A Black Hole Swallowing a Neutron Star May Have Been Observed by Scientists for the First Time

Scientists think they have observed a black hole swallowing a neutron star for the first time after detecting gravitational waves—ripples in space-time—from a location around 900 light years away from the Earth.

The gravitational waves were identified by the LIGO and Virgo observatories—in the United States and Italy respectively—on August 14, according to an international team of researchers.

Gravitational waves were predicted by Albert Einstein in 1916 as part of his general theory of relativity. However, they were not directly observed until 2015 when LIGO (Advanced Laser Interferometer Gravitational-wave Observatory) made a historic finding by detecting the gravitational waves created by a pair of colliding black holes.

Subsequently, scientists have made further gravitational wave observations—most of which have originated from colliding black holes. However, gravitational waves generated by the impact of two neutron stars have also been detected.

Now, researchers think they have observed a third source of origin for gravitational waves, completing their original wish-list: a black hole merging with a neutron star—incredibly dense, compact objects that are left behind as remnants when massive stars explode as supernovae.

"By observing these three types of events we will be able to better estimate the population size of these systems in the universe and also better understand how these binary systems 'get together' in the first place," Susan Scott, from the Australian National University, told Newsweek. "The collisions involving neutron stars will also enable us to probe the enigmatic nature of the composition of neutron stars—the densest stars in the universe—one of the major open problems in physics today."

While the final results of their observations are yet to be published in a peer-reviewed journal—and thus cannot be confirmed—the scientists say that the gravitational waves likely originated from a black hole-neutron star merger due to the size of the two objects involved.

"About 900 million years ago, this black hole ate a very dense star, known as a neutron star, like Pac-man—possibly snuffing out the star instantly," Scott said in a statement.

"Scientists have never detected a black hole smaller than five solar masses or a neutron star larger than about 2.5 times the mass of our Sun," Scott said. "Based on this experience, we're very confident that we've just detected a black hole gobbling up a neutron star."

Nevertheless, Scott notes there is a small chance that the gravitational waves were generated by an even more exotic phenomenon—the black hole swallowing a very light black hole.

"There is also the intriguing possibility that the lighter object could, in fact, be a very light black hole," she told Newsweek. "This would be a very exciting outcome and a great consolation prize for not confirming our first black hole collision with a neutron star."

"We are not aware of any black holes in the universe with masses less than about five solar masses," she said. "Since we currently estimate the smaller mass to be less than three solar masses, if it is a black hole, then it would be significantly lighter than any other black hole that we know about. This would raise many new and challenging questions such as, 'How does such a light black hole form?'"

Like neutron stars, black holes are also the remnants of stars that die as supernovae. However, the stars that produce black holes tend to be larger, according to NASA.

Objects such as neutron stars and black holes are able to generate the kinds of gravitational waves that we can detect on Earth.

"Gravitational waves are minute ripples in the very fabric of space and time," Scott said. "The largest ones, which we have any hope of detecting, are generated, for example, by very massive and very dense objects orbiting close to each other and moving at appreciable fractions of the speed of light, such as we have with binary systems of black holes and neutron stars executing their final death throes as they inspiral to their cataclysmic collision.

"Gravitational waves, which are not part of the electromagnetic spectrum, travel outwards from their source at the speed of light," Scott said.

This article was updated to include additional comments from Susan Scott.

blach hole-neutron star merger
Artist's illustration of a black hole swallowing a neutron star. Australian National University