M22 - Sagittarius Cluster

Messier 22 or M22 (also designated NGC 6656) is a globular cluster in the constellation Sagittarius. It has an apparent visual magnitude of 5.1 and its angular diameter is 24 arc-minutes. M22 lies at an estimated distance of 10,400 light years. The Equinox 2000 coordinates are RA= 18h 36.4m, Dec= -23° 54´ which makes M22 best seen during the summer. The Messier Summer Star Chart shows the position of all Messier objects visible during that season. As one of the more famous objects in the Messier Catalog, it is commonly known as the Sagittarius Cluster.

The image above shows the uncropped view of M22 through the Takahashi E-180 Astrograph (North is to right). A 3x enlargement of this image appears to the right.

In spite of its inclusion in the Messier Catalog, this globular cluster was actually discovered by Ihle in 1665. According to Recio-Blanco et al.(2005), the distance of M22 is 10,440 light years and its diameter is 100 light years. This makes M22 one of the nearest globular clusters. It is the brightest globular cluster in the Messier Catalog and third brightest in the entire sky. Only globular clusters Omega Centauri and 47 Tucanae are brifgter. The estimated mass of M22 is 500,000 solar masses and it contains 78 variable stars.

For more information, see the Messier Catalog as well as specific entries for M22 in Wikipedia and SEDS.

Messier's Description of M22

June 5, 1764
`Nebula, below the ecliptic, between the head and the bow of Sagittarius, near a star of 7th magnitude, 25 Sagittarii, according to Flamsteed, this nebula is round, it doesn't contain any star, and one can see it very well in an simple refractor of 3.5 feet; the star Lambda Sagittarii served for determination [of its position]. Abraham Ihle, a German, discovered it in 1665, while observing Saturn. M. Le Gentil observed it in 1747, and he made an engraving of it. Memoirs of the Academy, year 1759, page 470. Reviewed March 22, 1781; it is reported in the English Atlas Coelestis.' (diam. 6')

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