A study recently published by the Royal Astronomy Society suggests that the Hyades star cluster, a group of stars that is only about 150 light years away from Earth, could contain 2 or 3 black holes within it, which which would make them the closest to our planet (until now, the proximity record was held by Gaia BH1, a black hole located 1,560 light years away).
The research, in which scientists from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC) have participated, has used complex programs that simulate the movements and situation of the stars belonging to the closest known star cluster (the Hyades), and the result has been compared with the data collected by the European Gaia probe. This is how scientists have found that simulations that include black holes more accurately describe the structure observed for this cluster.
Star clusters are large groupings of stars of which two clearly differentiated categories are known. The so-called globular clusters have a spherical structure, contain tens of thousands (or in some cases even millions) of stars and are usually found in the external region that surrounds galaxies and is called the galactic halo. In the case of our galaxy, the Milky Way, about 200 of these clusters are known.
On the other hand, open clusters are much less compact and without a defined structure, and typically include hundreds or thousands of stars. Some of these clusters are relatively close to Earth. Specifically, the Hyades open cluster, located in the constellation of Taurus, contains more than 700 members and is about 147 light years away, making it the closest to our planet.
Astronomers suspect that black holes created at the time of the death of very massive stars could hide in the center of some star clusters. These suspicions arise especially from the study of gravitational waves, the vibrations of space-time that generate large masses in accelerated motion, typically mergers of pairs of very compact and massive objects such as black holes or neutron stars.
Star clusters present structures that, among other parameters, are defined by the so-called average mass radius, which is the size of the central part that contains half of the total mass of the group.
Due to their mass, the black holes that a cluster may contain sink inward, which in turn causes interactions that cause the massive stars in the cluster to remain further from the center. The result is that a cluster with black holes will tend to have a larger average mass radius value, that is, it will be less compact.
The analysis of the average mass radius has been one of the main pillars of the new study. Specifically, the values obtained from simulations have been compared with those observed by the Gaia satellite, the European space agency probe that, since 2013, is composing the most precise map ever generated of our galaxy and cataloging with extreme Accurate the position of more than 2 billion stars in the Milky Way (including those that make up the Hyades cluster).
The result of the study has been that the simulations only agree with the value of the mean mass radius observed for the Hyades if the cluster contains 2 or 3 black holes inside. On the contrary, without black holes the average mass radius obtained deviates by 30% from the real one.
The study also specifies that the results are compatible with the existence of black holes relatively close to the cluster, which could have been ejected from the interior due to mutual interactions. In this case, the black holes in question should be found no more than 200 light years from the center of the cluster and would have been ejected in the last 150 million years.
Black holes of stellar origin are created as a result of the explosive death of massive stars at the end of their lives, a phenomenon known as supernova. These stars, when they exhaust the nuclear fuel inside, collapse due to their enormous weight and the star falls towards its center at speeds equivalent to fractions of that of light.
The pressure in the heart of the dying star reaches such intensity that the matter is extremely compressed, giving rise, depending on the mass, to a neutron star or a black hole. NASA estimates that there must be about 100 million black holes generated by supernova explosions in our galaxy alone.
