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: M18 is one of the smaller open clusters within the Messier catalog at approximately seven to nine arc-minutes in apparent diameter. The cluster is relatively sparsely populated with less than 50 member stars and dominated by a handful of eighth and ninth magnitude stars well-dispersed and well-detached from the background sky with the brightest member (HIP 89831) being mag 8.67. The cluster has been estimated to be a mere 32 million years old and to lie at a distance of about 4,900 light-years away, thus suggesting a physical diameter of about thirteen thousand light-years across. Charles Messier discovered M18 in June, 1764.