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: Open cluster NGC 2099 is the brightest and most populous of the three Messier open clusters in Auriga with a magnitude of 5.6. Dominated by approximately 150 member stars ranging in magnitude from 10 to 13, this cluster lies approximately 4,400 light-years away and spans about 25 light-years. As revealed by the image below, the presence of various red giants indicates that this cluster is advanced in age and is estimated to be approximately 300 million years old. The cluster was first discovered by Giovanni Batista Hodierna in 1654 and later first observed by Messier in 1764. The cluster lies in nearly a vertical line with the other two primary clusters in Auriga (M36 and M38) being the middle cluster of the set. All three clusters are best observed using low magnifications during fall and winter where they are placed directly overhead when looking due east.