'Surprising sight': Astronomers discover grouping of small black holes 'in a sea of stars'

NASA astronomers found an unexpected sight while looking for a massive black hole. Initially, the astronomers thought that the globular cluster hosted an intermediate-mass black hole. These were the long-sought "missing link" between supermassive black holes and stellar-mass black holes. However, now they have discovered a very strong evidence for an invisible mass in the dense core of the globular cluster. 

A very strong evidence 

“We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not 'point-like' (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster”, said Eduardo Vitral of the Paris Institute of Astrophysics (IAP), in the press release. Vitral and Gary Mamon, who also come from IAP, used the velocities of stars in the cluster to determine the distribution of its total mass. This is the mass in the visible stars, faint stars and black holes. The more is the mass, the faster the star can travel. 

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They used previous estimates of the stars' tiny proper motions. This allowed them to determine their true velocities within the cluster. These precise measurements in the cluster's core could only be observed with Hubble over several years of observation. Data obtained from Hubble was added to well-calibrated proper motion measurements provided by the European Space Agency's Gaia space observatory. Mamon said, “We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes”. Vitral further added, “Ours 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”.

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In the end, it was concluded that the invisible component can only be made of the remnants of massive stars. This conclusion was made keeping in mind the mass, extent, and location. The corpses sank to the cluster's center after gravitational interactions with nearby less massive stars. This process is termed as ‘dynamical friction’. It takes place through an exchange of momentum. Also, the heavier stars are segregated in the cluster's core and lower-mass stars migrate to the cluster's periphery.

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(Image Credits: NASA.gov)