Open star clusters are widely distributed in our galaxy and represent a loose collection of stars which number from a few dozen to a few hundred stars and are weakly-held gravitationally. Perhaps the three most famous such open clusters are the Pleiades (M45) in Taurus, the Beehive (M44) in Cancer and the double cluster in Perseus. They are all characterized with a handful of hot and white prominent stars and nebular material surrounding these stars.

Note: The 1958 publication from the University of California at Berkeley entitled "Catalogue of Star Clusters and Associations" by Jiri Alter et al is a collection of 104 open star clusters identified from the Palomar Observatory Sky Survey (POSS) plates. As indicated below, the catalogue consists of many extremely old open star clusters which easily exceed one billion years in age and including perhaps the oldest open star cluster in our galaxy (Berkeley 17 at 10.06 to 10.08 billion years old).