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 6531 in Sagittarius depicted below is a rich dense cluster comprised of 57 member stars of varying magnitude and part of the Sagittarius OB1 stellar association. As revealed by the image below, the cluster is associated with a strongly concentrated core and a handful of other widely dispersed members. The cluster is well detached from the rich background sky and is dominated by SAO 186215 (mag 7.21) at its center. The cluster spans 13 to 15 arc-minutes in diameter. M21 lies at a distance of 2,200 to 4,250 light-years away (depending on the source) and is dated at a very modest 4.6 million years-old and as evidenced by the plethora of white hot stars in the image below. Less than one degree away lies the stunning emission and reflection Trifid Nebula to the cluster's southwest. The cluster is best observed using low magnifications (50-100x) during mid-summer as it approaches the southern meridian near midnight. NGC 6531 was discovered by Charles Messier on June 5th, 1764.

Note: The image below was taken with extremely poor seeing and will be revisited at the first available opportunity with much better seeing conditions!