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: As indicated by the image below, NGC 2331 in Gemini is an open cluster which is not very well detached from the background. It is comprised of 50 to 100 stars all of moderate brightness and spread across approximately 15 arc-minutes or half of the apparent diameter of the full moon. The cluster is not very well studied, for no estimates exist as to its physical distance or age. The cluster is best observed using low magnifications (50-100x) during winter when it is directly overhead and can be found to lie between Pollux (β-Gem, mag 1.22) and Mebsuta (ε-Gem, mag 2.99). NGC 2331 was discovered by William Herschel in 1785.