M22 - Globular Cluster in Sagittarius
Discovered by Abraham Ihle in 1665.
Messier 22 (M22, NGC 6656) is one of the brightest and remarkable clusters in the sky, and in paticular of those observable from mid-northern latitudes. It was the first of these objects to be discovered.
M22 is a very remarkable object. At 10,400 light years, it is one of the nearer globular clusters. At this distance, its 32' angular diameter, sligtly larger than that of the Full Moon, corresponds to a linear of about 97 light years; visually, it is still about 17'. 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 Messier's catalog), Omega Centauri (NGC 5139) and 47 Tucanae (NGC 104) - this is the ranking of the four brightest in the sky.
This cluster is notable because it contains a weak planetary nebula, discovered by the infrared satellite IRAS and cataloged as IRAS 18333-2357 or GJJC 1. This planetary was the second discovered in a globular cluster after Pease 1 in M15, and one of only four known planetary nebula in Milky Way globular clusters.
Recent Hubble Space Telescope investigations of M22 have led to the discovery of a considerable number of planet-sized objects which appear to float through this globular cluster; these objects may have masses of only 80 times that of Earth, and were discovered by so-called micro lensing effects, i.e. bending of light of background member stars of the cluster.
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.