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: The open cluster NGC 7039 in Cygnus depicted below is comprised of approximately 100 member stars which are not very well detached from the background sky owing to the fact the cluster is in a very rich section of Cygnus and is comprised of many mag 11-13 member stars near the core as well as a handful of brighter stars to the northeast (SAO 50547 at mag 6.69 and SAO 50561 at mag 7.83) and southwest (SAO 50531 at mag 8.82 and GSC 3588:1025 at mag 6.6) which effectively border the cluster. NGC 7039 spans 25 arc-minutes in diameter or nearly the apparent diameter of the full moon and is estimated to be relatively young at 66 million years-old while lying at a distance of 3,100 light-years away. The cluster is best observed using low magnifications (50-100x) during summer when it is directly overhead. NGC 7039 was discovered by John William Herschel (1792-1871) in 1829.