S0-102: The Star on the Brink

PArmed with all the stars, the S0-102 looks like an extreme sportswoman. It zips around the center of our Milky Way faster than anyone else. And she helped us understand the amazing object that is there. Because at the heart of the Milky Way lies a supermassive black hole, 4 million times heavier than the Sun.

The existence of these giant entities had been suspected for a long time, but only careful observation of stars close to the galactic center confirmed this hypothesis beyond a shadow of a doubt. Imagine our Milky Way as a disc with a large ball in the middle. In the disc, the stars, including our sun, are arranged in spiral arms. However, the center is located in what is known as the “bulge”, a spherical region that spans about 10,000 light years. There are many more stars there, much closer together than in the arms. It is in the center of all these stars that the black hole remains.

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And this is also where the star S0-102 is located, which, together with a group of other stars, orbits the center in a surprisingly short time. To travel the entire Milky Way, our sun needs about 220 million years. As American astronomer Andrea Ghez and his colleagues discovered in 2012, S0-102 manages to orbit the central black hole in just 11.5 years! The fastest of all known stars, it is also an extremely valuable source of information.

An unmasked black hole

Unlike the black hole itself—called “black” precisely because nothing, not even light, escapes from it—such a star remains observable. The motion of S0-102 then tells us about some of the properties of the black hole. It was stars like this that allowed us to be certain that a black hole exists out there.

The closer a star is to the object it orbits, the faster it orbits it. The same laws that govern the motion of the planets around a star apply here. And just as we manage to calculate, from the motion of the planets, the mass of the star around which they orbit, we utilize the orbits of stars like S0-102 to determine the mass of the object around which they orbit. As early as the late 1990s, it was calculated from observations of other stars that passed just as close to the center that there must be an extremely massive object there. Based on the size of the track, we had also derived an upper limit for its possible expansion.

4.1 million solar masses

The result ? In the center of the Milky Way, there is such a mass in such a small space that it can only be a black hole. As S0-102 completed its orbit around the galactic center, examination of the data left no doubt. With now two orbits fully observed, Ghez and his collaborators were able to estimate the mass of the black hole at 4.1 million solar masses.

There is no longer any hesitation: at the center of the Milky Way is an incredibly massive black hole. Likewise, we now know with certainty that such objects can also be found at the center of all other large galaxies. However, we have not yet understood how black holes with such gigantic masses can form. One thing is for sure, it has nothing to do with “normal” black holes caused by the collapse of a star, because such massive stars cannot exist.

Stars like S0-102 can be reassured: they will continue to be the focus of astronomers’ attention. They will certainly help us in the future to solve one or the other mystery. Unless they get too close to the black hole and the latter swallows them up…

This article is fromStars. A story of the universe in a hundred stars » by Florian FreistetterFlammarion editions, November 2020, 463 pages, 25 euros

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