A swarm of black holes

    Just 7,800 light years away, in the southern constellation of Ara (El Altar), is the globular cluster NGC6397. It has 400,000 stars and is one of the two closest to Earth.
    The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Topics of exciting research, astronomical adventures and scientific news about the Universe analyzed in depth.

    A compact group of stellar-type black holes has been detected at the core of a cluster of 400,000 stars, one of the closest globular clusters to Earth.

    Globular clusters are huge groupings of stars. In our Milky Way, more than a hundred of them are known and in a certain sense they resemble globe-shaped micro-galaxies (hence their name) that are found in the halo of the Milky Way orbiting around the galactic center. Its spherical morphology is produced by the gravity that has held the star cluster together since the initial moments of the formation of the Milky Way. They are thus groups of old stars; in fact, they may contain some of the first stars to form in the Galaxy.

    Just 7,800 light years away, in the southern constellation of Ara (El Altar), is the globular cluster NGC6397, one of the two closest to Earth and, therefore, one of the favorites of astronomers to study. the properties and structure of clusters. It contains about 400,000 stars, with extremely high stellar density in its central region. Moreover, studying the density profile, it is concluded that the central nucleus of this cluster has collapsed due to the intense gravitational effect that occurs in that area.


    As the center of NGC6397 collapsed, it is natural to think that a black hole has formed. The compact objects expected to be found in the nuclei of globular clusters are so-called ‘intermediate mass black holes’ (IMBH). Very few objects of this class are known. Both the supermassive black holes that inhabit the nuclei of galaxies, and the stellar-type ones that result from the collapse of an individual star, are much better understood. But intermediate mass black holes (IMBH) are very poorly studied, very little is known about them.

    Eduardo Vitral and Gary A. Mamon (from the Paris Institute of Astrophysics) undertook observations of NGC6397 to determine the characteristics of the intermediate-mass black hole that they assumed existed in its nucleus. They used data from the Hubble and Gaia space telescopes to measure the motions of individual stars in the cluster with extremely high precision. Since the distance to the cluster is well known, by observing the changes in the position of the stars, they calculated their velocities and their orbits within the cluster. And, from these orbits, they were able to deduce the structure of the material at the core of this huge stellar hotbed.


    Naturally, the stars of the cluster themselves do not allow their central region to be observed directly, but astronomers were surprised to observe that the nucleus of this cluster cannot be a small point of very high density, as would correspond to a black hole of intermediate mass.

    Instead, they found that this massive central zone must occupy an appreciable percentage of the cluster’s size and between 0.8 and 2% of its mass. Therefore, this region must be populated by a small swarm of compact objects: white dwarf stars, neutron stars and stellar-type black holes. They are the corpses left by the old stars after reaching the end of their lives, remnants that are of different types depending on the mass that each of these stars had during its life.

    Vitral and Mamon used theoretical arguments, based on the theory of stellar evolution, to deduce that, of the compact objects at the center of NGC6397, the most abundant must be stellar-type black holes.

    These observations perhaps disappoint by preventing studying the properties of the elusive intermediate-mass black holes, which was the initial goal of the work. But they immediately raise the fascinating possibility that there are many more globular clusters harboring swarms of ‘small’ black holes within them. If this is confirmed, these swarms would be very suitable places for the emission of gravitational waves that could be detected in the near future with the large observatories dedicated to these waves, such as LIGO and VIRGO.

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