“Only” 1,000 light years separate us: European Southern Observatory (ESO) astronomers have discovered a new black hole, the closest to Earth ever detected, located in a stellar system visible to the naked eye, according to a study released Wednesday.
It is a stellar black hole, resulting from the collapse of a massive star on itself, as big as at least four times the Sun. Unlike supermassive black holes, it is “silent” because it “does not interact violently with its environment” (it does not absorb matter): it is therefore “really black” and hidden in the stellar system “HR 6819 “, ESO said in a press release.
“We were totally surprised”
The ESO team was initially interested in HR 6819 as a binary star system, which includes a variety of stars more massive and brighter than the Sun. Thanks to the “FEROS” spectrograph (instrument which measures the radial speed of an object), located at the La Silla observatory in Chile, astronomers have noticed that one of the two stars is moving faster than the other, and orbiting in an unusual way, every forty days, around an invisible object.
By studying this trajectory, they were able to detect the presence of the black hole, and calculate its mass. “An invisible object with a mass of at least four times the Sun can only be a black hole,” says astrophysicist Thomas Rivinius, lead author of the study published in the journal Astronomy & Astrophysics.
“This star system contains the closest black hole to Earth that we know of,” he said. “We were totally surprised when we realized that it was the first stellar system visible to the naked eye with a black hole,” adds Petr Hadrava, of the Prague Academy of Sciences, co-author of the ‘study.
Few black holes detected in our galaxy
To date, astronomers have spotted only a few dozen black holes in our galaxy, most of which interact strongly with their environment, resulting in the emission of powerful X-rays and making them detectable. The center of our galaxy also includes in its center a supermassive black hole.
“We know few stellar black holes in the Milky Way, but we think there are a lot, because they are natural products of the evolution of massive stars”, comments Pierre Kervella, astronomer of the Paris Observatory – PSL. “As long as they are calm, that is to say that there is no accretion of matter, these objects are difficult to detect. If they are not in a system, we cannot see them, ”he continues.
The black hole of HR 6819 could thus be detected via “the gravitational attraction which it imprints on the star”, decrypts the astronomer, according to which this study is an “additional brick to understand the sources of gravitational waves” , detected in 2016.
The object discovered is located 1,000 light years away (several tens of millions of billions of kilometers), or “roughly 200 times further than the star closest to us, Alpha Centauri; which, on a stellar scale, is not far away ”.
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