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 2548 in Hydra is a bright cluster (magnitude 5.5) which can be visible under dark skies. It is comprised of at least 80 member stars with more than half being magnitude 13 or brighter. It is characterized with a relatively dense core which spans nearly 30 arc-minutes and extends further out to nearly one degree. It has been estimated to be approximately 300 million old and characteristically includes a few yellow giants. It has been determined to lie at a distance of approximately 1500 light-years away and to span about 23 light-years in diameter. The cluster was first discovered by Charles in Messier in 1771 who accidently recorded an erroneous position in his catalog and which required another 160 years to be rediscovered in 1934 by Oswald Thomas. Although the cluster lies in the constellation of Hydra, it is best located using ζ-Mon as a reference point and slewing three degrees southeast. The cluster is best observed during winter when it crosses the southern meridian around midnight.