M 22

This was probably the first globular cluster discovered, by A. Ihle in 1665. It is a very remarkable object; lying 10,000 light years distant, its 24' angular diameter correspond to a linear of about 65 light years. It is visible to the naked eye for observers at not too northern latitudes, as it is brighter than the Hercules globular cluster M13 and outshined only by the two bright southern globulars (not in Mesier's catalog), Omega Centauri (NGC 5139) and 47 Tucanae (NGC 104) - this is the ranking of the four brightest in the sky.

M22 is one of the nearer globular clusters at 10,000 light years. While Shapley and Pease counted 70,000 stars in this great stellar swarm, only the relatively small number of 32 variables has been identified, half of them already known to Bailey in 1902, among them a long-period Mira variable which is probably not a member. The brightest stars are about mag 11. The stars are spread over a region roughly 200 light years in diameter, and receding from us at 144 km/sec.

This cluster is notable because it contains a weak planetary nebula, discovered by the infrared satellite IRAS. Images of this planetary nebula can be retrieved here.

For the observer, it is of interest that M22 is less than 1 degree of the ecliptic, so that conjuctions with planets are frequently conspicuous.

Right ascension	18 : 33.3 (hours : minutes)
Declination	-23 : 58 (degrees : minutes)
Distance	10.0 (light-years*10^3)
Visual magnitude	5.9