The Canon (46 page)

For simplicity's sake, let us assume that our model founder star is made of pure hydrogen, with none of the other elemental efforts of the Big Bang adulterating it. Our exemplar star is an enormous condensation of hydrogen, hundreds of times more massive than the sun, and the electrons have been stripped from their protons, and all is a plasmic bisque. Gravity is tugging everything inward, toward an imaginary point at the center, and so the pileup of hydrogen particles is greater the more deeply you delve. At the superhot, high-pressure confines of the core, the hydrogen nuclei are swirled and squeezed, swirled and squeezed, until a critical threshold is surmounted, electromagnetic repulsion is defeated, and discrete hydrogen particles are fused into helium nuclei. The energy liberated by this thermonuclear fusion begins radiating outward, from the core toward the surface, and the billowing up of heat and light offers a counterbalance to the inward pull of gravity. In fact, the pulsing radiation, the bounty of fusion, is what keeps the star intact, keeps its inner layers from collapsing under the weight of those on top. But the effort is energy-intensive and cannibalistic.

As the Oxford chemist Peter Atkins has written, a star's appetite for hydrogen is "truly prodigious." Our sun, for example, fuses 700 million tons of hydrogen into helium every second, and in so doing radiates away pieces of itself each day, splashing warmth and light across the solar system, and My Very Educated Mother and her Nine (or eight) Pies, and their retinue of moons, and the asteroid belt, and Hale-Bopp, and Comet Kohoutek, too. Yet even though the sun has burned for 5 billion years, and even as it grows gaunter with every passing moment, it has enough hydrogen, packaged with just the right degree of density, to stay lit for another 5 billion years.

No such longevity for our voluptuous stellar forebear. Here, the great weight of its mountains of accreted matter heats up the core layers with staggering rapidity, accelerating the pace of fusion and quickly depleting the star's hydrogen stores, perhaps in as little as a couple of million years after the star first formed. Its hydrogen fuel exhausted and the stabilizing counterpressure of fusion energy momentarily cut off, the star again falls prey to gravity and sharply contracts. That downsizing in turn raises the temperature and density of the core, until the next thermonuclear threshold is crossed. Now the helium particles, the fruit of the previous act of fusion cuisine, start merging into carbon, infusing the star with a fresh burst of radiant energy that thwarts gravitational collapse. That is, until the helium, too, is spent, at which point another round of contraction ensues, followed by a new fusion event, and the creation, the so-called nucleosynthesis, of still heavier atoms at the stellar core. Onward through the periodic table of the elements the star marches in its struggle to stave off total collapse, hammering smaller nuclei into nitrogen, oxygen, sodium, phosphorus, potassium, calcium, silicon, yes, the familiar ingredients listed on nutrition labels or kicked up by bullies on the beach; onward through to the nucleosynthesis of iron and nickel, the elements with the most stable nuclear configurations of all. We're only about a quarter of the way through the atomic
table, and there are many other, heavier elements yet to be synthesized, but iron and nickel mark the end of the line for fusion power. If you fuse an iron nucleus with another nucleus, you won't release any energy. To the contrary, this heavy-handed union requires an
input
of energy. And the fluttering outward of liberated, radiant energy is what keeps a star from caving in on itself. The last good-bye is nigh.

At this stage, the star is a like a great ball of baklava, with a dense center of nickel and iron nuclei surrounded by thin shells of successively lighter elements that it had baked up through the ages but hadn't gotten around to cannibalizing. Lacking any radiant bulwark against gravity, the whole construct again condenses, and the core temperature soars to 8 billion degrees, hot enough to synthesize elements a bit beyond iron and nickel, yes, but to no avail for the star: its engine of thermonuclear stability, the release of radiant energy through fusion, is dead. The core begins to lose structure, and upper layers dive in toward lower layers. Violent photons of light ricochet in all directions, splitting apart any heavy nuclear particles that stand in their way. The star's interior goes into free fall, the stippled, plasmic strata of itself streaming helplessly toward that imaginary point at the center of the orb. In less than a second, a core the width of many suns is squeezed down to something the size of North America. The catastrophic contraction sends shock waves through the entire celestial body and blows out a halo of stellar matter "like a great spherical tsunami," as Peter Atkins puts it. Our star explodes as a supernova, and in those furious closing moments of its life, the real heavyweights of the elemental table are forged—platinum, thallium, bismuth, lead, tungsten, gold. The newborn particles are scattered into space, along with the many other, comparatively lighter elements that the star belly had built up before the whole star went belly-up.

For this sequined shrapnel, we can thank our lucky stars. By salting the young universe with heavy elements, particularly metals, the first few meganovas helped touch off a boom in stellar construction. The background gas was hot, Chuck Steidel explained, and it's difficult to get stars started from masses of overheated, overexcited gas. The metal particles bequeathed by the progenitor stars cooled the gaseous landscape enough for a multitude of nebulous eddies to begin condensing into stars, into clusters of stars, into bustling, burgeoning barrios of stars. "We think it went from massive stars to small galaxies relatively quickly, during the first billion or so years of the universe," said Steidel. Larger galaxies would then have formed through mergers and acquisitions, by collisions between galaxies or by one comparatively dense galaxy gravitationally sucking in the contents of a smaller galaxy. By 12 billion or so years ago—1.7 billion years after the Big Bang—the majority of the universe's galaxies had formed, including our Milky Way, although they would continue to sail ever outward and away from one another, buoyed on the expanding silk of space; and each galaxy would continue to evolve, its gathered goods to rotate around the midpoint of its mass, its stellar citizens to live out their lives at varying tempos and temperatures, depending on their mass and their proximity to other stars. In many galaxies, particularly spiral ones, we find flourishing stellar nurseries, comparatively thick patches of gas and dust from which new stars perpetually are condensing, the natal event often driven by the obliging and violent death of a massive older star that lived next door.

So it likely was with our solar system. Some 5 billion years ago, the shock waves of an exploding supernova and the concomitant expulsion of the star's salubrious heavy elements into interstellar space spurred a ragged cloud of gas and dust in one of the Milky Way's arms to begin condensing. As it contracted, the nebula began to spin (just as a figure skater spins by pulling in her arms) and to flatten into a disk (as a figure skater happily does not). Through several million gyrating years, the bulk of the mass was drawn by gravity toward the center of the pancake, forming a bulge of ever escalating heat and density, which finally burst into thermonuclear splendor. Still some discus matter remained around our newborn sun, a petticoat of gas, dust, and all the hundred-odd elements that Dmitri Mendeleev would later seat around his table. That matter formed clumps: the protoplanets and their protomoons. Closer in to the central orb, only aggregates of rock and metal could withstand the heat, and so the four inner planets—Mercury, Venus, Earth, and Mars—are balls of rock and metal and are designated the terrestrial planets, firma to their core. Farther out on the disk, it was frigid enough for water to freeze, and once the ice particles had formed they collided and gathered gas and dust in a veritable snowball effect, yielding the four outer planets, the so-called gas giants—Jupiter, Saturn, Uranus, and Neptune. As for Pluto and Sedna and others of their subcompact class, whether you consider them planets, dwarf planets, planetisimals, planet parodies, or Planters party mix, they were formed in the Kuiper belt, one of the coldest, flimsiest nethermost rims of the solar disk, where there was not enough there to make much of them. Pluto and Sedna are among the behemoths of the icy, rocky bodies in the belt, and still you could hide nearly 10 Plutos inside tiny Mercury, and maybe 150 inside the Earth.

Or sun is a good star, a stalwart star, and its life span is only halfway through. But when its hydrogen store starts running low, the sun will have just a few ploys for keeping its plasma aflame. In 5 billion years, having depleted the hydrogen at its dense core, the sun will start burning the hydrogen in its comparatively thin outer layers, puffing up as it does so to thirty times its current girth. That swollen sun will be a cooler sun, its radiance ruddier than today. Our sun will be a red giant, and woe to any earthlings who may be around to witness its bloated blush, for the planet on which they stand will likely be vaporized in the expansion. Our distant descendants would do best to abandon Earth well ahead of time, relocating to, say, one of the bigger moons of Jupiter or Saturn. Come the sun's expansion, places like Jupiter's Ganymede and the saturnine Titan will be transformed into far more clement places than they are today—their skies brightened, their ice stores thawed into liquid oceans and rivers. Titan even has a gas atmosphere that, while not currently breathable, in theory could be reconfigured to suit human respiration, and its scenic views of Saturn's rings are an obvious plus. Wherever the space farers alight, they may as well kick off their boots and settle into a comfortable chair. The sun will radiate as a red giant for another 2 billion years.

And then? Then it's time to decamp and head for a whole new solar system. Our star lacks sufficient mass to explode, and instead it will simply sputter into barren obscurity. After the hydrogen shell has been exhausted, the core will contract sharply, and its upper layers will start sloughing off into space. In the end, all that will remain is a dense, smoldering ember of carbon and oxygen barely bigger than Earth. The once mighty Ra and plucky
pro tem
red giant will have become a white dwarf; and though it can no longer generate fusion power, by sheer heat it still glows, and it will glow in this guise for the rest of all time.

The sun and other midsize stars can build from the Bang basics a handful of the elements we bio-Legos demand, notably carbon, oxygen, and nitrogen. The frequency with which ordinary stars concoct oxygen partly explains why oxygen is the third commonest element in the universe, after hydrogen and helium; and the combined commonness of hydrogen and oxygen explains why there is water, water everywhere, though only on Earth in abundant drops to drink. But stars of modest means and restrained temperaments keep most of what they make to themselves and supply only trace quantities to the universal inventory of post-helium parts, the weighty elements from which animate matter is made. The overwhelming bulk of our mortal cargo—the carbon in our cells, the calcium in our bones, the iron in our blood, the electrolytes of sodium and potassium that allow our hearts to beat and our brain cells to fire—was stoked in the furnaces of far larger stars than ours and splattered into the cosmic compost when those stars exploded. "We are star stuff, a part of the cosmos," said Alex Filippenko. "I'm not just speaking generically or metaphorically here. The specific atoms in every cell of your body, my body, my son's body, the body of your pet cat, were cooked up inside massive stars. To me, that is one of the most amazing conclusions in the history of science, and I want everybody to know about it."

The gaseous nebula from which our solar system formed very likely had been enriched several times over with star stuff, with the luxurious carnage of multiple supernovas that had exploded nearby over the course of the last 10 billion years. Each round of enrichment had enhanced the chance that the cloud at last would cool, and swirl, and condense into a skirted star, and the skirt would prove elementally weighty enough to yield the rocky, complex inner planets on which life could make a deal. Not on eenie or meenie, and Moe, I don't think so. But behind curtain number three, the full monty, it's mine.

We know that there is life on Earth, and that at least one species among its phylogenetic plentitude is, if not always sensible or reliable, certainly very clever at inventing tools, especially tools that allow us to engage in animated, disembodied forms of communication while simultaneously driving, jaywalking, or attending our daughter's piano recital. We are such indefatigable telecommunicators that the world and its 6.5 billion content providers don't feel like enough, and we can't help but wonder, Who else can we call? Are there other beings, on other worlds, and will we ever be able to contact them, or they us? Are we alone, or one of millions of habited planets in the galaxy, or billions in the universe? Will it ever stop feeling so hard and so hollow to ask and to ask and to ask again? Is there any evidence one way or the other for extraterrestrial life? What do astronomers think, and does their thinking on this most cosmic of all questions have any more moment than a five-year-old's musical Milky Way dream?

The answers to these questions are a mix of bad news, no news, and good news. The bad news is, no, we can't yet contact any extraterrestrial beings, not even with the sort of miraculous long-distance connections by which presidents ring up astronauts to banter about space food and mountaineers trapped in a storm on the top of Everest call their loved ones to discuss the slim odds of their being home for dinner. If we could, don't you think they'd already be working as customer service representatives under suspiciously bland names like Hank or Sherry?

Most of the news from the space-alien front is, alas, no news, or rather we-don't-know news. We have no evidence one way or another about whether there is life on other worlds. None. After the initial flurry of excitement in the 1990s over the possibility that we had detected signs of past or current microbial life on Mars, the evidence fell apart. There is no credible evidence that extraterrestrials have ever visited Earth or abducted any earthlings or searched any earthly body cavities for their inscrutable, nefarious purposes. Aliens have yet to respond to the recordings we included aboard the first two
Voyager
spacecraft, launched in 1977—of wistful greetings in fifty-five languages; the music of Bach, Beethoven, Louis Armstrong, Peruvian pan pipers, Azerbaijani balaban players; and whales singing, chimpanzees grunting, and a train whistle passing in exemplary Doppler form. Is there life on other planets? We don't know yay or nay, there's no proof either way, so on this topic scientists can have nothing to say—can they?