Black hole discovered inside largest globular cluster

By analyzing more than 500 images taken by the Hubble Space Telescope (HST) over two decades, astronomers have measured the speeds of more than 1.4 million stars in the globular cluster Omega Centauri. They found that the innermost part of the cluster contains seven fast-moving stars. Their motion indicates the presence of a black hole.

Maximilian Häberle from the Max Planck Institute for Astronomy in Germany points out that these seven stars should not be in the cluster at all. They are moving so fast that they should be thrown out and escape the cluster. The most likely explanation is the presence of a very massive object that attracts them with gravity, keeping them close to the center. This is probably a black hole with a mass of 8,200 times greater than the mass of the Sun. The existence of such a black hole in the Omega Centauri cluster has been postulated before. For example, in 2008 based on observations by the Hubble Telescope and the ground-based Gemini telescope. This time, however, the evidence is stronger.

Intermediate-mass black holes are rare. They are considered to be the missing link in black hole evolution. Currently, only a few candidate black holes of such masses are known. Most known black holes are so-called supermassive black holes, located at the centers of most galaxies (mass counted in millions or even billions of solar masses), or stellar black holes (mass of the order of a dozen or so solar masses, much less than 100 solar masses).

Omega Centauri is a globular cluster of stars. It is visible from Earth with the naked eye and is comparable in size to the full moon in the sky. Physically, it is 17,000 light-years away from us.

It was first listed in an astronomical catalogue almost 2,000 years ago, by Ptolemy, as a single star. In 1677, Edmond Halley referred to it as a nebula, and in the 1830s, John Herschel determined that it was a globular cluster of stars. Globular clusters are compact clusters of stars, containing up to a million stars bound together by gravity. The name comes from the spherical shape of such a cluster. Globular clusters occur on the outskirts and also in the central regions of many galaxies, including the Milky Way. Omega Centauri is much more massive than typical objects of this type – it has a mass about 10 times greater than other large globular clusters. It can be compared in mass to even a small dwarf galaxy. It contains almost 10 million stars.