Astrophysicists think that the supermassive black holes (SMBH) we see in the modern Universe are the result of a long drawn-out process. That process is referred to as the hierarchical black hole mergers framework. It explains how some SMBH can reach such staggering masses. Some of the most massive appear to have billions of solar masses.

The idea is that these SMBH start out as seeds, and those seeds are stellar-mass black holes. Two processes create them. One is when massive stars between about 8 and 50 solar masses reach the end of their lives of fusion. They either explode as supernovae, and their cores collapse, forming black holes, or they collapse directly into black holes. The other is when two neutron stars merge.

In between stellar mass black holes and SMBH are intermediate mass black holes (IMBH.) These black holes range in mass from about 100 solar masses to 100,000 solar masses.…