Discovered by Charles Messier in 1764.
M21 is a relatively small open cluster of approximately 60 stars. It is located near the Trifid Nebula (M20) at a distance of approximately 4,200 light years. The estimated age of the cluster is 4.6 million years which makes it a very young stellar object. The brighter stars in the cluster are blue giants of spectral type BO and are consuming their stellar fuel at a rapid pace.