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 6811 in Cygnus depicted below is a cluster comprised of approximately 70 member stars which are of uniform brightness and very well detached from the background sky. The cluster spans 15 arc-minutes in diameter or half the apparent diameter of the full moon. Depending on the source, the cluster lies between 2940 and 3960 light-years away (heliocentric distance) with an intermediate age of approximately 700 million years old and as evidenced by the presence of some red giants and predominance of white hot stars in the image below. The small galaxy to the bottom right of the image below is PGC 2277519 (mag 16.7, 0.6'x0.3'). The cluster is best observed using low magnifications (50-100x) during summer when it is directly overhead and can be found to lie just to the northeast of δ-Cyg (mag 2.90). NGC 6811 was discovered by John Herschel in 1829.