Messier 21

M21 is an open cluster located in the southern constellation Sagittarius. It has an apparent magnitude of 6.5. The cluster lies at an approximate distance of 4,250 light-years from Earth. Its designation in the New General Catalogue is NGC 6531. It is also known as OCl 26.0, MWSC 2796, GC 372, C 1801-225. M21 consists mainly of small, faint stars, but is also home to a few blue giants. The cluster contains about 57 confirmed members but is pretty densely packed. It is classified as of Trumpler class I 3 r, which means that is a detached cluster with strong central concentration (I), consists of both bright and faint stars (3), and contains more than 100 stars (r). The stars of M21 are believed to be members of the Sagittarius OB1 Association, a group of massive O and B-type stars that share a common motion, age and origin. The eight of the 10 brightest stars in the cluster are spectroscopic binaries with periods shorter than 6 days. It is only 4.6 million years old, which makes it relatively young for an open star cluster. It is only 10 per cent of the age of Messier 45, the famous Pleiades cluster in Taurus, and 1 per cent the age of the Sun. The cluster contains about 35 stars with a visual magnitude between 8 and 12.

The cluster can be found using the Teapot asterism, formed by the brightest stars in Sagittarius, to first find either of the two famous nebulae and then use them to locate M21 at right ascension 18h 04.6m and declination -22°30′. The best time of year to observe M21 is in the months of June, July and August.

The open cluster was discovered by Charles Messier himself on June 5, 1764.  Credits: Messier Objects, NASA, Universe Today, Wikipedia.