The closest black hole to the solar system ever spotted may be just 1,000 light-years away. This newfound dark neighbor is at least 4.2 times as massive as the sun, and lives with two ordinary stars whose funny orbits gave the black hole’s presence away, astronomers report May 6 in Astronomy & Astrophysics.
Astronomers expect the Milky Way to harbor between 100 million and a billion black holes with masses between a few and 100 times the sun’s. But most of those black holes are invisible. “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 dozen small black holes that have been spotted so far interact violently with their environments, gobbling up gas from a companion star and heating the gas until it emits X-rays (SN: 4/4/18). The previous nearest known black hole, called V616 Mon, emits X-rays from about 3,200 light-years away.
The new neighbor black hole, called HR 6819, is not actively eating and so is invisible, the researchers say. But it appears to have two companions: one star that the black hole orbits every 40 days that is heavier and hotter than the sun as well as a more distant, massive star orbiting the star-black hole pair that is rotating 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 near enough and its stars are bright enough that on a dark, clear night in the Southern Hemisphere, the stars can be seen with the naked eye, the scientists say.
There could be many other unseen black holes of similar mass in the Milky Way, says ESO astronomer Marianne Heida, who is based in Garching, Germany. “It would be a little bit too convenient, if there’s only one in the Milky Way, that it’s right next door,” she says.
Rivinius and his colleagues first spotted the black hole by accident more than 15 years ago. A team led by Stanislav Štefl, then of ESO in Santiago, observed the system in 2004 as part of a study of fast-spinning pairs of stars. The team suspected there might be a third, invisible object locked 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 seems weirdly heavy, about 68 times the mass of the sun (SN: 11/27/19). But Rivinius’ team thinks that, like HR 6819, it’s probably a smaller black hole with two companion stars instead.
It’s possible that both HR 6819 and LB-1 are more ordinary than they appear, says astrophysicist J.J. Eldridge at the University of Auckland in New Zealand. She thinks that instead of a black hole, the systems could involve a third massive 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.”