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| M21, The Diamond Ring Cluster (NGC 6531) |
| Open cluster M21 was discovered by Charles Messier, who cataloged it on June 5, 1764. M21 is a cluster which shows a fairly strong concentration toward its center. It gets its name from a distinctive semi-circle of stars with a singular bright "jewel" star. M21 has a membership of maybe 57 stars, 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. As it is situated close to the Trifid Nebula (M20) 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. |
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10"
F6.3 Schmidt Cassegrain Telescope, (1600mm F.L.) Modified Canon
300D camera with Baader IR-UV filter and coma corrector, placed at the scopes prime focus |