Messier 22, NGC 6656

The globular cluster Messier 22 in Sagittarius, also known as NGC 6656, is one of the brightest in the sky, and among the first to be discovered.

Discovery and Observation

The first recorded sighting of M 22 is usually credited to the obscure German astronomer Abraham Ihle in 1665, but it may have been spotted previously by Hevelius. This globular was included in Edmond Halley's list of 6 nebulous objects published 1715. It was also observed by De Cheseaux, Le Gentil, and by Nicholas Louis de Lacaille, who included it in his catalog of southern objects as Lacaille I.12.

Charles Messier, who cataloged M 22 on June 5, 1764, stated that it is also included in John Bevis' English Atlas. M 22 was one of the first globular clusters to be studied by Harlow Shapley in 1930. Shapley counted 70,000 stars in this great stellar swarm.

Messier 22 is considered to be one of the finest globular clusters in the sky. With a visual magnitude of 5.1, it can be seen with the naked eye on a clear night, and is the third-brightest in the sky (after Omega Centauri and 47 Tucanae).

M 22 is a distinctly elliptical globular cluster with a diameter of 32', slightly larger than the full Moon. It contains approximately 500,000 stars; the brightest are about mag 11. The relatively loose 5' core is a mass of faint, well-resolved stars. Stars are in distributed over its halo along a multitude of arcs and streams, and in several clumps.

M 22 lies less than 1 degree off the ecliptic, so conjunctions with planets are frequent and conspicuous. The globular's setting on the northern fringe of the Great Sagittarius Star Cloud adds to the richness of the scene.

Physical Properties

At a distance of 10,400 light years, M 22 is one of the nearest globulars. At this distance, its angular diameter corresponds to a linear dimension of about 97 light years. It has an absolute magnitude of -8.5, and luminosity of 210,000 suns - both average for a globular cluster. M 22 is receding from us at about 149 km/sec.

M 22 is one of only four globulars known to contain a planetary nebula (the others are M 15, NGC 6441 and Palomar 6). This planetary nebula was discovered by the infrared satellite IRAS, and cataloged as IRAS 18333-2357 or GJJC 1.

Recent Hubble Space Telescope investigations of M 22 have discovered a number of planet-sized objects which appear to float through this globular cluster. These objects were discovered by micro-lensing effects, i.e. the bending of light of background member stars of the cluster, and may have masses of only 80 times that of Earth.