A new type of black hole discovered around a star

SPACE – The bestiary of black holes is growing. For the first time, a dormant black hole of stellar mass has been discovered orbiting another star, still far enough away not to swallow its companion.

This new type of black hole, long predicted by theory but very difficult to detect as it is well hidden, was revealed after six years of observation with the Very Large Telescope (VLT) from the European Southern Observatory (ESO) in Chile, according to a study published Monday, July 18 in Nature astronomy.

The observed system lurks in the Magellanic Cloud, a dwarf galaxy near the Milky Way. It is like the other leg of a binary system of two stars orbiting each other, one of which, dead, has turned into a black hole and the other is still alive, as you can see in the image above.

You can observe the system thanks to the reconstruction image at the top of the article and video above. It is composed of a blue star whose mass is 25 times greater than that of the Sun and the black hole whose mass is nine times greater. ESO specifies that the image is not in scale: the blue star is actually 200,000 times larger than the black hole.

“A needle in a haystack”

“We found a needle in a haystack,” said Tomer Shenar, its lead author, in a statement. For three years, several candidates for the title of “sleeping black hole” had presented themselves, but so far no one had been accepted by this international team of astronomers, dubbed by ESO “the police for the black holes”.

Star mass black holes – incomparably smaller than their supermassive big brothers – are massive stars (between 5 and 50 times the mass of the Sun) at the end of their lives that collapse into themselves.

These objects are so dense and their gravity so powerful that not even light can escape: they are therefore by definition invisible. Scientists can nevertheless observe the substance circulating around it before it is swallowed there … except when the black hole “sleeps”, on a diet.

A black hole that can be detected with X-rays

In the binary systems already observed, the star that has turned into a black hole is close enough to its companion star to “steal” its substance from it (we are talking about “growth”), explains Hugues Sana from the university in Louvain to AFP. (KU Leuven), Belgium, one of the authors of the study.

Once captured, this material emits detectable X-rays. But here the black hole emits no one, and with good reason: “The living star (about 25 times the mass of the Sun) is far enough away not to be eaten. It currently remains in equilibrium on this orbit, ”which lasts 14 days, the astronomer continues.

A balance that can not hold, according to him. “The living star will grow, and at this point part of its surface will be engulfed by the black hole”, which will then emit X-rays and therefore come out of its dormant state.

A dance game

But how do you know such an object exists? Imagine a pair of dancers holding hands as you observe in the dark. One has a black suit, the other a light suit: you only see the dance of the other, but still you know that he has a dance partner, thanks to the study of movement, ”explains Hugues Sana.

In astronomy, just as Jupiter and the Sun orbit each other, we can measure the respective masses of a binary system by observing these movements.

To make sure that the phantom object was actually a black hole, the researchers continued to eliminate and exclude several scenarios, such as a star losing its envelope. “The only plausible explanation is that it is a black hole because no other star can reproduce this observational data,” the researcher sums up.

According to newer models, about 2% of the massive stars in our galaxy are likely to have a black hole around them, or about 100 million, according to Hugues Sana. “Currently, we only know about ten of them, all detected thanks to their x-rays, so we are missing some!”

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