It has long been understood that Earth’s elements are of cosmic origin. Carbon and oxygen atoms in our bodies, for example, come from the interior of stars, where they were formed under high pressure and heat. They were later spewed into the universe in supernova explosions. It is literally true, as the late astronomer Carl Sagan was fond of saying, that we are all star stuff.
But new evidence suggests that gold and other heavy elements don’t come from supernovas, but from the neutron-star smashups.
“We are all star stuff, and our jewelry is colliding-star stuff,” said Edo Berger, an astronomer who led the research at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Neutron stars are the collapsed cores of stars that have exploded in a supernova. A neutron star might be roughly the diameter of the District but contain as much mass as our sun, or more, with all of it crammed together by the force of gravity until even the atoms have collapsed, leaving the object with the density of an atomic nucleus.
A teaspoon full of neutron-star material would weigh, on Earth, about 5 billion tons.
Most of these cosmic fruitcakes are solitary wanderers, but some are paired up, as remnants of binary stars. They can orbit each other for a billion years but will gradually drift closer and closer, spiraling together in obedience to Einstein’s laws of general relativity. One day, they will catastrophically merge.
This sort of thing isn’t a routine matter at the galactic level. 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. But the universe is big, containing many billions of galaxies, and so astronomers doing an all-sky survey will occasionally see one of these rare events. 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 a galaxy 3.9 billion light years away in the constellation Leo.
The burst lasted only two-tenths of a second. Astronomers scrambled to reobserve that tiny patch of space with a powerful telescope in Chile and 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 in the explosion caused by merging neutron stars.