Astronomers strike gold – and platinum – as they watch two neutron stars collide

An illustration of a neutron star collision from 130 million years ago that produced gold, platinum and other heavy metals. (NSF/LIGO/Sonoma State University/A. Simonnet)

In a highly anticipated first, scientists said they’ve detected the collision of two neutron stars and confirmed that these cataclysmic events are indeed a source of gold, platinum and other heavy elements in the universe.

The powerful smash-up produced gravitational waves that were picked up by the LIGO and Virgo observatories. It also emitted a broad swath of electromagnetic radiation that could be seen by more traditional telescopes, including ones that capture visible light.

By studying the gravitational waves, gamma rays, X-rays, ultraviolet light, infrared, radio waves and visible light from a single event, the scientists said they have embarked on the era of multi-messenger astronomy — one that promises a far deeper understanding of some of the most powerful and elusive phenomena in the cosmos.

“This is the beginning,” said Duncan Brown, a gravitational-wave astronomer at Syracuse University and member of the LIGO Scientific Collaboration. “This is the beginning of bringing the entire human toolkit of observations, of gravitational waves and electromagnetic waves, to bear on understanding our universe and where we live.”

The findings, described in a suite of papers released Monday, mark the first time the larger astronomical community has been able to study a gravitational-wave event.

Laura Cadonati of Georgia Tech, LIGO’s deputy spokeswoman, said at a briefing in Washington, D.C., that the combined information from gravitational waves and light was “bigger than the sum of its parts.”

The improvement, she said, “is equivalent to the transition from looking at a black-and-white picture of a volcano to sitting in a 3D IMAX movie that shows the explosion of Mount Vesuvius.”

Previously, LIGO and its European partner Virgo have picked up only collisions between black holes — events that can’t be detected with telescopes because not even light can escape a black hole’s powerful gravitational pull.

But scientists with the LIGO-Virgo collaboration have been itching to find a collision between two neutron stars because it would produce both gravitational and electromagnetic waves.

Neutron stars are the corpses of massive stars whose cores collapsed in supernova explosions. While they’re not that big, they’re incredibly dense, packing a sun’s worth of mass into the size of a city. A teaspoon of neutron-star stuff weighs around a billion or so tons.

The cosmic crash described Monday occurred about 130 million light-years away in the constellation Hydra, said David Reitze, executive director of the LIGO Laboratory at the California Institute of Technology. After a long dance toward each other, two neutron stars — one somewhere around 1.1 solar masses, the other weighing in the neighborhood of 1.6 suns — finally collided, converting some of their combined mass into gravitational waves.

These waves are ripples created by objects as they accelerate or decelerate, rather like the wake made by a boat moving through the water. Albert Einstein predicted their existence 101 years ago as part of his general theory of relativity, and LIGO scientists won a Nobel Prize this month for detecting gravitational waves and proving Einstein correct.

“It’s good the Nobel Prize was awarded once already to LIGO, because it may be in the future again,” Avi Loeb, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who was not involved in the work, said with a laugh. “This is very significant.”

Earth’s first inkling of the neutron-star collision came on Aug. 17, after LIGO’s twin detectors in Hanford, Wash., and Livingston, La., measured a powerful gravitational “chirp.”

This signal, dubbed GW170817, looked very different from the waves produced by colliding black holes, Reitze said. Since neutron stars are less massive than black holes, a doomed pair takes longer to complete its final death spiral and packs many more waves into a single event.

Around that time, NASA’s Fermi Gamma-ray Space Telescope picked up a powerful flash of high-energy gamma rays. (The gamma rays are produced after the gravitational waves, but Fermi was the first to send out an alert.)

Both Fermi and the LIGO and Virgo detectors were able to identify a patch of sky that was the likely source of their event — and those two areas overlapped.

“This produced the exact reaction you might expect: the astronomical equivalent of stopping traffic while we all stopped to go and get a look,” said University of California, Santa Barbara astronomer Andy Howell, a staff scientist at the Las Cumbres Observatory, whose global network of telescopes also followed the event.

Within hours, astronomers were training their telescopes on that promising region, looking for X-rays, ultraviolet waves, optical light, infrared light and radio waves. Each of these bands of the electromagnetic spectrum yields different kinds of information about the source, allowing the researchers to study this neutron-star collision in unprecedented detail.

Among their discoveries: Infrared and optical cameras found signs that heavy elements such as gold, platinum and neodymium had been produced by this powerful event. While nickel, copper, iron and other elements can be produced by supernovas, scientists have long suspected that many elements heavier than iron are often born from the collision between neutron stars.

“They’re really cosmic foundries for heavy elements like gold, platinum, uranium,” Reitze said. “That’s pretty amazing.”

The findings could explain the mysterious origins of a type of gamma-ray burst known as “short-hard,” which produces brief but highly energetic gamma-ray bursts — powerful flashes of light.

“That just solved a longstanding problem in astrophysics,” Brown said.

Marcelle Soares-Santos, an astrophysicist at Brandeis University who also studied the event, said the findings could help illuminate a deep mystery of cosmology: the nature of dark energy.

Researchers track the expansion rate of the universe using two different methods — the predictable luminosity of “standard candle” supernovas and the cosmic microwave background, the afterglow from the Big Bang. The problem is, these two methods produce slightly different expansion rates.

Gravitational wave astronomy could serve as the tiebreaker between the two. Its findings could reveal that our “standard candles” need recalibration — or could hint that there is some previously unknown physics at play.

“Both cases would be very interesting, very exciting,” Soares-Santos said.

As for the two neutron stars, the scientists could not say whether the pair had merged to form a bigger neutron star, collapsed into a black hole or met some other, unknown end.

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