Messier 21 or M21 (also designated NGC 6531) is an open cluster in the constellation Sagittarius. It has an apparent visual magnitude of 6.5 and its angular diameter is 13 arc-minutes. M21 lies at an estimated distance of 4250 light years. The Equinox 2000 coordinates are RA= 18h 04.6m, Dec= -22° 30´ which makes M21 best seen during the summer. The Messier Summer Star Chart shows the position of all Messier objects visible during that season.

The image above shows the uncropped view of M21 through the Takahashi E-180 Astrograph (North is to right). A 3x enlargement of this image appears to the right.

This open cluster was discovered by Messier in 1764. According to Kharchenko et al. (2005), the distance of M21 is 3930 light years and its diameter is 20 light years. It contains 105 stars and its estimated age is 4-8 million years.

For more information, see the Messier Catalog as well as specific entries for M21 in Wikipedia and SEDS.

Messier's Description of M21

June 5, 1764
`Star cluster, near the preceding [M20]: The nearest neighboring known star to these two clusters is 11 Sagittarii, 7 mag, according to Flamsteed. The stars of both these clusters are of 8-9 magnitude, enveloped in nebulosity.'

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