Astronomers have discovered the closest black hole to our solar system found to date, located "just" 1,000 light-years away.
According to a study published in the journal Astronomy & Astrophysics, the black hole forms part of a triple star system known as HR 6819, which is so close, in astronomical terms, that you can see it without binoculars or telescopes from the southern hemisphere on a dark, clear night.
"We were totally surprised when we realised that this is the first stellar system with a black hole that can be seen with the unaided eye," Petr Hadrava, a co-author of the study from the Academy of Sciences of the Czech Republic, said in a statement.
The previous record holder is a black hole candidate known as V616 Mon that could be as near as 1,200 light-years away, although most astronomers consider a figure of 3,000 light-years to be more realistic.
Black holes are astronomical objects that are so massive and dense that nothing, not even light, can escape their gravitational pull.
First predicted by Albert Einstein's theory of general relativity, black holes contain a singularity, a single point of infinite density and gravity where space and time as we understand them break down. Surrounding the singularity is the event horizon, the boundary beyond which nothing can escape.
Stellar-mass black holes, those with masses tens of times that of the sun, form after massive stars die as supernova explosions and collapse into themselves under the influence of gravity. Other black holes meanwhile, can form when incredibly dense star remnants, called neutron stars, collide. Similarly the merger of two black holes, or one neutron star and a black hole, can also generate a new, larger black hole. In addition, there are supermassive black holes, thought to be present at the center of many galaxies, whose origins are more mysterious.
"Stellar-mass black holes must not be confused with the supermassive black holes lurking at the center of most massive galaxies," Dietrich Baade, an author of the study from the European Southern Observatory (ESO), told Newsweek. "For instance, [the one] in the Milky Way has a mass of 4.2 million suns. Supermassive black holes formed early in the universe and probably keep growing through the accretion of stars and mergers with other supermassive black holes."
Astronomers think that there are vast numbers of black holes in the galaxy, but to date, only a couple of dozen have been identified.
"If you take the age of the Milky Way, the number of stars it contains, and the life expectancy of these stars, it is only a back-of-the-envelope effort to realize that there must be very many stellar-mass black holes in the galaxy," Baade said. "More complex models predict between 100,000,000 and 1,000,000,000 of them."
Black holes cannot be observed directly with telescopes that detect X-rays, light, or other forms of electromagnetic radiation. However, scientists can infer their presence by looking for their effects on the matter that surrounds them. Any matter in the immediate vicinity of a black hole will gradually be drawn inwards in a process known as accretion, creating an "accretion disk" of orbiting material. For example, if a star ventures too close, it will be consumed by the black hole, violently ripped apart by its powerful gravitational forces.
Nearly all of the black holes that have been identified so far have revealed themselves due to their strong interactions with their immediate environment. However, the authors of the latest study say that the black hole they identified in HR 6819 is one of the first stellar-mass black holes that does not interact violently with its environment, appearing truly black so to speak. This makes it extremely difficult to detect.
The team only discovered the black hole after observing its two companion stars using a telescope at the ESO's La Silla Observatory in Chile. Initially, they were monitoring the pair as a part of a study on double-star systems, but were shocked to uncover the previously hidden black hole while analyzing the data they had collected.
"We found that there was a third object whirling around one of the two luminous stars that weighs in at five suns or more," Baade said. "That fairly massive star's velocity changes with a period of 40 days. However, in spite of the strong gravitational pull exerted on this star, the third object does not emit any appreciable amount of light. Therefore, it can only be a black hole. The exciting thing is that it is one of the first—perhaps even the very first—absolutely dull black holes that do not make themselves known through the violence in their immediate neighborhood."
"Other black holes were detected because gas that is transferred to them from a companion star heats up to very high temperatures and radiates strongly in X-rays, which are readily observed," Baade said. "The new black hole is really black because it is not fed by its companion. This makes it so much more difficult to discover: instead of a single X-ray image, it takes many observations suitably distributed over a long time to detect periodic velocity changes."
According to Baade, the latest discovery is surprising for two main reasons: Firstly, the fact that the team found possibly the first instance of a true black, or "non-accreting," black hole. And secondly, that it was discovered so nearby, relatively speaking. This indicates that there are many more similar black holes to be found in the future, the researchers say, with this system likely representing just "the tip of the iceberg."
Roberto Saglia, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Germany, who was not involved in the latest study, told Newsweek that the important aspect of this research is the detection a "non-active" stellar mass black hole.
"Most stellar mass black holes are first discovered because they have a hot accretion disk around them that shines in the X-ray/ultraviolet range and is detected by X-ray satellites. Here there is no X-ray emission and the inference of the presence of a black hole comes just from dynamical measurements," he said.
"This is important, because we expect from stellar evolution that many more stellar mass black holes should be around compared to the number of detected ones," he said. "This system provides an alternative way to probe this 'unseen' family of black holes, as gravitational wave detections can also provide."