Swarm of smaller black holes found in NGC 6397 where lone, massive one was thought to exist

Astronomers have discovered a concentration of smaller black holes lurking at the heart of the globular cluster NGC 6397 instead of a single massive black hole. As per a NASA statement on the finding, globular clusters are dense stellar systems hosting stars that are closely packed together. While all globular clusters are fairly 'old' from an astronomical point of view, the NGC 6397 is peculiar – researchers think it may be almost as old as the universe itself. NASA states that the cluster resides 78,000 light-years away making it one of the closest globular clusters to Earth. The NGC 6397 is also known as a core-collapsed cluster due to its immensely dense nucleus.Astronomers had initially thought the globular cluster contained an intermediate-mass black hole. According to researcher Eduardo Vitral of the Paris Institute of Astrophysics (IAP) in France, they have found very strong evidence of an invisible mass in the dense core of the globular cluster. Surprisingly to the team, this extra mass was not 'point-like' – as is expected for a solitary massive black hole – but rather extended to a certain percent of the size of the cluster itself.[caption id="attachment_9336461" align="alignnone" width="1280"]

NGC 6397 is a globular cluster in the constellation Ara. The cluster contains around 4,00,000 stars, and can be seen with the naked eye under good observing conditions. Image: NASA[/caption]Vitral and Gary Mamon, also of IAP, used the velocities of stars in the cluster to determine the distribution of its total mass, as well as in faint stars and black holes, to detect the elusive hidden mass. Mamon added that their analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or extremely elongated.Researchers concluded that the invisible component can only be made of the remnants of massive stars. They added that the stellar corpses, "progressively sank to the cluster's center after gravitational interactions with nearby less massive stars."Vitral added that theirs is the first study to provide both the mass and the extent of what appears to be a collection of mostly black holes in the center of a core-collapsed globular cluster.Researchers note that the discovery raises the possibility that mergers of these tightly packed holes in globular clusters may be an important source of gravitational waves, ripples through spacetime.