An international team of astronomers has discovered the closest black hole to Earth, located just 1,000 light-years away. The discovery was serendipitous because they weren't even hunting for it, they were actually studying binary stars and stumbled upon it. What's even more exciting is it's the first black hole with two stellar companions that are visible to the naked eye.

The triple system is known as HR6819. As reported in the journal Astronomy & Astrophysics, the team discovered that one of the stars in the system is in a 40-day dance with an invisible object estimated to be about 4.2 times the mass of the Sun, while the second star orbits further out. The mystery object must be a black hole as no other object would create such a system without emitting any light.

“An invisible object with a mass at least 4 times that of the Sun can only be a black hole,” lead author ESO scientist Thomas Rivinius said in a statement. “This system contains the nearest black hole to Earth that we know of.”

The discovery is very exciting and not just because the system is so close to us. More often than not, black holes are discovered while they are active. Stellar-sized black holes such as this one are usually spotted due to powerful X-ray flares released during interactions rather than when they are just passively existing. Statistically black holes should be very common, the end product of large supernovae that exploded a long time ago, so we should see more of them, if we knew what to look for.

“There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” Rivinius continued.

This system also has the potential to explain other triple systems we know of, among them LB-1, located 2,300 light-years away. LB-1 appears to have a small evolved star orbiting a very large black hole. The black hole is a bit too big to have formed from a supernova; it could have formed from the merger of two smaller black holes. HR6819 could be the blueprint for it.

“We realised that another system, called LB-1, may also be such a triple, though we'd need more observations to say for sure,” said Marianne Heida, a postdoctoral fellow at ESO and co-author of the paper. "LB-1 is a bit further away from Earth but still pretty close in astronomical terms, so that means that probably many more of these systems exist. By finding and studying them we can learn a lot about the formation and evolution of those rare stars that begin their lives with more than about 8 times the mass of the Sun and end them in a supernova explosion that leaves behind a black hole."

HR6819 being the first stellar system with a black hole that can be seen unaided is a particularly exciting discovery. You can see the stars for yourself in the Southern Hemisphere. They are in the constellation Telescopium and are visible in low light pollution regions.