Gold May Form When Neutron Stars Collide

July 18, 2013 at 08:44


Rumpelstiltskin would be jealous. A recently observed flash in the distant universe suggests that smacking two dense, dead stars together can create gold in vast amounts – with a mass 10 times that of the moon. The finding may help settle a debate about whether colliding stars or supernovae are the main sources of heavy metals in the universe.

“We see a signature that we interpret as the production of very heavy elements – gold, platinum, lead – exactly the kind of material whose origin was unclear,” says Edo Berger of the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

After the big bang, the universe contained only hydrogen, helium and lithium. Most of the other elements are built up in the cores of massive stars, and released when stars die. But stars lack the energy and the spare neutrons to be able to forge elements heavier than iron.

One idea often put forward to explain how such elements are made is that supernovae explosions of massive stars produce a powerful, fast-moving wind of freed neutrons and protons, which can convert lighter atomic nuclei released during the explosion into those of heavier elements.

But computer simulations of the process did not always produce the proportions seen in nature of certain elements. Some researchers suggested that neutron stars, the dense balls of mostly neutrons that are left over after a supernova, could build heavy elements more efficiently when they collide.

“Because it’s two neutron stars being collided, there’s an overabundance of neutrons to create these heavier elements,” says Berger. “What’s really been missing from this debate is actual data to settle this question once and for all.”

Berger and colleagues saw an opportunity to test this idea on 3 June, when NASA’s Swift satellite caught a brief burst of energetic gamma-ray photons radiating from a galaxy 3.9 billion light years away.

No one is sure what causes short gamma-ray bursts, but one of the most likely culprits is a pair of neutron stars that spiral into each other and eventually collide, forming a black hole. The gamma-ray burst is the final flash as any superheated material left over from the collision falls into the black hole.

“We’ve been lacking a smoking gun telling us that this is what’s happening,” says Berger. “I think we finally have that smoking gun.”

The team used the Hubble Space Telescope to observe the same spot in the sky 9.4 days after the burst, and saw an afterglow in visible light and long-wavelength infrared light.

The infrared glow is a signature of the decay of radioactive elements, says Berger. Based on its brightness, the team estimates that about 3 per cent of the neutron stars’ combined material was tossed outwards in the collision and escaped the black hole. The ejected neutrons that produced the unstable radioactive elements should have also created huge amounts of stable, heavy elements such as gold, platinum and lead.

“This is the first time that we’ve seen this signature,” says Berger. “People have speculated about this for a long time, but we’ve never actually seen it before.”

Given the amounts of gold and other elements that the collision probably produced, and how often similar collisions are expected to happen in the Milky Way, the team argues that neutron star smash-ups can account for almost all of the universe’s heavy elements.

“It’s possible that supernovae still produce a small contribution, but they do not appear to be the dominant process,” says Berger.

Daniel Kasen of the University of California, Berkeley, thinks the idea is intriguing, but he points out that we don’t know for sure how common neutron star mergers are.

“For reasonable rates, if they’re ejecting that much mass, it’s plausible or likely that they’re ejecting a big chunk of the heavy elements of the universe,” he says. “But it would be helpful if they get more data points.