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Object description
The open star cluster Messier 21 northeast of the Trifid Nebula (Messier 20) and the dark clouds LDN 235/239

Short objekt description:

Messier 21 is an open star cluster in the constellation Sagittarius, located just southwest of the Trifine Nebula (M 20,). The cluster contains about 70 stars and is classified as Type I 3 m according to Shapley. At a distance of about 4,250 light years from the solar system, the true diameter is about 16 light years. The extensions of M 20 can be seen in our image on the lower right.

Most of the cluster members are relatively faint, but a few blue and hot giant stars of spectral type B are concentrated in the centre, estimated to be about 4.5 million years old. The star cluster was discovered by Charles Messier on 5 June 1764.

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