Gold mine. Colliding neutron stars may create precious metals. CREDIT: STEPHAN ROSSWOG AND RICHARD WEST, UNIVERSITY OF LEICESTER

Most elements in the universe heavier than hydrogen and helium are produced by nuclear fusion inside stars. But the heavy metals like gold and platinum are not synthesized during a normal star's lifetime. Instead, astronomers believe, heavy elements form by rapidly adding neutrons to existing lower-mass atoms like iron. This can only happen in exotic circumstances, where temperatures and densities are extremely high. These conditions exist during a supernova, the explosion of a massive star that has spent its fuel.

Another possible occasion is the collision of two neutron stars, the collapsed remnants of supernovae. When two neutron stars tightly orbit each other, they slowly spiral inward, and a merger is unavoidable. But the theory that heavy metals came from merging neutron stars had never been put to the test of exhaustive computer simulations.

Using a new supercomputer at the United Kingdom Astrophysical Fluids Facility, physicist Stephen Rosswog of the University of Leicester, together with colleagues from Leicester and Basel University in Switzerland, simulated the catastrophic impact of two neutron stars. Although the superdense stars eventually merge into a single black hole, some debris is ejected. In this extremely dense and hot matter, energetic neutrons can turn base elements into precious metals, according to the simulations. The simulations, presented on 5 April at the U.K. National Astronomy Meeting in Cambridge, match the observed abundances of gold and platinum in the universe.

These are the first detailed computer simulations to test the neutron star theory. But, says Craig Wheeler of the University of Texas, Austin, there are alternative models. "The general belief is that [rapid neutron capture] takes place somehow in supernova explosions. That has been hard to prove or rule out."

--GOVERT SCHILLING

Related sites

More images and movies from the computer simulation
UK Astrophysical Fluids Facility
Alan Calder's page on neutron star mergers
Primer on the origin of heavy elements