A new type of black hole discovered around a star

ESO/L. Calcada A dormant stellar-mass black hole orbiting another star has been discovered by scientists.

ESO/L. Calcada

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 because it is well hidden, was revealed after six years of observation with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile, according to a study published on Monday, July 18 in Natural astronomy.

The observed system lurks in the Magellanic Cloud, a dwarf galaxy near the Milky Way. It is like the second 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 the video above. It is composed of a blue star whose mass is 25 times that of the Sun and the black hole whose mass is nine times greater. ESO specifies that the image is not to 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,” Tomer Shenar, its lead author, said in a statement. For three years, several candidates for the title of “sleeping black hole” had presented themselves, but none had so far been accepted by this international team of astronomers, dubbed by ESO the “black hole police”.

Stellar 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, which collapse in on themselves.

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

A black hole detectable with X-rays

In the binary systems already observed, the star that has become a black hole is close enough to its companion star to “steal” its matter from it (we speak of “accretion”), explains Hugues Sana from the University of Louvain to AFP. (KU Leuven), Belgium, one of the authors of the study.

When this material is trapped, it emits x-rays which can be detected. But here the black hole emits none, 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 cannot 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 emerge from its dormant state.

A dance game

But how do you know such an object exists? “Imagine a pair of dancers holding hands that you observe in the dark. One has a black suit, the other a light suit: you only see the dance of the other, yet you know 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 motions.

To be sure that the ghost object was indeed a black hole, the researchers continued to eliminate and rule out several scenarios, such as a star losing its envelope. “The only reasonable explanation is that it is a black hole, because no other star can reproduce these observational data”, summarizes the researcher.

According to recent 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. “At the moment we only know about ten of them, all discovered thanks to their X-rays, so we’re missing some!”

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