[<< wikibooks] Messier Index/M21
Messier 21 or M21 (also designated NGC 6531) is an w:open cluster of w:stars in the w:constellation of Sagittarius. It was discovered and catalogued by w:Charles Messier on w:June 5, w:1764.
M21 is a relatively young cluster of a mere 4.6 million w:years of age. It is tightly packed but contains about 57 w:stars. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars. With a magnitude of 6.5, M21 is not visible to the naked eye; however, with the smallest binoculars it can be easily spotted on a dark night. 

== External links ==
Messier 21, SEDS Messier pages