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: NGC 6633 is a mid-size open cluster in Ophiuchus with an apparent diameter of 27 arc-minutes and is dwarfed by other much larger open clusters within Ophiuchus such as Melotte 186 (240 arc-minutes), IC 4665 (70 arc-minutes) and Cr 350 (45 arc-minutes). NGC 6633 is dominated by many eighth and ninth magnitude stars with the brightest member (HD 169959) being mag 7.76. The approximately thirty stars which comprise the cluster are both well-dispersed and well-detached from the background sky. The cluster has been estimated to be somewhat advanced at 660 million years of age and to lie at a distance of about 1,040 light-years away. Swiss astronomer Philippe Loys de Cheseaux is credited with its discovery during 1745-1746 and which was later independently rediscovered by Caroline Heschel in 1783.