Astronomers targeted this special globular cluster NGC 6397 because it is 7,800 light-years away from Earth and is one of the closest globular clusters on Earth. More importantly, scientists believe that globular star clusters are ideal places to find medium-sized black holes, because their cores are densely packed with stars.
Globular clusters are large spherical collections of stars orbiting the periphery of a galaxy. These star clusters are very old, sometimes almost as old as the universe itself. A globular cluster with a collapsed core, such as NGC 6397, is old enough that more massive stars are drawn toward the center of the cluster, while younger stars move toward the periphery. This gives the globular cluster a very dense core.
Since it is impossible to observe these black holes directly, astronomers measured how the stars in the cluster move (their speed) to find the distribution of mass in the cluster. It is found that the place where the star moves faster is the area with concentrated mass. However, the distribution of these stars is not limited to a point-like center in the nucleus, which is caused by the existence of a medium-sized black hole. Instead, the quality seems to be spread out more randomly, extending to a few percent of the cluster size.
Therefore, based on the evolution of stars, astronomers have concluded that stellar remains in the form of stellar mass black holes are scattered throughout the inner regions of globular clusters. NGC 6397 can accommodate more than 20 of the "lightest" black holes.
Like many scientific discoveries, the discovery of the concentration of black holes in globular clusters has also caused more problems. One of the key questions is: Will these black holes merge into a compact quarter to produce gravitational waves? Perhaps scientists will discover whether the black holes colliding with each other will produce the gravitational fluctuations recently discovered by some instruments, perhaps they will discover something that they did not even plan.
Bottom line: Astronomers look for a medium-sized black hole in the center of a globular cluster, and instead find a large number of smaller black holes.
Source: Does NGC 6397 contain intermediate-mass black holes or more diffuse internal subclusters?
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