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 open cluster M103 is the second major such cluster in Cassiopeia following in the footsteps of NGC 457. The cluster contains approximately 170 member stars spanning 15 light-years across and, as illustrated by the image below, is dominated by a handful of bright stars ranging in magnitude from 7.3 to 8.3. The cluster was first discovered by Pierre Mechain in 1781 and represents one of the latest entries in Charles Messier's catalog. M103 is best observed using either binoculars or a rich-field refractor and low magnifications. The cluster is very easy to locate since it lies within a degree of δ-Cassiopeiae (Ruchbach, mag 2.65) and is best observed during late summer and early fall when it is directly overhead around midnight.