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Composed of two stars and a black hole having 4,2 times the mass of the Sun, the HR system 6819 is a thousand light years away.

Posted today at 2:03 p.m., updated at 4:15 pm Reading Time 3 min.

It sounds like a vaudeville - the story of a couple with a third thief hiding in a closet - but in an astronomical transcription. The couple in question is made up of two stars located in the Telescope, a constellation visible from the southern hemisphere. Until there, nothing exciting, because the Milky Way is full of binary systems.

More, as revealed by an international study published Wednesday 6 in Astronomy & Astrophysics, analysis of the measurements made by the Feros spectrograph - instrument installed on the telescope of 2,2 meters in diameter than the European Southern Observatory (THAT) owns on the Chilean site of La Silla - showed that something was wrong in the HR duo 6819 : one of the two stars seemed to dance tango with a third object.

Triple systems are not an incongruity in astronomy. So, the famous Alpha Centauri is not a simple star but a trio made up of Alpha Centauri A, of Alpha Centauri B and Proxima Centauri, the closest star to the Sun. Boredom, with HR 6819, is that the third member did not appear anywhere.

One could have imagined that it was a neutron star but, by studying the orbit of his partner, the researchers calculated that the mysterious unknown had an imposing mass, equivalent to 4,2 times that of our Sun. And therefore too important to be that of a neutron star.

As Thomas Rivinius explains, ESO astronomer and first author of the article, another conclusion was logically imposed : "An invisible object with a mass at least four times that of the Sun can only be a black hole. "Like HR 6819 is about a thousand light years away from us, this makes its third member the closest black hole to the Solar System.

We can be reassured however, even if this distance is only a hundredth of the diameter of our galaxy, She is respectable enough so that there is no fear of meeting this black hole one of these four mornings (to compare, Proxima Centauri is located 4,2 light years away from us).

Obviously it’s not a giant black hole, like the one whose photograph unveiled a year ago went around the world. These behemoths, whose mass is equivalent to several million or even several billion times that of the Sun, are lying in the center of galaxies. The third thief of HR 6819 East, For its part, a black hole says stellar because it appears after the death of a massive star.

What remains of the latter after the final explosion - the supernova - remains very important in terms of mass and this stellar corpse then collapses under its own weight. As no force is able to oppose this collapse, the mass is concentrated more and more and then a black hole is born, a peculiarity of space-time, hence nothing, not even the light, does not escape.

Given the large amount of massive stars that have been born and died in the Milky Way since its inception, the number of stellar black holes in our galaxy is estimated to be in the hundreds of millions, even a billion. However, we only know about twenty ! Invisible by definition, emitting no electromagnetic waves by themselves, black holes are left, indeed, very difficult to capture.

The rare ones that are known generally betray themselves by tearing away material from what surrounds them. Falling into the black hole, it heats up enormously and produces energetic X-rays detectable by space telescopes. On the other hand, black holes which do not have this type of interaction and are probably the most numerous, remain desperately elusive. The authors of the study therefore recommend paying more attention to double systems ... which could well be triple.

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Source: https://www.lemonde.fr/passeurdesciences/article/2020/05/06/decouverte-du-trou-noir-le-plus-proche-de-nous_6038853_5470970.html

World news – FR – Discovery of the nearest black hole to the Solar System

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