As part of the Gaia survey, astronomers discovered a double star consisting of an extremely metal-poor giant and a dark object with a mass 33 times greater than the Sun. It turns out it’s a black hole, or more accurately, a record-breaking stellar-mass black hole in the Milky Way. Moreover, it is not far from us by galactic standards.

As part of the Gaia survey, astronomers are collecting the most detailed map of stars in the Milky Way. Three data releases have been released through 2024; The next Gaia DR4 will not be released before the end of 2025. Among other things, the new version is expected to have many more detected binaries; Among them, scientists will be able to find systems in which one of the components is a black hole. During a preliminary data check on such systems, researchers discovered an unusual pair. In light of the significance of the discovery, they decided to publish the results of the calculations now.

One remarkable object turned out to be a binary system that will now be called Gaia BH3, although it was previously known as a star. It is located in the region of the constellation Orel, approximately 1.92 thousand light-years away from us (590 parsecs). Judging by the parameters, the bright component of the system is a metal-poor giant. It orbits a dark object with a period of 11.6 Earth years.

Astronomers noted this because the estimated mass of this companion is higher than 30 solar masses, while the other 1.5 million potential binaries do not exceed 20 solar masses. As a result of accurate calculations, it was revealed that the mass of the star was 0.76±0.05 solar masses and the mass of the dark body was 32.7±0.82 solar masses. The study was published in the journal Astronomy and Astrophysics.

This object’s luminosity is too low to be considered a star and its mass is too large for a neutron star. That leaves only three options: a black hole, a pair of black holes, or a black hole paired with a compact object. Theoretically, the presence of a pair of objects can be detected by oscillations. However, according to calculations, these fluctuations are too small to be seen in Gaia data. So, although the authors of the study could not refute the hypothesis of a pair of compact objects, they still adhere to the simplest explanation: this is a “stellar” black hole with a mass 33 times greater than the Sun.

Previously, astronomers had only “seen” such black holes through the gravitational waves resulting from their collisions. Their range is from 30 to 83 solar masses. The main problem is that such a magnitude is quite difficult to explain by stellar origin. Stars 30 times more massive than the Sun lose much of this mass during their evolution due to the strong stellar wind. As a result, they produce black holes with a mass of less than 20 solar masses.

Before the discovery of BH3, the largest black hole in the Milky Way was Cygnus X-1, which was about 20 solar masses. However, this does not mean that there are no large “stellar” black holes in the Galaxy. They are very difficult to detect because most do not interact with the companion at all.

With all this said, there is only one “solution” that will allow you to get a massive black hole from a massive star. For this to happen, the massive star must be a giant with low metallicity.

First, it reduces the percentage of mass lost during evolution. Secondly, the probability of merging with a companion is less due to the smaller radius. Third, when collapsing into a black hole, such stars have a much weaker “push” or even no “push” that can disable the second companion. At the same time, the potentially permissible metallicity of the star remains a matter of debate. According to some models, even massive stars with the metallicity of the Sun can turn into black holes of 30 solar masses.

The low metallicity of the star in the BH3 binary increases the possibility that the black hole was formed from a star with the same low metallicity. However, the authors of the new study noted that they could not exclude the formation scenario of this black hole from two small black holes.

If it formed in an environment saturated with objects, there is a possibility that it became “attached” to the star as it passed by the black hole. This version can be supported by the fact that, judging by the parameters of its motion, BH3 can be included in the star ED-2, a possible remnant of a recently discovered globular cluster.

Now the global community of astronomers can begin observing BH3 immediately. Fortunately, by galactic standards, it is not too far from the Solar System, making observation easier. Studying BH3 and comparing its parameters with BH1, BH2 and black holes detected by gravitational waves will help understand the formation and evolution of such objects.