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 6871 in Cygnus depicted below is a cluster comprised of less than 50 member stars which are interspersed and well detached from the background sky. The cluster is dominated by a pair of double star systems comprised of SAO 69402 (mag 6.77) and SAO 69403 (mag 7.36) for the northern pair and SAO 69405 (mag 7.86) and SAO 69404 (mag 8.86) which make up the southern pair. The cluster spans 20 arc-minutes in diameter and lies 5135 light-years away. NGC 6871 has been studied fairly intensely owing, in part, to the presence of eclipsing binaries (ex. V453 Cyg). Its distance has been estimated to be around 5135 light-years while its age is a mere 9.1 million years old. NGC 6871 is surrounded by nebulosity including LBN 180 to the north, LBN 182 to the east and LBN 179 and 174 to the south and southwest. The small patch of nebulosity near the bottom right corner is the nebula GN 20.03.0. 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 south of 27-Cyg (mag 5.38). NGC 6871 was discovered by Friedrich Georg Wilhelm von Struve (1793-1864) in 1825.

Note: SAO 69402 (mag 6.77) referred to above is also the brightest Wolf-Rayet star (WR 133) visible in the northern hemisphere!