Most of these cosmic fruitcakes are solitary wanderers, but some are paired up, as remnants of binary stars. They will orbit a common point in space and gradually drift closer and closer, spiraling toward one another in obedience to Einstein's laws of general relativity. One day, they will collide.
In the Milky Way galaxy, with hundreds of billions of stars, such a neutron-star collision is likely to happen about once every 100,000 years, Berger said.
Astronomers are fortunate in that the universe is so vast, containing many billions of galaxies, that any all-sky survey might possibly see something even as rare as a neutron-star collision. So it was that on June 3, NASA's Swift space telescope observed a flash of light, called a short gamma-ray burst (GRB), in an extremely distant galaxy in the constellation Leo.
Astronomers scrambled to reobserve that tiny patch of space with a powerful telescope in Chile and with the Hubble Space Telescope.
They saw something glowing where they'd earlier seen the GRB. After comparing their observations with theoretical models, the astronomers concluded that they were seeing the radioactive afterglow from a huge quantity of heavy metals formed by a neutron-star smashup.
This observation potentially explains this type of short-duration GRB. These flashes of light can briefly outshine an entire galaxy. The June event, in a galaxy 3.9 billion light-years away, lasted only two-tenths of a second.
Although neutron-star collisions had been proposed as a source of such GRBs, now there is a direct observation.
"When they make contact, several exciting things happen very quickly," Berger said. "Most of the material actually collapses to form a black hole. Some of the material then gets sucked into the black hole. That is the event that causes the gamma-ray burst. Some of the material gets spewed out into space. That material, since it came from neutron stars, is very rich in neutrons, and as a result, is very efficient at forming these heavy elements, including gold."