LIGO Announcement: Colliding Neutron Stars Discovered for First Time, Sheds Light on Origin of Gold

An artist's rendering of the collision of two neutron stars. Carnegie Institution for Science

Scientists announced on Monday a first-of-its-kind detection: They have heard the echo of two neutron stars colliding, a giant explosion of matter about 130 million light-years away. Scientists have also confirmed that these mergers are capable of producing heavy metals, including gold and platinum.

The results were published in a suite of papers in journals like Nature, Science and Astrophysical Journal Letters and were announced during a pair of press conferences. For the first time, the discovery draws on not just gravitational wave data but also observations made using a range of different types of light.

"The signals that were picked up were of a kind that we'd been hoping to find since the very early days of the project," Peter Saulson, a physicist at Syracuse University who has been involved with the Laser Interferometer Gravitational-Wave Observatory (LIGO) since 1981, tells Newsweek. He adds that the data produced by a neutron star merger is both a very strong signal and very distinctive. "When we see it, we will know that we've seen it."

And now, the scientists say, we've seen it.

The story begins 11 billion years ago, when two stars were born in a galaxy we call NGC 4993. Each weighed about 10 times as much as our own sun. First one then the other burst in a brilliant explosion called a supernova, flinging much of their contents out into the galaxy and squishing all the rest to become neutron stars, a super-dense star with just a little more mass than our sun—crammed into a space about the size of Manhattan.

Drawn inexorably inward by their own gravity, the two neutron stars began to circle each other as if headed down a drain, approaching faster and faster all the time, while contorting space-time around them and spitting out larger and larger gravitational waves faster and faster.

Finally, they slammed into each other at one third the speed of light, creating one last gravitational wave. The collision flung neutron-rich material into space in a flash of light called a kilonova, and that material decayed to form several heavy metals—including gold and platinum. Where the drain would have been, a giant stream of high-energy gamma rays shot off in both directions for less than two seconds.

And the gravitational waves that marked the stars' last moments rippled out through the universe, traveling millions of light-years until they reached a few patches of Earth where scientists have built enormous, super-sensitive ears tuned to just these apocalypses—the twin LIGO detectors.

The 100-second-long signal arrived at 8:41 a.m. ET on August 17, just three days after the machines sensed their fourth black hole collision, four days before a total solar eclipse swept across America and eight days before LIGO turned off to begin a year of upgrades. Just two seconds later, the Fermi spacecraft registered a short burst of high-energy gamma radiation coming from the same celestial neighborhood.

"We realized, OK, this is the day we've been waiting for," says Raffaella Margutti, an astrophysicist at Northwestern University who studies explosions in space. Within hours, astronomers around the world had received an alert and turned 70 telescopes to a patch of the sky.

That was crucial because all of the data suggesting this was a neutron star collision came from these astronomers. "With just the LIGO piece, all you would be able to say is that two sources merged that were likely roughly this mass," says Ben Shappee, an astronomer with the Carnegie Institution for Science who happened to have time reserved at telescopes in Chile on August 17. He and his colleagues obtained the first images of the event's kilonova, a bluish burst of light where none had been before.

The kilonova produced by the explosion appeared as a burst of bluish light. Carnegie Institution for Science

Observations went on for two weeks after the initial detection, until NGC 4993 slipped behind the sun, and will continue in December when the galaxy comes out of hiding. Those observations cover a variety of types of light and will help scientists better understand the details of the event.

Unusually for a high-profile scientific publication, the papers describing the new data were not made available in advance to journalists to read and to seek comment from unaffiliated experts. (Although unaffiliated experts would be hard to find: One project member estimates that about 30 percent of the entire astronomical community is involved with the suite of papers.)

That, plus the observations still to come, mean scientists still have plenty of questions about what precisely took place, so far away, to two tiny dense stars with a metallic sheen.