Gold seen in neutron star collision debris

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Dead stars make good alchemists.

Images snapped by the Hubble Space Telescope suggest that gold may have been generated by a violent neutron star collision that also yielded lead, platinum, uranium and other heavy elements.

The stellar smashup was detected on June 3, when NASA’s Swift satellite observed a gamma-ray burst 3.9 billion light-years away. Astrophysicists believe that a crash between two neutron stars, the dense, neutron-rich cores left over after massive stars explode, released the 0.2-second flash of energy.

Images snapped by the Hubble Space Telescope nine days later saw evidence for a bounty of heavy elements amounting to about 1 percent of the sun’s mass and including several moon masses of gold, says Edo Berger of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Given the amount of gold and the fact that these collisions probably happen once every 10,000 or 100,000 years in any given galaxy, such crashes could account for all of the gold in the universe, he says.

Berger’s team posted its findings June 17 on arxiv.org.

The idea that neutron star collisions create heavy elements was suggested in the 1970s by James Lattimer, now at Stony Brook University in New York, and colleagues. Back then, Lattimer says, there were few observations of neutron stars and computers were slow. So, Lattimer says, “the calculations were fairly crude, and I’m not sure people believed the model that much.”

Instead, most astrophysicists thought elements heavier than iron — those that don’t form via fusion within a star — resulted from massive stars collapsing in supernovas. But computer simulations have had a tough time showing that this is possible, Lattimer says.

Although Lattimer says it’s “comforting” that the findings fit with the neutron star theory, he notes the results are “still fairly speculative.” The Hubble images show a glow of infrared light that Berger’s team says is characteristic of radiation emitted by the radioactive decay of heavy elements. Yet the team can’t rule out that the light was produced by the gamma-ray burst itself, Lattimer says.

Upcoming missions such as the Advanced Laser Interferometer Gravitational-Wave Observatory will make it easier to find short gamma-ray bursts and may confirm whether they are triggered by neutron stars, says astrophysicist Neil Gehrels of NASA’s Goddard Space Flight Center in Greenbelt, Md.

And new telescopes such as the Large Synoptic Survey Telescope that monitor larger swaths of sky will make it easier to distinguish between a gamma-ray burst’s afterglow and shine coming from heavy elements. Finding signs of heavy elements, which require a lot of neutrons, in association with short gamma-ray bursts is the best evidence yet that neutron star collisions make heavy elements.

This animation shows the collision of two neutron stars, which may unleash short gamma-ray bursts. Such collisions also eject material that may be the source of all the universe’s gold.
Credit: Dana Berry/SkyWorks Digital Inc.