A star orbits our black hole in just 4 years

It is an understatement to say that the star S4716 leads an eventful life. In orbit around the supermassive black hole in the center of our galaxy, it orbits it in just four years at a speed of … 8,000 kilometers per second! This is the shortest orbit ever observed for a star orbiting a supermassive black hole. This record was recorded by researchers from the University of Cologne, Germany, and Masaryk University in Brno, Czech Republic. Their work is the subject of an article in The Astrophysical Journal.

A surprising cluster of stars

S4716 is fearless: it approaches only 100 AU (one AU, for astronomical unit, equivalent to 149,597,870 kilometers) from the largest monster in the Milky Way, Sagittarius A *, whose very first image was unveiled on 12 May 2022 by Global Cooperation Event Horizon Telescope (EHT). This is a minimal distance by astronomical standards. If the S4716 is closest to the black hole, it’s not the only one playing with fire (or, shall we say, with the laws of gravity). It is actually part of a very dense star cluster, known as the S cluster. Discovered by Andreas Eckart and Reinhard Genzel in 1996, it is home to more than a hundred stars with variable brightness and mass, dancing wildly from the supermassive black hole at. center of our galaxy. One of them, the S62, even reaches a speed of 20,000 kilometers per second!

“The interesting aspect of this cluster is its young age”explains to Science and the Future Florian Peissker, astrophysicist at the Department of Physics at the University of Cologne and lead author of the study. “The stars are so young that they must have formed close to the black hole. Which amazes us, for a supermassive black hole is far from being considered a favorable place for star birth. To the point that if I were only to to name a place where I would never see a star born, I would choose the environment for a supermassive black hole. ” The study of a star like S4716 should just help to better understand the history of these stars that appeared in such an extreme environment.

Unexpected sight

As fascinating as it is, the S-cluster unfortunately tends to give researchers a lot of trouble. It is one of its stars, dunce S2, whose mass reaches the equivalent of 14 times that of the Sun and blocks the view of the black hole’s direct environment. Florian Peissker compares her “to an adult sitting in front of us in a movie theater” which would prevent us from watching the movie. “But in brief moments we can observe the surroundings of the black hole”, the expert continues. In addition, S2 is a very crowded area, which requires the development of a large database to identify the stars individually. Achieving the mass of the S4716, its rotational speed, or even the distance it is from Sagittarius A *, thus required more than twenty years of work, performed by means of five different telescopes.

Such efforts should in any case provide their share of valuable information, as the identification of a star orbiting the central black hole in the Milky Way in just four years is judged by scientists as “spectacular and quite unexpected”. “This shows us that certain phenomena still escape us completely, as the presence of the star necessarily involves processes that have led it to its present location.”, delights Florian Peissker. One hypothesis is that the S4716 moved inward and possibly approached other stars and objects in the S-cluster, causing its orbit to narrow significantly.

But by flirting so much with Sagittarius *, shouldn’t the S4716 end up being swallowed up by the latter? “It’s a plausible fate. There are theoretical models that predict that the star may be disturbed over time and therefore end up being engulfed.”

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