theresult would be an unimaginably compacted core. Atoms would literally be crushed together,their electrons forced into the nucleus, forming neutrons. You would have a neutron star.
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.
On a table beside the telescope were stacks of photos and papers relevant to his pursuits,and he showed me some of them now. If you have ever looked through popular astronomicalpublications, and at some time you must have, you will know that they are generally full ofrichly luminous color photos of distant nebulae and the like—fairy-lit clouds of celestial lightof the most delicate and moving splendor. Evans’s working images are nothing like that. Theyare just blurry black-and-white photos with little points of haloed brightness. One he showedme depicted a swarm of stars with a trifling flare that I had to put close to my face to see.
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”
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.”
“There is actually a certain value in not finding anything,” he said. “It helps cosmologists towork out the rate at which galaxies are evolving. It’s one of those rare areas where theabsence of evidenceis evidence.”
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 Thorne, “he did not understand the laws of physics well enough to be able tosubstantiate his ideas.” Zwicky’s talent was for big ideas. Others—Baade mostly—were leftto do the mathematical sweeping up.
Looking for supernovae is mostly a matter of not finding them. From 1980 to 1996 heaveraged two discoveries a year—not a huge payoff for hundreds of nights of peering andpeering. Once he found three in fifteen days, but another time he went three years withoutfinding any at all.
Surprisingly little of the universe is visible to us when we incline our heads to the sky. Onlyabout 6,000 stars are visible to the naked eye from Earth, and only about 2,000 can be seenfrom any one spot. With binoculars the number of stars you can see from a single locationrises to about 50,000, and with a small two-inch telescope it leaps to 300,000. With a sixteen-inch telescope, such as Evans uses, you begin to count not in stars but in galaxies. From hisdeck, Evans supposes he can see between 50,000 and 100,000 galaxies, each containing tensof billions of stars. These are of course respectable numbers, but even with so much to take in,supernovae are extremely rare. A star can burn for billions of years, but it dies just once andquickly, and only a few dying stars explode. Most expire quietly, like a campfire at dawn. In atypical galaxy, consisting of a hundred billion stars, a supernova will occur on average onceevery two or three hundred years. Finding a supernova therefore was a little bit like standingon the observation platform of the Empire State Building with a telescope and searchingwindows around Manhattan in the hope of finding, let us say, someone lighting a twenty-first-birthday cake.
At the time Evans had a ten-inch telescope—a very respectable size for amateur stargazingbut hardly the sort of thing with which to do serious cosmology—and he was proposing tofind one of the universe’s rarer phenomena. In the whole of astronomical history before Evansstarted looking in 1980, fewer than sixty supernovae had been found. (At the time I visitedhim, in August of 2001, he had just recorded his thirty-fourth visual discovery; a thirty-fifthfollowed three months later and a thirty-sixth in early 2003.)Evans, however, had certain advantages. Most observers, like most people generally, are inthe northern hemisphere, so he had a lot of sky largely to himself, especially at first. He alsohad speed and his uncanny memory. Large telescopes are cumbersome things, and much oftheir operational time is consumed with being maneuvered into position. Evans could swinghis little sixteen-inch telescope around like a tail gunner in a dogfight, spending no more thana couple of seconds on any particular point in the sky. In consequence, he could observeperhaps four hundred galaxies in an evening while a large professional telescope would belucky to do fifty or sixty.
Zwicky also was the first to recognize that there wasn’t nearly enough visible mass in theuniverse to hold galaxies together and that there must be some other gravitational influence—what we now call dark matter. One thing he failed to see was that if a neutron star shrankenough it would become so dense that even light couldn’t escape its immense gravitationalpull. You would have a black hole. Unfortunately, Zwicky was held in such disdain by mostof his colleagues that his ideas attracted almost no notice. When, five years later, the greatRobert Oppenheimer turned his attention to neutron stars in a landmark paper, he made not asingle reference to any of Zwicky’s work even though Zwicky had been working for years onthe same problem in an office just down the hall. Zwicky’s deductions concerning dark matterwouldn’t attract serious attention for nearly four decades. We can only assume that he did alot of pushups in this period.
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.
WHEN THE SKIES are clear and the Moon is not too bright, the Reverend Robert Evans, aquiet and cheerful man, lugs a bulky telescope onto the back deck of his home in the BlueMountains of Australia, about fifty miles west of Sydney, and does an extraordinary thing. Helooks deep into the past and finds dying stars.
-Alexander Pope
“I just seem to have a knack for memorizing star fields,” he told me, with a franklyapologetic look, when I visited him and his wife, Elaine, in their picture-book bungalow on atranquil edge of the village of Hazelbrook, out where Sydney finally ends and the boundlessAustralian bush begins. “I’m not particularly good at other things,” he added. “I don’tremember names well.”
So when a hopeful and softspoken minister got in touch to ask if they had any usable fieldcharts for hunting supernovae, the astronomical community thought he was out of his mind.
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.”
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:
On January 15, 1934, the journal Physical Review published a very concise abstract of apresentation that had been conducted by Zwicky and Baade the previous month at StanfordUniversity. Despite its extreme brevity—one paragraph of twenty-four lines—the abstractcontained an enormous amount of new science: it provided the first reference to supernovaeand to neutron stars; convincingly explained their method of formation; correctly calculatedthe scale of their explosiveness; and, as a kind of concluding bonus, connected supernovaexplosions to the production of a mysterious new phenomenon called cosmic rays, which hadrecently been found swarming through the universe. These ideas were revolutionary to say theleast. Neutron stars wouldn’t be confirmed for thirty-four years. The cosmic rays notion,though considered plausible, hasn’t been verified yet. Altogether, the abstract was, in thewords of Caltech astrophysicist Kip S. Thorne, “one of the most prescient documents in thehistory of physics and astronomy.”
Four billion years later people began to wonder how it had all happened. And it is there thatour story next takes us.
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 comfortably safe 169,000 light-years away.
Supernovae do much more than simply impart a sense of wonder. They come 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.
“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.”
For the next 500 million years the young Earth continued to be pelted relentlessly bycomets, meteorites, and other galactic debris, which brought water to fill the oceans and thecomponents 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.
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.
“Or where he’s put things,” called Elaine from the kitchen.
Interestingly, Zwicky had almost no understanding of why any of this would happen.
But the universe is vast, and supernovae are normally much too far away to harm us. In fact,most are so unimaginably distant that their light reaches us as no more than the faintesttwinkle. For the month or so that they are visible, all that distinguishes them from the otherstars in the sky is that they occupy a point of space that wasn’t filled before. It is theseanomalous, very occasional pricks in the crowded dome of the night sky that the ReverendEvans finds.
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.
Eventually the clumps grew large enough to be called planetesimals. As these endlesslybumped and collided, they fractured or split or recombined in endless random permutations,but in everyencounter there was a winner, and some of the winners grew big enough todominate the orbit around which they traveled.
