|Right Ascension||18 : 36.4 (h:m)
|Declination||-23 : 54 (deg:m)
|Visual Brightness||5.1 (mag)
|Apparent Dimension||24.0 (arc min)
Discovered by Abraham Ihle in 1665.
This was probably the first globular cluster discovered, by Abraham Ihle in 1665. According to Kenneth Glyn Jones, it is supposed that Hevelius may have seen it even earlier. This globular was included in Halley's list of 6 objects published 1715, and observed by De Cheseaux (his No. 17) and Le Gentil as well as by Abbe Nicholas Louis de la Caille, who included it in his catalog of southern objects as Lacaille I.12. Messier states that it is also in Bevis' English Atlas.
M22 is a very remarkable object; lying about 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 Messier'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 10,400 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; we have images of this planetary nebula from George Jacoby's planetary nebulae sampler.
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.
Last Modification: 9 Dec 1999, 22:58 MET