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. |