Globular clusters are spherical star formations gravitating around galactic cores. During their evolution, lighter stars tend to move away from the centre, which is left populated largely with stars of higher mass. But globular cluster NGC 6101 (pictured) is peculiar. Its stars are just too evenly distributed — devoid of the mass segregation expected for its size and age. Using extensive simulations of the dynamic evolution of the cluster over thirteen billion years, Miklos Peuten and colleagues were able to figure out the reason why: the lack of segregation is due to the existence of hundreds of black holes.
So where do these black holes come from? We know that they are the remnants of explosions of massive stars, but current theories predict that these black holes would be kicked away from their parent star. The simulations of Peuten et al. account for the current star distribution in NGC 6101, including hundreds of black holes of stellar mass. But they also suggest that the kick black holes receive after birth is lower than expected.