The Plume of Gold Ejected by a Cosmic Collision

Astronomers say the merger of two neutron stars produced about 10,000 Earth-masses of precious heavy elements.

NSF / LIGO / Sonoma State University /A. Simonnet

Some 130 million years ago, in another galaxy, two neutron stars spiraled closer and closer together until they smashed into each other in spectacular fashion. The violent collision produced gravitational waves, cosmic ripples powerful enough to stretch and squeeze the fabric of the universe. There was a brief flash of light a million trillion times as bright as the sun, and then a hot cloud of radioactive debris. The afterglow hung for several days, shifting from bright blue to dull red as the ejected material cooled in the emptiness of space.

Astronomers detected the aftermath of the merger on Earth on August 17. For the first time, they could see the source of universe-warping forces Albert Einstein predicted a century ago. Unlike with black-hole collisions, they had visible proof, and it looked like a bright jewel in the night sky.

But the merger of two neutron stars is more than fireworks. It’s a factory.

Using infrared telescopes, astronomers studied the spectra—the chemical composition of cosmic objects—of the collision and found that the plume ejected by the merger contained a host of newly formed heavy chemical elements, including gold, silver, platinum, and others. Scientists estimate the amount of cosmic bling totals about 10,000 Earth-masses of heavy elements.

Neutron stars are the collapsed cores of dead stars, the sole survivors of supernovae. They are the densest known objects in the universe; one neutron star measures about the size of a bustling city, but has about the same mass as our sun. A teaspoon of its contents would weigh about 10 million tons.

When neutron stars merge, they release a fire hose of neutrons. Heated to extreme temperatures, the neutrons bombard surrounding atoms, and form heavy elements. The baby elements go on to become part of other objects of the universe, like stars and planets, including our own.

The discovery confirms a long-standing astronomical theory. Astronomers have suspected for decades that neutron-star mergers were responsible for the production of most of the heavy elements found in the universe. The lightest of the elements, like hydrogen, helium, and lithium, came from the Big Bang. Heavier elements, like carbon and oxygen, came later, fused in the hearts of stars. Some even heavier elements erupted from supernovae. But computer simulations showed these explosions weren’t powerful enough to forge some of the elements that are heavier than iron, like the precious metals. The universe needed another kind of explosion called a kilonova, which shines 1,000 times brighter than a typical supernova.

Until this summer, astronomers only had theoretical models for such an event. Now, they say the new data suggests neutron-star mergers could account for about half of all elements heavier than iron in the universe.

“I think this can prove our idea that most of these elements are made in neutron-star mergers,” said Enrico Ramirez-Ruiz, a theoretical astrophysicist at the University of California, Santa Cruz, who worked on the discovery, in a press release Monday. “We are seeing the heavy elements like gold and platinum being made in real time.”

Scientists will spend years poring over the data from this discovery. We’re just getting to know these cosmic collisions and the science behind them. But we have been long acquainted with their effects. The aftermath of neutron-star mergers is all around us, in our mines, computers, toasters, wedding bands—the list goes on.