M21 is a compact open cluster in the constellation of Sagittarius, which is positioned less than one degree northeast of the Trifid Nebula (M20). It contains about 60 stars, mostly of them white but a sprinkling of blue giants, set in a tightly packed area covering 13 arc minutes. With an apparent mag. of +6.5, M21 is a nice sight in binoculars and small telescopes but compact enough that larger scopes also show it well, especially at low magnifications.
M21 was discovered - along with M20 - by Charles Messier on June 5, 1764. It's best seen from southern and equatorial regions during the months of June, July and August.
The ideal starting point to find M21 is the teapot asterism that forms the core star grouping of Sagittarius. The top three stars of the teapot are Kaus Borealis (λ Sgr - mag. +2.8), Kaus Media (δ Sgr - mag. +2.7) and φ Sgr (mag. +3.2). Just over 6 degrees north of an imaginary line connecting φ Sgr and Kaus Borealis is M20, with M21 located 0.75 degrees northeast of M20.
M21 is a fine binocular object with the brightest stars resolvable. It appears compact and misty with a sprinkling of starlight, particularly when using averted vision. Through a 80mm (3.1-inch) scope, the cluster is a wonderful sight with many stars revealed. In total, there are some 35 stars between magnitudes +8 and +12 and a medum size 150mm (6-inch) or 200mm (8-inch) scope will show them all. The stars are packed tightly together.
M21 is a relatively young cluster at only 4.6 million years old. It's located 4,250 light-years from Earth and has an actual diameter of 16 light-years.
M21 Data Table
|Object Type||Open cluster|
|RA (J2000)||18h 04m 13s|
|DEC (J2000)||-22d 29m 24s|
|Apparent Size (arc mins)||13 x 13|
|Age (years)||4.6 Million|
|Number of Stars||60|