Send in the clouds: the lives of molecules in space

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.

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