galaxy cluster: An accumulation of
galaxies that are gravitationally
bound to each other. Galaxy clusters
vary in population from small (20
members) to immense (10,000+
members). Our galaxy, the Milky
Way, belongs to a small cluster
known as the Local Group. 
 Galaxy clusters are both regular and
irregular. Irregular clusters are not
spherically symmetric and contain a
mixture of all types of galaxies.
Regular clusters tend to be
spherically shaped and are dominated
by elliptical galaxies.