There is, however, a well-marked type to which many of the richest clusters belong; these are the globular clusters.
These clusters present many unsolved problems. Thus Perrine, from an examination of ten globular clusters (including Messier 13 and Centauri), has found in each case that the stars can be separated into two classes of magnitudes.
The oldest globular clusters contain only stars less massive than 0.7 solar masses.
In the ensuing 70 years, only three more planetary nebulae have been discovered in globular clusters.