The first direct discovery of a black hole in a young star cluster

Astronomers have directly detected a black hole in a very small star cluster outside our galaxy for the first time, in a way that promises new discoveries of these hard-to-accurate objects, according to a study Thursday.

“There are a lot of black holes in the universe, but we don’t know them because we can’t see them,” Sarah Saracino, an astrophysicist at the university’s Astrophysical Research Institute, told AFP from Liverpool in Great Britain.

Their black color simply reflects the fact that these stars are, by definition, invisible. Its gravitational force, the force that pushes man to the surface of the Earth, is so strong that not even light can escape.

They can be detected indirectly, by the radiation emitted at their limits when they absorb matter, or by the resulting gravitational waves, for example, by merging two black holes. And if not directly, then when the proximity of the black hole to a nearby star disrupts the orbit of the latter.

Thanks to this technique, the team led by Sarah Saracino discovered a black hole with a mass of about 11 suns, located in the star cluster NGC 1850 of the Large Magellanic Cloud, a galaxy “close” to the Milky Way, about 160,000 light-years away. This “small” black hole slightly distorts its nearby star, which “weighs” five solar masses.

Photo by AFP

“This is the first time we’ve detected one with this technology in a very small group” outside our Milky Way, says the scientist, whose study appears in Monthly Notices of the Royal Astronomical Society.

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To find the rare bird, the scientists used MUSE, a broad-field spectrometer, installed just a few years ago on the European Southern Observatory’s (ESO) Very Large Telescope in Chile.

It made it possible to “observe a densely populated area,” according to Sebastian Kaman, co-author of the study, quoted in an ESO press release, “with information on thousands of stars simultaneously, ten times more than any other instrument.”

Sarah Sarachino explains that the cluster’s relative youth – which is less than 100 million years old – is an asset, because “there are a completely different kind of black holes, meaning they formed very recently”. They did not have time to be expelled, ‘as with very old star constellations’, especially not to interact with each other.

The fact that they are small and still “light” is of more interest to scientists seeking to characterize the full range of black holes. From “stellar” masses, such as that identified by Sarah Saracino’s team, to supermassive several million solar masses, including “intermediaries”, the existence of which is still disputed.

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