Why was this black hole so hard to find?

Scientists knew they existed, but this is the first time they have discovered a “sleeping” black hole. And for good reason, they are very complicated to detect.

SPACE – This is the first one. A dormant black hole has been discovered by a team of international experts. They published their research in Natural astronomy, last July 18. Named VFTS 243, it took them 6 years of observation with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile to find this black hole in the Large Magellanic Cloud, a galaxy neighboring our own.

Finding a sleeping black hole, a real headache

“We have identified a needle in a haystack”, says Tomer Shenar, lead author of the study. Generally, we see a black hole when a star orbits it. The material that makes up this star is partially trapped by the black hole, and it is by detecting the movements of this material, which emits X-rays, that it can be seen in space. But here the black hole emits no one, and for good reason: “The living star (about 25 times the mass of the Sun) is far enough away not to be eaten,” explains Hugues Sana from the University of Louvain and co-author of the study.

To find VFTS 243, the collaboration searched for nearly 1,000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, looking for any that might have black holes as companions. But it is extremely difficult to identify these companions as black holes, as there are many other possibilities.

“As a researcher who has uncovered potential black holes in recent years, I was extremely skeptical of this discovery”, says Tomer Shenar. This skepticism was shared by co-author Kareem El-Badry of the Center for Astrophysics | Harvard & Smithsonian in the USA, which Tomer Shenar calls “black hole destroyer”. ” When Tomer asked me to check his results, I had my doubts. But I couldn’t find a plausible explanation for the data that didn’t involve a black hole. », explains Kareem El-Badry.

Different types of black holes

There are mainly two types of black holes. The one discovered is a stellar-mass black hole. This type of black hole has a mass that can be up to 20 times that of the Sun. It forms when the core of a star collapses, often causing an explosion called a supernova. But the phenomenon can vary, evidenced by VFTS 243, which has another peculiarity: the researchers did not detect any signs of explosion.

Maïca Clavel, CNRS astrophysicist at the Institute of Planetology and Astrophysics in Grenoble explains that this is a phenomenon that has already been theorized: “There are papers that say that if the star is really very massive, there is no freedom for the material to escape, and therefore everything really falls at once. »

The second type of black hole is said to be supermassive. They are at least a million times the mass of the Sun. If scientists do not know very well how they were formed, the main thing to remember is that they are at the center of galaxies. Ours for example, the one at the center of the Milky Way is called Sagittarius A*

A supermassive black hole is powerful enough to swallow a star, but rest assured, when it comes to our galaxy and specifically the Sun, there is not much to worry about, as Maïca Clavel details: “You really have to get extremely close to the black hole to be sucked in. However, the supermassive black hole at the center of our galaxy is located 27,000 light years away. So if we went at the speed of light towards Sagittarius A*, it would take 27,000 years to get there. In addition, our sun orbits the center of our galaxy at about 220 km/s. So it is not going in the right direction nor at the right speed. There is therefore no risk of us encountering the black hole at the center of our galaxy. »

There would be a third type of black hole located between those of stellar mass and supermassive. They are called intermediate black holes, and they would be a few thousand solar masses. Unfortunately, we never discovered any.

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