Galaxies can also have satellites in the form of dwarf galaxies and globular clusters. Large elliptical galaxies typically have an extensive system of globular clusters. Open clusters are very different from globular clusters. Astronomers are searching for exoplanets of stars in globular star clusters. M87 has an abnormally large population of globular clusters. The Sombrero Galaxy has a relatively large number of globular clusters. This indicates that planets are relatively rare in globular clusters. Currently there are approximately 130 millisecond pulsars known in globular clusters. Astronomers have searched for black holes within globular clusters since the 1970s. This is a hypothesis for the formation of globular clusters. The latter mechanism may be responsible for the formation of globular clusters. Cacti whose stems are even smaller may be described as globular (or globose). In this sense they have some similarity to the much smaller globular clusters. This star-forming period is relatively brief, compared with the age of many globular clusters. This is a list of globular clusters.