It’s one of the great fun facts of astronomy: most of the elements that make up our bodies, including carbon, oxygen, nitrogen, calcium—pretty much everything, in fact, aside from the H in our H2O—didn’t exist when the universe was young. Back then, there was nothing but hydrogen, helium and lithium. Heavier elements, on up to iron, were forged later by the heat and pressure deep inside stars. “We are all star stuff,” as Carl Sagan loved to say, in his inimitably geeky way.
Even stars can’t make elements as heavy as gold, however. For that, you need some sort of powerful shockwave, and until now it’s been unclear what could set it off. But a team of Harvard astronomers has come up a possible answer. The gold in our fillings and our jewelry and in Fort Knox may have been created during titanic collisions between neutron stars, the unimaginably dense husks left over after a massive star dies. “People like me have gathered a lot circumstantial evidence,” he says Edo Berger, lead author of a study just submitted to The Astrophysical Journal, “but until now it’s been a debate without actual data.”
That all changed last month when NASA’s orbiting Swift telescope initially spotted a burst of high-energy gamma rays that lasted just two-tenths of a second. Gamma-ray bursts are popping off all the time in the universe, but while longer-lasting bursts are caused by supernovas, or exploding stars, the short ones haven’t really been explained.
When this one flashed, however, astronomers quickly trained the twin, Chile-based Magellan telescopes on the spot, and caught a glow of visible light, which let them gauge the distance to whatever had exploded at 3.9 billion light-years from Earth. When they aimed the Hubble at that piece of sky a week later, the light was still there, but it had faded until it shone only in the infrared part of the electromagnetic spectrum.
The light’s characteristics can best be explained, say Berger, by a burst of brand new atoms totaling more than 3,000 times the mass of the Earth. Some are radioactive, which causes the glow. Some are atoms of platinum and lead and other heavy elements. And some, totaling several times the mass of the Moon, are pure gold.
The best explanation for the burst itself, Berger argues, is the collision of two neutron stars. “It’s catastrophic and it happens very fast,” he says. “Plus theorists think neutrons are crucial to the formation of these heavy elements, and neutron stars have plenty of neutrons.” That last bit is a no-brainer, but it’s less obvious to a lay person that the burst’s short duration means it must have happened in a small volume of space—much smaller than a supernova, for example, which is the competing theory for how gold is made.
Another point in favor of neutron stars, says Berger: “The rate of collisions in the Milky Way is one every 100,000 years, on average, and that’s just enough to explain the abundance of gold in our galaxy.
Actually, “rarity” might be a better word. A few moons’ worth of gold sounds like a huge amount—but if it comes along only once every 100,000 years or so, and then gets distribited through a galaxy made of 100 billion stars, with hundreds of billions of planets, stretching across 100 thousand trillion miles of space, it doesn’t go a long way. No wonder the stuff is so precious.