In the past few years, a debate has erupted. Many astronomers now believe that the space-quaking merger of two neutron stars can forge the universe’s supply of heavy elements. Others hold that even if garden-variety supernovas can’t do the trick, more exotic examples might still be able to. To settle the argument, astrophysicists are searching for clues everywhere, from alchemical computer simulations to gamma-ray telescopes to the manganese crust of the deep ocean. And the race is on to make an observation that would seal the deal—catching one of the cosmos’s rarest mints with its assembly line still running.
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In 1957, the physicists Margaret and Geoffrey Burbidge, William Fowler, and Fred Hoyle laid out a set of recipes for how the lives and deaths of stars could fill in almost every slot in the periodic table. That implied that humans, or at least the elements making up our bodies, were once stardust. So was gold—somehow.
“The problem itself is rather old, and now for a long time has been the last stardust secret,” said Anna Frebel, an astronomer at the Massachusetts Institute of Technology.
The Big Bang left behind hydrogen, helium, and lithium. Stars then fused these elements into progressively heavier elements. But the process stops at iron, which is among the most stable elements. Nuclei bigger than iron are so positively charged, and so difficult to bring together, that fusion no longer returns more energy than you have to put in.
To make heavy elements more reliably, you can bombard iron nuclei with charge-free neutrons. The new neutrons often make the nucleus unstable. In this case, a neutron will decay into a proton (popping out both an electron and an antineutrino). The net increase of a proton leads to a new, heavier element.
When additional neutrons are thrown into a nucleus more slowly than it can decay, the process is called slow neutron capture, or the s process. This makes elements such as strontium, barium and lead. But when neutrons land on a nucleus faster than they decay, rapid neutron capture—the r process—occurs, beefing up nuclei to form heavy elements including uranium and gold.
In order to coax out the r-process elements, the Burbidges and their colleagues recognized, you would need a few things. First, you have to have a relatively pure, unadulterated source of neutrons. You also need heavy “seed” nuclei (such as iron) to capture those neutrons. You need to bring them together in a hot, dense (but not too dense) environment. And you want all this to happen during an explosive event that will scatter the products out into space.
To many astronomers, those requirements implicate one specific kind of object: a supernova.
A supernova erupts when a massive star, having fused its core into progressively heavier elements, reaches iron. Then fusion stops paying off, and the star’s atmosphere crashes down. A sun’s worth of mass collapses into a sphere only about a dozen kilometers in radius. Then, when the core reaches the density of nuclear matter, it holds firm. Energy rebounds outward, ripping apart the star in a supernova explosion visible from billions of light years away.