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 IC 4725 in Sagittarius depicted below is a rich dense cluster comprised of about 100 member stars which are of varying brightness and well detached from the rich background sky and includes a handful of giants of spectral type M. The cluster spans 32 arc-minutes in diameter, lies at a distance of 2,000 light-years away while spanning 19 light-years across. M25 is dated to be about 90 million years-old, as evidenced by the predominance of white hot stars in the image below, and based on the presence of a δ-Cepheid variable (U Sgr). The cluster is best observed using low magnifications (50-100x) during mid-summer as it approaches the southern meridian near midnight. IC 4725 was discovered by Swiss astronomer and mathematician Philippe Loys de Cheseaux (1718-51) in 1745 and later observed by Charles Messier in 1764.