Send in the clouds: the lives of molecules in space

Natural History, Dec, 2004 by Neil deGrasse Tyson

For nearly all of the first 400 millennia after the birth of the universe, space was a hot stew of fast-moving, naked atomic nuclei with no electrons to call their own. The simplest chemical reactions were still just a distant dream, and the earliest stirrings of life on Earth lay 10 billion years in the future.

Ninety percent of the nuclei brewed by the big bang were hydrogen, most of the rest were helium, and a trifling fraction were lithium: the makings of the simplest elements. Not until the ambient temperature in the expanding universe had cooled from trillions down to about 3,000 degrees Kelvin did the nuclei capture electrons. In so doing, they turned themselves into legal atoms and introduced the possibility of chemistry. As the universe continued to grow bigger and cooler, the atoms gathered into ever larger structures-gas clouds in which the earliest molecules, hydrogen ([H.sub.2]) and lithium hydride (LiH), assembled themselves from the earliest ingredients available in the universe. Those gas clouds spawned the first stars, whose masses were each about a hundred times that of our Sun. And at the core of each star raged a thermonuclear furnace, hell-bent on making chemical elements far heavier than the first and simplest three.

When those titanic first stars exhausted their fuel supplies, they blew themselves to smithereens and scattered their elemental entrails across the cosmos. Powered by the energy of their own explosions, they made yet heavier elements. Atom-rich clouds of gas, capable of ambitious chemistry, now gathered in space.

Fast forward to galaxies, the principal organizers of visible matter in the universe--and within them, gas clouds pre-enriched by the flotsam of the earliest exploding stars. Soon those galaxies would host generation after generation of exploding stars, and generation after generation of chemical enrichment--the wellspring of those cryptic little boxes that make up the periodic table of the elements.

Absent this epic drama, life on Earth--or anywhere else--would simply not exist. The chemistry of life, indeed the chemistry of anything at all, requires that elements make molecules. Problem is, molecules don't get made, and can't survive, in thermonuclear furnaces or stellar explosions. They need a cooler, calmer environment. So how in the world did the universe get to be the molecule-rich place we now inhabit?

Return, for a moment, to the element factory deep within a first-generation high-mass star.

There in the core, at temperatures in excess of 10 million degrees, fast-moving hydrogen nuclei (single protons) randomly slam into each other, giving rise to a series of nuclear reactions that, at the end of the day, yield mostly helium and a lot of energy. So long as the star is "on," the energy released by its nuclear reactions generates enough outward pressure to keep the star's enormous mass from collapsing under its own weight. Eventually, though, the star simply runs out of hydrogen fuel. What remains is a ball of helium, which just sits there with nothing to do. Poor helium. It demands a tenfold increase in temperature before it will fuse into heavier elements.

Lacking an energy source, the core collapses and, in so doing, heats up. At about 100 million degrees, the particles speed up and the helium nuclei finally fuse, slamming together fast enough to combine into heavier elements. When they fuse, the reaction releases enough energy to halt further collapse--at least for a while. Fused helium nuclei spend a bit of time as intermediate products (beryllium, for instance), but eventually three helium nuclei end up becoming a single carbon nucleus. (Much later, when carbon becomes a complete atom with its complement of electrons in place, it reigns as the most chemically fruitful atom in the periodic table.)

Meanwhile, back inside the star, fusion proceeds apace. Eventually the hot zone runs out of helium, leaving behind a ball of carbon surrounded by a shell of helium that is itself surrounded by the rest of the star. Now the core collapses again. When its temperature rises to about 600 million degrees, the carbon, too, starts slamming into its neighbors--fusing into heavier elements via more and more complex nuclear pathways, all the while giving off enough energy to stave off further collapse. The factory is now in full swing, making nitrogen, oxygen, sodium, magnesium, silicon.

Down the periodic table we go, until iron. The buck stops at iron, the final element to be fused in the core of first-generation stars. If you fuse iron, or anything heavier, the reaction absorbs energy instead of emitting it. But stars are in the business of making energy, so it's a bad day for a star when it finds itself staring at a ball of iron in its core. Without a source of energy to balance the inexorable force of its own gravity, the stark core swiftly collapses. Within seconds, the collapse and the attendant rapid rise in temperature trigger a monstrous explosion: a supernova. Now there's plenty of energy to make elements heavier than iron.