M22 is a 6th magnitude globular star cluster in Sagittarius. It ranks among the top six globular star clusters in the sky, and is the third most luminous globular cluster in the Milky Way. It is one of the nearest of all of the globular clusters, if not the closest, located less than 10,000 light years away. The total population of the cluster is thought to be at least half a million stars. The cluster is more than 50 light years in diameter. M22 is approaching us at a rate of about 89 miles per second. The brightest stars shine at magnitude 10.7. A four-inch scope will easily show the brightest stars in the cluster as individual pin points of light. An 8-inch to 10-inch telescope at 250x will resolve stars right in the core of the cluster.
Internal homogeneity of metallicity has been found in many globular clusters. This metallicity homogeneity within globular clusters is remarkable because it sets them apart from other stellar systems. This suggests that star formation in globular clusters occurred in a different fashion to star formation in other stellar systems, for example, dwarf spheroidal galaxies. However, there have been claims that M22 is one of the few globular clusters that does exhibit metallicity variations. The existance of such variations in M22 remains controversial. Differential reddening across the cluster complicates observations, and makes it difficult to confirm such metallicity variations.
M22 was probably discovered by a little known German astronomer Abraham Ihle in 1665. William Herschel is thought to be the first observer to recognize that M22 was a dense cluster of stars. Charles Messier included it in his first list published in 1769.
This is an RGB composite CCD image taken with an SBIG ST-8E CCD using a Takahashi FS-128 refractor. This image was taken from an observing site near Prescott, Arizona.
M22 (NGC 6656)
RA: 18h 36m 26.2s Dec: -23d 53' 54"
July 2, 2000
Image by Sid Leach
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Description of equipment used to acquire images.
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