Messier 21 (NGC 6531) is an open cluster located northeast of the Trifid Nebula (M 20) in Sagittarius. Charles Messier discovered M 21 in June of 1764 while he was observing the Trifid Nebula.
Messier 21 is a compact, round cluster containing 57 stars of 7th magnitude and fainter, concentrated into an area 13' across. With a total magnitude of 6.5, Messier 21 can be easily spotted with binoculars on a dark night. A double star of 9th and 10th magnitude lies at the center of the group. Two strings of fairly bright stars trail toward the Trifid Nebula, less than half a degree from M 21 to the southwest.
Estimates place this cluster at a distance of 4,200 light years, closer than the Trifid Nebula. Its true diameter is about 20 light years, and it has an absolute magnitude of -5.9, corresponding to a luminosity of 20,000 Suns. The brightest star in M 21 is a hot, luminous class B0 giant, so M 21 must be a very young group, merely 4.6 million years of age.