For the first time, astronomers witnessed a black hole swallowing a neutron star, the densest object in the universe, in fractions of a second.
Ten days later, they observed the same thing, on the other side of the universe. In both cases, a neutron star – a teaspoon of which would weigh a billion tons – orbits closer and closer to that point of no return, a black hole, until the two collide and the neutron star disappears in a gulp.
Astronomers observed the last 500 orbits before the stars were swallowed, a process that took much less than a minute and briefly generated as much energy as all the light in the observable universe.
“It was a quick big dump and it was gone,” said study co-author Patrick Brady, an astrophysicist at the University of Wisconsin. The black hole “has a nice dinner from a neutron star and it gets a little bigger.”
The energy bursts from the collisions were discovered when detectors on Earth detected gravitational waves, waves of cosmic energy that travel through time and space and initially mentioned by Albert Einstein. The waves came from a distance of more than a billion light-years. They were detected in January 2020, but the study of more than 100 scientists that analyzed and interpreted the data was published in the Astrophysical Journal Letters.
Although astronomers had seen gravitational waves from collisions between two black holes and between two neutron stars, this is the first time they have seen collisions between a black hole and a neutron star.
Neutron stars are the corpses of massive stars, what remains after a giant star dies in a supernova explosion. They are so dense that they are about 1.5 or 2 times the mass of our Sun, but condensed to about 10 kilometers (6 miles) wide. Some black holes, known as stellar black holes, are created when an even larger star implodes, creating something with such powerful gravity that not even light can escape.
Scientists think there must be many more of these neutron star-black hole collisions, but they have not detected others in our galaxy.