They're two of the most mysterious and least-known about objects in the universe.
But an international team of scientists have for the first time detected the moment a black hole and a neutron star collided in space, not just once but twice.
The collision was detected more than 1 billion light years away from Earth, using specialist gravitational wave observatories in the US and Italy.
The detections were recorded in January 2020, with one of the collisions involving a black hole nine times the size of the Sun and a neutron star that was double the Sun's mass.
A neutron star is a large star that has undergone a supernova, or star explosion, where it collapses under its own gravitational field, but is not heavy enough to form a black hole in its own right.
More than 1000 scientists from across the world worked on the project to help detect the collision, including about 100 Australians.
Among them was Australian National University distinguished professor Susan Scott, who said the collision's detection was a major step forward.
"A black hole is the densest object in the universe, and we don't know what happens in the centre, and then with a neutron star, it's another of the densest objects in the universe, and we can't produce the conditions of the star on earth because it's so dense," she said.
"By watching the objects collide together we can hope to better understand the materials that make up a neutron star."
While the collision is between two of the largest objects in the universe, it's still the black hole that comes on top after they crash together.
"In this case, as the neutron star got closer and it approached the event horizon of the black hole, it gets swallowed up," Professor Scott said.
"It is quite surprising to get two close up.
"Each collision isn't just the coming together of two massive and dense objects. It's really like Pac-Man, with a black hole swallowing its companion neutron star hole."
Due to the density of both the black hole and the neutron stars, gravitational wave observatories were able to detect the collision even from as far away as 1 billion light years.
While neutron stars are not in our solar system, the stars have been detected in the Milky Way.
The discovery comes just a few years after the first recorded instance of two black holes colliding in 2015 and the first detection of two neutron stars crashing into each other in 2017.
Professor Scott said the two types of celestial objects had never been recorded as crashing into one another, which made the discovery even more significant.
"We feel like this is just the start, this is a new window on the universe that has never been accessed before," she said. "We want to go on observing these systems colliding because we feel there's going to be so much more understanding of these mysterious objects."
The findings are published in The Astrophysics Journal Letters.
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