How fast is the supermassive black hole in the center of the Milky Way spinning?

Einstein argued that a black hole is definedthree properties: mass, spin and electric charge. The charge of the black hole should be close to zero, since the matter captured by the black hole is electrically neutral. The mass of a black hole determines the size of its event horizon and can be measured in several ways, from the brightness of the material around it to the orbital motion of nearby stars. But the rotation of a black hole is much more difficult to study. We don't really know how fast the supermassive black hole at the center of the Milky Way is spinning. We know that stars and planets rotate on their axis, and exactly the same thing happens with black holes. The only difference is that black holes have no physical surface like stars and planets. The spin of a black hole, like its mass, is a space-time property and determines how space around a black hole bends, so to measure the spin of a black hole, you need to study how matter behaves near it.

Supermassive black hole in the center of the Milky Way is a mystery to scientists

How do black holes rotate?

In a study published inAstrophysical Journal Letters, the rotation of some supermassive black holes has been measured. This is because with the help of several active black holes, researchers can study the X-rays emitted by their accretion disks. The X-ray beam of light from the disk receives a pulse of energy from rotation, and by measuring this momentum, spin can be determined.

But there is another way - to get straightimage of a black hole, as astronomers did with the one in the center of the elliptical galaxy Messier 87 (M87) in the constellation Virgo. The ring of light that we see in the image is brighter on the side rotating towards us. Read more about how the researchers managed to get the first ever image of a black hole, read our material.

That same famous shot of a black hole. One side of the photo is brighter due to its rotation.

For even more fascinating articles on how astronomers study the observable Universe, read our channel in Yandex.Zen. There are regularly published articles that are not on the site!

But despite these amazing results, the backthe black hole that orbits the center of the Milky Way galaxy, researchers still do not know. The problem is that our black hole is not very active, and it is much smaller than the one located in the center of the elliptical galaxy M87. Partly for this reason, astronomers cannot measure its rotation by observing light near a black hole. The good news is that in the new study, scientists are proposing a new way to measure the spin of a supermassive black hole in the heart of our galaxy.

Their method uses a property known as Lense-Thirring effect (frame dragging) with which you canto observe how the precession of the orbital plane of the test mass revolves around a massive rotating body, or as the precession of the gyroscope's axis of rotation in the vicinity of such a body. When the mass rotates, it slightly bends the space around it. We know this is real because scientists have measured the Lens-Terring effect of the Earth.

X-rays from a black hole tell astronomers about its rotation. Photo: NASA / JPL-Caltech

Rotating a black hole has the same effectand by measuring it, the rotation of the black hole can be determined. Considering that it is impossible to launch a probe into orbit around a black hole, as it was with Earth, the new method seems to be the most real solution to the problem.

Secrets of the Milky Way

Our galaxy hides many secrets, butmost of them are concentrated around a supermassive black hole, next to which a large number of stars revolve. By the way, I wrote about the most interesting of them in this article. Some forty of these stars, known as S-stars, orbit perilous trajectories as they approach the black hole. However, due to the Lense-Thirring effect, their orbits should shift over time. Initially, the authors of the new study suggested that by measuring these shifts, it would be possible to measure the spin of the black hole. Astronomers logically believed that the more spin, the greater the displacement of the orbit.

An S-star cluster orbiting a black hole in our galaxy.

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However, during the study, after carefulstudying the orbits of S-stars, the Linse-Terring effect was not found. Given the fact that the orbits of these stars are well known to astronomers, we know that the black hole at the center of our galaxy must rotate slowly. Thus, the team determined that the spin of a black hole in the very heart of our galaxy can be no more than 0.1 on a scale from 0 to 1, which means that it rotates less than 10% of the maximum possible spin for a black hole. For comparison, the spin of the black hole M87 is at least 0.4.