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 2437 is one of three major open clusters in Puppis (along with M47 and M93). The cluster is relatively young with an estimated age of 300 million years old. It has an apparent diameter similar to that of the full moon, spanning 30 light-years across and comprised of approximately 500 member stars including many between magnitude 10 and 13. What makes this cluster most unique is the appearance of a planetary nebula (NGC 2438) within the same field of view. However, this is an optical illusion, for studies have confirmed that this nebula not only is not a member of the cluster but lies closer to us than the cluster itself. The cluster was first discovered by Charles Messier in 1771 and reported during the second revision of his catalog. The cluster lies to the east of Sirius and within the immediate vicinity of M47. NGC 2437 is best observed in winter as it approaches the southern meridian just before midnight.