M15 is one of the brighter globular clusters and has captured the interest of astronomers for several reasons. M15 packs one of the densest cores known for a globular cluster. M15 has undergone the phenomenon of core collapse, not an uncommon event in the evolution of globular clusters. As clusters evolve, low mass star populations are depleted by tidal stripping, collisions and mergers. The more massive stars, including binary systems and neutron stars will gravitate to the center of the cluster. The cluster will then readjust itself gravitationally speaking. In the process of rearranging its structure stars are drawn to the center leading to an extremely compact core. Although the true nature of the compact center is still obscure some evidence gives support for an intermediate mass black hole in M15 of between 500 and 3500 solar masses. Approximately 20% of Milky Way globular clusters have undergone core collapse.
M15 contains a high number of pulsars (radio wave emitting neutron stars). These once massive stars are remnants of supernova events from a time when the cluster was much younger. M15 is one of only 4 known globular clusters to contain a planetary Nebula (Pease I). Although arguments have been made that Pease I is not a true member of M15 radial velocities of both nebula and cluster seem to match and provide the best evidence for membership within the cluster.