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 1039 is one of three prominent open clusters in the constellation of Perseus (the other two being NGC 884 and NGC 869 which form the famous "Double in Perseus"). With an apparent diameter of 35 arc-minutes, just slightly greater than that of the full moon, this cluster has been estimated to be approximately 180 million years of age and to span 14 million light-years in diameter. The cluster is comprised of approximately 100 member stars including multiple bright members whose magnitudes range from 7.3 to 9.0. The cluster is best observed during winter using low magnifications (50-100x) and is located a few degrees due north of Algol (β-Per). Although independently discovered by Messier in 1764, it is believed to have also been observed by Giovanni Batista Hodierna over 100 years earlier.