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 2244 in Monoceros depicted below lies at the core of the large and expansive Rosette emission nebula and, quite often, the NGC 2244 designation is also associated with the nebula itself. Comprised of over 100 member stars which are interspersed and without a central concentration, the brigher 0B-association member stars help illuminate the surrounding gas. These stars have blown away the gas in the immediate vicinity with their intense radiation, thus leaving behind a hole within the nebula. NGC 2244 spans approximately 23 arc-minutes in diameter and lies 4715 light-years away. In addition to the double classification (NGC 2239 and NGC 2244), a number of other classifications exist including NGC 2237 and NGC 2238 to the immediate west of the cluster as well as NGC 2246 to the northeast representing various small portions of nebulosity involving the Rosette nebula itself. Also of interest are the various Blok globules visible in the periphery and which often lead to the formation of binary and multiple star systems. NGC 2246 was discovered by John Flamsteed (1646-1719) in 1690.