An international team of astronomers has discovered a black hole just a thousand light-years from Earth. This black hole is closer to the solar system than any other known to date. It was discovered due to the fact that it is part of a triple system of stars, so it was possible to prove the existence of an invisible object in it by tracking the movements of its two companion stars. Astronomers expect many black holes to be discovered in the same way in the future. The results of the work are published in the journal Astronomy & Astrophysics…
The triple system in question, called HR 6819, is located in the constellation of Telescope and is so close to us that in the southern hemisphere its stars can be seen in the dark cloudless sky without binoculars or a telescope. The group initially observed the system as part of their study of binary star systems. However, when astronomers analyzed their observations, they realized that a third, previously undetected body was present in the HR 6819 system. Observations with the FEROS spectrograph at the MPG / ESO 2.2-meter telescope in La Silla showed that one of the two visible stars orbits an invisible companion in 40 days, and the second is at a great distance from this inner pair.
The black hole in HR 6819 is one of the first discovered stellar mass black holes that do not actively interact with their surroundings and therefore remain truly invisible. The team was able to detect its presence and calculate its mass by examining the orbit of the inner pair star. Its mass was at least 4 times the mass of the Sun, and such a massive object can only be invisible as a black hole.
To date, astronomers have found only a couple of dozen black holes in our Galaxy. Almost all of them actively interact with their surroundings and betray their presence with the powerful X-ray radiation that arises at the same time. But according to scientists, during the lifetime of the Milky Way, many more stars should have turned into black holes at the end of their evolution – hundreds of millions are counted. The discovery of an invisible black hole in HR 6819 means that many other black holes may be hidden in this way.
In particular, astronomers suspect that a black hole may be lurking in another system with two visible stars, known as LB-1. To prove this, however, additional observations are required. LB-1 is slightly farther from Earth, but still pretty close by astronomical standards. If it is possible to find and study a significant number of such systems, this will help to learn more about the formation and evolution of rare stars that begin their life with a mass of tens of solar masses, and end with a supernova explosion, after which a black hole remains.
The study of triple systems with an inner pair and a third distant star is also of interest for explaining the events of stellar mergers, accompanied by the release of enormous energy. These events emit gravitational waves powerful enough to be detected on Earth. Some researchers believe that such mergers can occur in systems with a configuration similar to HR 6819 or LB-1, but in which the inner pair consists of two black holes or a black hole and a neutron star. A distant outer object can gravitationally act on an inner pair, initiating its merger. Although HR 6819 and LB-1 have only one black hole and no neutron stars, these systems can still help to understand exactly how the dynamics of triple star systems containing compact massive objects works.
Source: ESO Found Closest Black Hole to Earth // ESO