You can 'see' the closest known black hole to Earth with the naked eye

There is a black hole in our backyard. Astronomers have found the closest black hole yet at just 1000 light years from Earth, close enough to see the stars that orbit it without a telescope.

Marianne Heida at the European Southern Observatory in Germany, and her colleagues spotted this black hole completely by accident. It is part of a system called HR 6819 that also contains two stars. The team was looking for pairs of stars in which one was a type that rotates so quickly it flings off material from its equator, creating a kind of ring from its own plasma.

They found HR 6819, which has one of those plus one normal star, but the normal star appeared to be orbiting an empty spot of space once every 40 days. That turned out to be a black hole at least four times as massive as the sun, invisible because it is not actively devouring any material.

“There must be a bunch of them closer by that we haven’t found yet, but this is the closest that we know,” says Heida. “Based on the number of stars in the Milky Way, we expect that there are about 100 million of these small black holes, and we’ve only found less than 100 of them.” If black holes were spread evenly throughout the galaxy, the closest should be just 30 to 40 light years away, she says.

Not only is this the closest black hole we’ve found, it’s also the first one that’s been spotted in a triple system and one of very few inactive black holes that have been spotted. Most black holes are probably practically invisible like this one, so finding more like this can help us get a handle on exactly how many black holes there are in our galaxy.

Luckily, this black hole is far enough away that we don’t need to worry about it. “Earth is not in any danger,” says Heida. “Given that there are two stars that are much closer than we are, and they are not falling in, we won’t fall in.”

But if you want to keep an eye on it, the system can be seen in the sky in the southern hemisphere, in the constellation Telescopium.

Astronomy & Astrophysics DOI: 10.1051/0004-6361/202038020