Astronomers have discovered the closest black hole to Earth, embedded in a star system that is visible with the naked eye.
The black hole is located about 1,000 light years away in a three-body system called HR 6819, which also contains two bright stars that can be spotted as a single light by skywatchers in the Southern Hemisphere. The black hole in HR 6819 is about three times closer than the next nearest known black hole, V616 Monocerotis.
The discovery is especially exciting because this “unseen object” hints at the “existence of an entire population of quiet black holes,” according to a study published on Wednesday in the journal Astronomy & Astrophysics. “Loud,” or active, black holes emit light due to close tidal interactions with stars or gas clouds. Conversely, quiet black holes are dark, making them extremely difficult to detect.
“That it is so close to Earth makes it special in the sense that it means these things must be quite common,” said Thomas Rivinius, an astronomer at the European Southern Observatory (ESO) who led the new study, in an email. “Our neighbourhood is nothing special, so if it exists here, chances are it exists everywhere.”
So if the black hole in HR 6819 is not emitting any light, how did Rivinius and his colleagues find it? As it turns out, the discovery was serendipitous.
“We were looking for something quite different, expecting only a normal binary star system,” Rivinius said. “We hoped detailed observations would have helped us to understand why the two normal stars in there are quite different.”
The stars are visible to the naked eye both because they are relatively close to our solar system and because they are about six times as massive as the Sun, making the whole system fairly bright.
Rivinius and his colleagues wanted to figure out why one of the stars was spinning rapidly while the other seemed to rotate rather sluggishly. “To explain this difference is what we were after and then we realized that there are actually three objects,” Rivinius said.
The researchers were on the verge of solving the mystery when tragedy struck in 2014. Astronomer Stanislav Štefl, who had been leading the project, died in a car accident, and was widely mourned by the astronomical community. The team included a dedication to Štefl’s memory in the study, writing, “in sadness and grateful appreciation of his never-tiring alertness that also triggered this work.”
As a result, the quest to study HR 6819 was placed on hiatus for several years, until the recent discovery of another star system called LB-1, located 7,000 light years away, that appeared to have a similar multi-body architecture.
Inspired by the discovery, Rivinius and his colleagues resumed observations of HR 6819 for several months using the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. By tracking the stellar movements, they were able to confirm that one star is in close proximity to a companion black hole, orbiting it every 40 days, and that the other star follows a much more distant orbit around this inner pair. Based on the movements of the stars, this black hole is at least four times as massive as the Sun.
Scientists have long theorized that these quiet black holes, which are formed by the explosive deaths of large stars, must be common in the universe. But it has proved difficult to back up this theory observationally because these objects are so dark.
The scientists who discovered LB-1 suggested it might be a binary system containing a star and a gigantic black hole, but Rivinius’ team thinks LB-1 may well be another three-body system like HR 6819, containing a stellar-mass black hole and two stars.
The new research not only sheds light on the existence of these quiet black holes, it could also help scientists better understand the cosmic events that produce gravitational waves, which are ripples in the fabric of spacetime.
Since 2015, gravitational wave observatories have detected many of these ripples, most of which are produced by mergers of two black holes into one compact object.
The stars in HR 6819 are not big enough to become black holes when they die, so it’s unlikely that this particular system will produce a black hole merger that emits the kind of gravitational waves that would be detectable on Earth. In the future, the outer star will expand into a red giant, shed its outer layers, and become a type of dead star called a white dwarf. The inner star will probably go through the same giant phase, though it could eventually end up uniting with the black hole.
“Being so close to a companion black hole means that in that supergiant state, the black hole will start accreting matter from the other star,” Rivinius explained. “It may even grow so much as to engulf the black hole. What happens then is hard to predict, but it is possible that the star and the black hole merge.”
But though HR 6819 is not destined to produce a merger of two black holes, it could help constrain details about systems that create these mind-boggling, wave-emitting unions because they are all descended from multi-body configurations of massive stellar objects.
Rivinius and his colleagues hope to study the system further to examine its individual constituent parts with ESO’s Very Large Telescope Interferometer. They also want to search for even the dimmest glow of light from the black hole, which might indicate that the object is feeding on nearby gas. This accretion process may be totally different from the radiant X-ray light emitted by loud and active black holes that are consuming stars and other large masses.
“Of course, having developed an eye for it,” Rivinius concluded, “it's worth looking at the catalogs, if there are any other objects we might want to have a closer look at.”