M21 (NGC 6531)
Messier 21 (NGC 6531) is an open cluster located in the constellation Sagittarius, in the Sagittarius Arm of the Milky Way Galaxy in the Local Group of galaxies. M21 is 3,900 light years away from Earth.
M21 is best viewed during late summer, is magnitude 6.5, and can be viewed with binoculars. M21 is 14' in apparent size. For reference, the full moon is 30' (arc minutes) in size.
* The naked eye can see up to magnitude 8 objects under ideal dark sky conditions.