SEPTEMBER 2009 – NO. 35
The Fallen Sky
The story of the solar system begins with dust
The story of the solar system — and all that follows, from meteorites and those who cherish them — begins with dust. Stars died. They exploded. There were, researchers have conjectured, perhaps up to ten supernovae whose traces we can read in the solar system today. When such stars explode, the remains gather together at a far distance to start again as dust, entrained with capes of gas.
Supernovae occur when large stars — those about 25 times heftier than our sun — can no longer burn. They've run out of fuel, one element after another having been gobbled up through hunger and change. After it has burned hydrogen, helium, carbon, neon, and oxygen in successively shorter ages, comes the last day in the life of such a star, a few hours fueled by silicon. Within the star, iron is about to be shredded like a puffball because iron makes less energy than it needs in order to be a fusion source. One ordinary second ends it all: the star collapses, sending shock waves slamming through it, and the star just rips apart.
Previously, the star had expelled carbon gas, which eventually condenses into diamonds. "Countless trillions of diamonds," as scientist Harry Y. McSween, Jr. puts it, "but not a single one will ever grace a pendant or seal a marriage vow." The supernova itself casts out, among other things, a rare form of xenon that infuses into those interstellar diamonds. And some of these micro-diamonds make their way into meteorites that land on Earth. It took two decades of painstaking work by chemist Edward Anders and other scientists to coax from meteorites the traces of all that interstellar material. "The ... grains ...." McSween writes, "are the grime of a dozen other stars and the smoke of at least one supernova." Oxygen, plutonium, and iodine are forged in the hearts of dying stars. Gold too. All that sent forth in a flash of iron demise.
Stardust soared from explosions brighter than galaxies. Gas billowed. Shock waves with their linty freight hit other particles and gases already lingering in one small pocket of the Milky Way. Gravity rescued them.
About 4.6 billion years ago this cloud of dust and gas began slowly to contract, rotate, and flatten. The heart of the presolar nebula began to quicken, and within a few thousand years a protostar — not yet fusing hydrogen — went through phases of contraction, expansion, and varied brightness. It took some 10,000 years for outward gas pressure to balance against gravitational contraction, then another ten million years for the protostar to condense just enough to create what became our sun, whose interior pressures forced a sustained hydrogen fusion reaction at ten million degrees.
When I first began reading in detail about the development of the solar system, news arrived that researcher Eric Feigelson and others had used the orbiting Chandra X-ray Observatory to stare deep into the star womb of Orion, where each winter the green wing of that nebula — some 1,500 light-years away — flies into the view through my backyard telescope. In northern Utah, where I've lived since 2002, I'll stand outside in my parka hunched over the scope, just staring into the nebula as it rises over the Bear River Range, my face cold, my nose running. I love seeing the Orion Nebula. I love knowing that I'm looking back at a version of what preceded our own solar system. Feigelson found that Orion's young stars — stars like our own in its childhood — flare up in the X-ray portion of the spectrum, producing isotopes of beryllium-10, calcium-41, and aluminum-26, all substances found in meteorites and once thought to be made only by supernovae. I've also learned that at least one huge star in its old age shed some mass in an interstellar wind rich in aluminum-26, which also could have precipitated the contraction of materials that became our solar system. Not long after shedding its aluminum-26, the star went supernova. So the presolar nebula contained stuff cast off by massive stars as well as the remains of a supernova — or more than one — and after our young sun formed, it too gifted all manner of energy and isotopes.
The presolar nebula's contraction, the outward pressure of the protostar, then of the sun, and the rotation of the entire ensemble led to a complicated dance of creation. (I use the word "creation" — it's a good word — in a nondeistic sense.) Like a figure skater's scratch spin — legs crossed, arms pulling in, beautiful dervish, quicker and quicker on ice, the greater the contraction, the faster the spin — this not-quite-a-solar-system drew in more particles and gas molecules into faster orbits. Working against the contraction was, some say, a magnetic field generated by the shrinking cloud itself, which may have slowed the solar system's formation.
What material didn't flow inward to the warming center accreted in "small, loosely bound dustballs," researcher David Kring says. Particles clumped, bumped, stuck, and flew. Grains and flecks became pebbles. Electrostatic attraction overcame the shredding violence of impacts, swirling gases, and solar winds. Data from the Stardust space probe, which returned actual samples of a comet, suggest that in addition to stuff falling toward the center, gas pressure and turbulence pushed heated materials outward along the plane of the developing solar system to the chillier reaches of this messy, mixed-up cloud. Such stuff can get out beyond the so-called snow line, where it's cold enough for ice to form in space. Scientists are finding evidence that isotopes varied in different parts of the proto-solar system, which means conditions varied not only by temperature but by composition of materials.
Grains, flecks, and pebbles accreted into rocks. They became careering piles of rocks, then boulders upon boulders. Once these objects were more than a half-mile wide, gravity superseded chance accretion, electrostatic attraction, and drag, thereby speeding up formation of the planetesimals. These minor planets, the asteroids, were born. From them would come meteorites. Easily told in a few sentences, this transformation from dusty disk to solar system actually took millions of years.
On days so cold that water vapor freezes directly in the air — ice crystals appearing as if by magic in a clear sky, catching sunlight in a dazzling curtain of glitter — I can almost believe I'm at the beginning, floating through the outer reaches of the solar system. I think of what Kring said about hot dust that cooled rapidly: that those grains "condensed like snow from the air." On winter afternoons in Utah, on a hike, say, with my partner Kathe, I sometimes try to imagine that I'm standing on a minor planet billions of years ago instead of in a snowy draw flanked by juniper and Douglas fir. I try to imagine that I'm looking toward Neptune, toward Pluto, toward the distant Oort cloud with its encircling moat of billions of comets.
Snow falling from a cold, cloudless sky has a name. It's called "diamond dust."
Excerpt from THE FALLEN SKY: An Intimate History of Shooting Stars by Christopher Cokinos (Tarcher/Penguin hardcover, 2009).
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