Messier 21 is an open cluster located in the constellation of Sagittarius a half of a degree northeast from M20, the Trifid Nebula.

M21 lies 4,250 light-years away, and subtends an apparent angle of 16 arcminutes, which corresponds to a true size of 18 light-years in space. It shines at magnitude 5.9.

The cluster contains 57 member stars, the brightest of which is a magnitude 7.2 spectral-type BO hot young blue star.

M21 is a young cluster with an estimated age of 4.6 million years and is part of the Sagittarius OB1 stellar association.

Charles Messier discovered M21 on June 5, 1764.

North is to the top in the above image.

M21 Catalogs: M21, NGC 6531 Object Type: Open Cluster Magnitude: 5.9v Size: 16' Constellation: Sagittarius Image Field Centered At: RA: 18h 04m 01s Dec: -22° 32' 04"