Astronomers have observed for the first time a black hole eating a neutron star. And it was immediately hit twice. A team of more than a thousand astronomers captured gravitational waves from two different collisions between a black hole and a neutron star. In such a collision, the black hole swallows the neutron star. Not only do the observations provide evidence for the existence of those events, which have been suspected for some time. It also helps astronomers understand how collisions between compact celestial bodies occur and how often they occur.
Black holes and neutron stars are both remnants of massive stars. Only a star that is at least eight times as massive as the sun has enough gravity and pressure to die in a massive explosion. As soon as such a star dies, its core collapses and the outer layers are rapidly flung out into space. That’s a supernova. From the core of the lighter stars that die in this dramatic way, a compact ball of neutrons remains. Such a so-called neutron star typically has more mass than the sun, but then crammed into a sphere with a radius of several tens of kilometers. The core of a more massive star that dies as a supernova meets an even more extreme fate. It ends up as a black hole. Black holes are so dense that anything that comes close to them disappears into them.
Like waves in a pond
The detectors LIGO (in the US) and Virgo (in Italy) became world famous in 2015 when they first detected gravitational waves from a collision between black holes. Such waves are created by collisions between compact celestial bodies, ripples that move through the universe like waves in a pond after a rock is thrown into it, and strong enough to absorb here on Earth. Dozens of gravitational waves from collisions between black holes and two from collisions between neutron stars have been observed.
Now the last type of collision that was still missing, the mixed collision between a neutron star and a black hole, can also be added to the list. In January last year, LIGO, Virgo and Kagra (in Japan) caught two curious gravitational waves. The analysis of the waves shows that the masses of the colliding objects correspond to those of a black hole and a neutron star. The Astronomers published the results in this week Astrophysical Journal Letters.
“The observations don’t surprise me at all,” says Chris van den Broeck. He is a physicist at the Dutch particle institute Nikhef and co-author of the article. “It would be crazy if we saw multiple collisions between black holes and between neutron stars, but never a mixed collision of a black hole and a neutron star. But the only way to prove its existence is to observe it.”
The neutron stars that collided with the black holes about a billion years ago (that’s how long it takes for the waves to reach the detectors on Earth) met a spectacular fate – like anything that comes too close to a black hole. First, the heavenly bodies revolved around each other for millions of years. During that dance they moved closer and faster towards each other. As the neutron star approached the black hole, the black hole’s gravity pulled harder on the part of the neutron star closer to the black hole than the part farther from it. The neutron star tore apart. Most of them disappeared into the black hole. A small part may have escaped.
The two gravitational waves were not easy to find
Christiaan Brinkerink astronomer
Christiaan Brinkerink, astronomer at Radboud University and not involved in this research, is enthusiastic. “The two gravitational waves were not easy to find: they are very weak and come from dozens of galaxies.”
Importantly, the observations help astronomers to understand how mixed collisions between black holes and neutron stars occur, Brinkerink believes. “We have computer models that simulate that and it also follows how often they should occur. Now we can start comparing our models with reality.”
“And this is just the beginning”, responds Van den Broeck. “LIGO, Virgo and Kagra are currently undergoing a technical upgrade. I expect that we will experience collisions between a neutron star and a black hole much more often.”