Messier 21 — Open Cluster in Sagittarius

M21 is a young, moderately compact open star cluster in the constellation Sagittarius, located about 4,250 light-years from Earth. It contains roughly 57 stars and spans about 13 light-years across. The cluster is estimated to be only about 4.6 million years old, making it one of the youngest Messier open clusters. Its brightest stars are hot blue-white giants of spectral type B0. M21 lies just 0.7 degrees northeast of the much more famous Trifid Nebula (M20), and the two objects make an attractive pair in a wide-field view.

Located just 0.7 degrees northeast of M20 (Trifid Nebula) in Sagittarius. At magnitude 6.5, it is visible in binoculars as a small bright patch near M20. A telescope at 50-75x shows a compact group of bright stars. The cluster is best appreciated at moderate magnification (75-100x), which resolves the individual members while keeping the cluster compact and rich-looking. Its proximity to M20 makes it easy to find and a natural companion to the Trifid. Best observed from July through September.

Discovered by Charles Messier on June 5, 1764, on the same night he observed M20. Messier described it as a cluster of small stars near M20. The cluster has been confirmed as physically unrelated to the Trifid Nebula — they are at similar distances but are distinct objects.