M15 may look like an ordinary globular cluster, but it is not.
Several of its features are of particular interest to astronomers. One is the planetary nebula (the large bright spot to one side) where a dying star has ejected a shell of gas.
M15 is a ΓÇ£collapsedΓÇ¥ cluster. The central two arc seconds are much brighter than those of most clusters; this means the center of M15 has an unusually large number of stars. Why has M15ΓÇÖs center collapsed when other globular clusters have not? There are several possibilitiesΓÇöthat the center contains a black hole is one of the more excitingΓÇöbut a complete answer is not possible without more information (see Hubble image).
