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.
The observation potentially explains this type of short-duration GRB. These flashes of light can briefly outshine an entire galaxy.
Although neutron-star collisions had been proposed as a source of such GRBs, now there is a direct observation.
“We now have kind of a smoking gun,” said Berger.
A pair of neutron stars can orbit each other for a billion years before colliding, and then, Berger said, “several exciting things happen very quickly.” Most of the material collapses to form a black hole. Some of it is spewed into space. That material is rich in neutrons, which drives the formation of heavier and heavier elements, the way mud piles up on an off-road vehicle.
“You need a lot of neutrons to throw at some seed nucleus to build it up to something heavy like gold or lead or platinum,” said Daniel Kasen, a University of California at Berkeley astrophysicist.
These explosions make an astonishing amount of heavy atoms. Kasen did a back-of-the-envelope calculation for the June 3 event and found that it produced about 20 Earth-masses worth of gold. That’s enough gold to fill 100 trillion oil tankers, he said.
And if platinum is your thing, then rejoice: These collisions create seven times as much platinum as gold.
“I’m partial to the name ‘blingnova’ to describe this kind of event, since what we are seeing is basically an ostentatious glimmering of riches,” Kasen said.
A lingering question: Does this mean gold and other metals can’t be produced in a supernova? Berger’s calculations show that the neutron-star origin can account for all the gold in the universe.
At a news conference Wednesday, he responded to a question by saying he could not rule out the possibility that some gold comes from garden-variety supernovas. But he said, “There is no real need to invoke another mechanism.”
There’s still a lot that must be done with those gold atoms before they wind up on someone’s front tooth. The gold is basically dust in the wind, atomized, until it moves into a cloud of material that can coalesce, through the force of gravity, into a solar system of planets with a star at the center.
Early in Earth’s history, asteroids containing gold continued to bombard the surface. Over a billion years or more, the planet’s geological processes concentrated the atoms of gold so that they could form veins and nuggets. Gold is chemically nonreactive, almost inert, and doesn’t want to bond with other elements.
“It’s a process of distillation. That’s what planets do,” said Robert Hazen, a mineralogist at the Carnegie Institution of Washington. Gold is rare on Earth — about one part per billion in the Earth’s crust, Hazen said.
Most of Earth’s gold is trapped in the planet’s core, he said. And, he added, there’s a long-standing conjecture that at the very center of the Earth is a small core that’s pure gold.
So, does Earth have a heart of gold? They haven’t found a way to check on that — yet.