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.