Monday, January 26, 2009

Messier 22

Messier 22, or M22, is an elliptical globular cluster in the constellation Sagittarius near the Galactic bulge region. It is one of the brightest globulars that is visible in the night sky.

M22 is one of the nearer globular clusters to Earth at a distance of about 10,600 light-years away. It spans a spatial diameter of 99 ± 9 light-years. 32 variable stars have been recorded in M22. It is projected in front of the galactic bulge and is therefore useful for its microlensing effect on the background stars in the bulge. Despite its relative proximity to us, this metal-poor cluster's light is limited by dust extinction, giving it an apparent magnitude of 5.5 making it the brightest globular cluster in the norther hemisphere.

Messier 22 was one of the first globulars to be discovered in 1665 by Abraham Ihle and it was included in Charles Messier's catalog of comet-like objects on June 5, 1764. It was one of the first globular clusters to be carefully studied in 1930 first by Harlow Shapley, who discovered roughly 70,000 stars and found it had a dense core. Then Halton Arp and William G. Melbourne continued to study the M22 in 1959.