M21, Open Cluster in Sagittarius

Image by Jim Dixon
M21 is a cluster which shows quite a strong concentration toward its center. Therefore, it is classified by Woldemar Götz as of Trumpler class I 3 r (strong concentration to the center, large range in brightness, i.e. bright and faint stars, and richly populated), while Trumpler, according to Kenneth Glyn Jones, classified it I 3 p (i.e., poor, or under 50 stars).

According to Burnham, S.N. Svolopoulos, in 1953, has demonstrated the membership of 57 stars (making it Trumpler type I 3 m), the brightest of which are giants of spectral type B0. This implies that this cluster is very young: the Sky Catalog 2000 gives an estimated 4.6 million years, and states that this cluster is part of the Sagittarius OB1 stellar association.
The distance of this cluster is discordantly given by the sources: Mallas/Kreimer give 3,000 light years, Burnham 2,200, while Kenneth Glyn Jones and the Sky Catalog 2000 have 4,250 light years. It is interesting that all sources have different distances for the Trifid Nebula M20, but discord which is closer to us, the cluster M21 or the Trifid nebula.

Open cluster M21 was discovered by Charles Messier, who cataloged it on June 5, 1764.