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In only a thousand light years from Earth is a black hole. And it is quite peculiar because it is an invisible black hole, that is, it does not interact violently with its environment and does not emit radiation, so astronomical instruments cannot detect it in the same way that other black holes are studied. It is also the closest to the Solar System that had been discovered now, although more than 20 have been counted in our galaxy.

The newly discovered black hole is quite small, has a mass equivalent to that of four suns, and is found in the constellation Telescopium. It is part of a triple system called HR 6819, made up of two stars and this black hole . Although the black hole is not visible, the two stars can be seen from the southern hemisphere on a dark and clear night, with a simple telescope or even with the naked eye.

" Until now, the closest black hole to Earth was considered to be V616 Monocerotis , which the most optimists placed at 1,300 light years. However, other scientists estimate that it is three times as far away," explains Petr Hadrava to THE WORLD. , scientist emeritus of the Academy of Sciences of the Czech Republic and one of the discoverers of the black hole in HR 6819, the details of which are published this Wednesday in the magazine Astronomy & Astrophysics .

The finding was made from Chile by a team of astronomers from the European Southern Observatory (ESO) and from other research institutes using the 2.2-meter MPG / ESO telescope at the La Silla Observatory. They believe that this system is just the tip of the iceberg and they hope to find many other such black holes in the future.

Sky region, in the constellation Telescopium where the HR 6819 system is located, consisting of two stars and a black hole. The image has been obtained by combining shots from the Digitized Sky Survey 2

The discovery of this unexpected black hole was made while analyzing the images of that double star system. Observations with the FEROS instrument of that telescope revealed that there was a third object that was not visible. One of the stars orbited that invisible object every 40 days and the other star was further away. "We were very surprised when we realized that this is the first star system with a black hole that can be seen with the naked eye," says Hadrava.

An invisible black hole

According to Hadrava, this is the first confirmed case of a stellar black hole that does not interact violently with its environment. "Although we have no doubts that these types of black holes are very abundant, there have been no confirmed cases so far. It is difficult to find them and verify their existence," he says.

To calculate its mass, astronomers studied the orbit of one of its stars: "An invisible object with a mass at least four times that of our sun can only be a black hole," says Thomas Rivinius, ESO researcher and leader of this investigation.

The other black holes discovered in our galaxy interact with their environment and are locatable thanks to their powerful X-ray emissions. However, scientists believe that there are many more invisible black holes because in the Milky Way, many other stars end up becoming holes. blacks at the end of their lives, after exploding. So they point out that finding this black hole can help them find many others hidden in the galaxy. "It must make hundreds of millions of black holes and we have only discovered a few," says Rivinius.

So far, this finding has given them a clue to find a possible second system with another black hole. It's called LB-1, it's further away and they think it could also be a triple system, but they will have to make more observations first.

When it comes to the number of known black holes in our galaxy, Hadrava notes that there is no precise number they can offer because they are not sure if some of the binary systems they have studied contain a black hole or a neutron star. As new observations and new studies are made, he adds, astronomers often change their minds about the nature of an individual object. "However, the lists of black hole candidates in different collections typically contain between 20 and 50 objects."

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