The closest black hole to our solar system has been discovered to be only 1,000 light-years away. This unexpected dark neighbor has at least 4.2 times the mass of the sun and lives with two regular stars whose unusual orbits disclosed the black hole’s presence, according to astronomers writing in Astronomy & Astrophysics.
Astronomers predict that the Milky Way will include between 100 million and a billion black holes with masses ranging from a few to 100 times that of the sun. However, the majority of these black holes are unseen. “If it’s lonely out there without a companion, you’ll never find it,” says astrophysicist Thomas Rivinius of the European Southern Observatory in Santiago, Chile.
The few hundred small black holes discovered so far interact strongly with their surroundings, gobbling up gas from a companion star and heating the gas to the point where it emits X-rays. V616 Mon, the previous closest known black hole, emits X-rays from around 3,200 light-years away.
According to the researchers, the new neighbor black hole, known as HR 6819, is not actively consuming and thus unseen. However, it appears to have two companions: a star that the black hole rounds every 40 days and is heavier and hotter than the sun, and a more distant, massive star that orbits the star-black hole combination and is whirling so fast that it’s almost breaking apart. The motions of those two stars first suggested something weighing at least four solar masses must be orbiting with them, unseen.
“We would have seen it if it was a normal star,” Rivinius says. “If it’s not a normal star, the only thing it can be otherwise is a black hole.” HR 6819 is close enough and its stars are bright enough that they may be viewed with the naked eye on a dark, clear night in the Southern Hemisphere, according to the scientists.
According to ESO astronomer Marianne Heida, who is headquartered in Garching, Germany, there could be many other undetected black holes of similar mass in the Milky Way. “If there’s only one in the Milky Way, it’d be a little too convenient if it’s right next door,” she says.
Rivinius and his colleagues discovered the black hole by chance almost 15 years ago. In 2004, a team led by Stanislav Tefl of the European Southern Observatory in Santiago observed the system as part of a study of fast-spinning pairs of stars. The researchers assumed that a third, invisible object was involved in an orbital dance with the two visible stars.
But in 2014, before the team could publish the observations, Štefl died in a car accident. “I consider this part of his legacy, this work,” Rivinius says.
The team picked the research back up after another black hole, called LB-1, was reported in November 2019. That black hole appears to be orbiting a more ordinary star and looks to be very hefty, weighing around 68 times the mass of the sun. Rivinius’ team believes that, like HR 6819, it is most likely a tiny black hole with two partner stars.
According to J.J. Eldridge, an astrophysicist at the University of Auckland in New Zealand, both HR 6819 and LB-1 may be more common than they look. Instead of a black hole, she believes the systems may include a third large star with a disk around it. Because of the way that the observations are made and the complexity of the objects’ orbits, “it would be really, really difficult to disentangle,” she says. “The interpretation of a black hole is more interesting, but also may not be correct.”