M21 - NGC 6531

Messier 21 (NGC 6531) is a relatively young and compact open cluster in the constellation Sagittarius, located approximately 4,250 light-years from Earth. The cluster contains about 57 confirmed member stars within a diameter of roughly 13 light-years and shines at an apparent magnitude of about 6.5. Its estimated age of around 4.6 million years makes it one of the younger Messier open clusters, and its stellar population is dominated by hot blue B-type stars, with a few early O-type stars that are among the most luminous members. M21 was discovered by Charles Messier on June 5, 1764, during the same observation session in which he cataloged the nearby Trifid Nebula (M20). The two objects are separated by less than 1 degree of sky and are often observed together, though they are not physically related. M21 lies at a different distance and is not embedded within the Trifid's gas and dust. Through binoculars, M21 appears as a bright, compact knot of stars that stands out from the rich Milky Way background. A small telescope at moderate magnification reveals a pleasing group of 20 or so brighter stars in a roughly triangular arrangement, with fainter members filling in the background. While M21 is not among the most spectacular open clusters, its youth, compact form, and proximity to the Trifid Nebula make it a natural target for observers touring the rich Sagittarius star fields during summer evenings. It serves as a fine example of a young cluster whose stars have not yet had time to disperse.

Contains about 57 stars within 13 light-years diameter at a distance of 4,250 light-years, with a very young age of approximately 4.6 million years.