Astronomers discover closest black hole to Earth

Anew-found black hole may be the closest black hole to Earth, and you can spot its cosmic home in the night sky without a telescope. The black hole, which is lurking 1,000 light years from Earth in the southern constellation of Telescopium, belongs to a system with two companion stars that are bright enough to observe with the naked eye. But you won’t be able to see the black hole itself; the massive object has such a strong gravitational pull that nothing, not even light, can escape it.

Astronomers discovered this black hole while studying what they thought was just a binary star system, or two stars that orbit a common centre of mass. They were using the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile to observe the binary, known as HR 6819, as part of a broader study on double star systems. When they analysed their observations, the researchers were shocked to learn that a third object was hiding in the system: a black hole.

Although the astronomers could not directly observe the black hole, they were able to infer its presence based on its gravitational interactions with the other two objects in the system. By observing the system for several months they were able to map out the stars’ orbits and figure out that another massive, unseen object must be acting in the system.

The observations also showed that one of the two stars orbits the invisible object every 40 days, while the other star hangs out by itself at a much greater distance from the black hole. They calculated that the object is a stellar-mass black hole, a black hole that forms from the collapse of a dying star, that’s about four times the mass of the Sun. “An invisible object with a mass at least four times that of the Sun can only be a black hole,” said Thomas Rivinius, who led the new study. “This system contains the nearest black hole to Earth that we know of.”

The black hole in HR 6819 is one of the first stellar-mass black holes found in our galaxy that does not release bright X-rays while violently interacting with its companion stars, and the discovery could help researchers find other similarly ‘quiet’ black holes in the Milky Way.

An artist’s impression shows the orbits in the HR 6819 triple system, which consists of a binary star pair in which the stars (blue) orbit a black hole (red)