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: Collinder 399 in Vulpecula is a large asterism comprised of approximately 40 member stars with an apparent diameter of 90 arc-minutes. The asterism is best known for the appearance of an upside-down coat hanger formation thanks to the eight or nine brightest member stars. There exists some confusion as to whether this collection of stars is an open cluster or simply an asterism. The cluster was first observed by Al-Sufi in 964 AD and was never assigned an NGC designation perhaps owing to its large apparent diameter. Lying at a distance of 420 light-years away, the asterism is best observed using a pair of binoculars during late summer and early fall when the constellation of Vulpecula is high overhead following the end of astro twilight.