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 6494 in Sagittarius depicted below is a rich dense cluster comprised of over 100 member stars which are of uniform brightness and well detached from the rich background sky. The cluster spans 30 arc-minutes in diameter or 15-20 light-years across. M23 lies at a distance of 2,150 light-years away and is dated at a mere 220-300 million years-old and as evidenced by the predominance of white hot stars in the image below. The cluster is best observed using low magnifications (50-100x) during mid-summer as it approaches the southern meridian near midnight. NGC 6494 was discovered by Charles Messier in 1764.