M22 is a magnificent globular cluster located in the constellation of Sagittarius and one of the best objects of its type in the night sky. With a magnitude of +5.1, the cluster is visible to the naked eye under dark skies and also the brightest globular in the Messier catalogue. Only the two great southern globulars, Omega Centauri (NGC 5139) and 47 Tucanae (NGC 104) are more brilliant. Both of these are positioned far too south in the sky to have been seen by Messier. With an apparent diameter extending 32 arc minutes, M22 covers more sky than the Full Moon.
The main reason why M22 appears so large and bright is because it's close at only 10,400 light-years. It was probably the first globular to have been discovered - by Abraham Ihle in 1665 - although it has been suggested that Hevelius may have seen it earlier. M22 was included in Edmund Halley's list of 6 objects published in 1715 and then catalogued by Charles Messier on June 5, 1764. M22 is an easy object to locate as it's positioned 2.5 degrees northeast of the top star of the teapot asterism of Sagittarius, Kaus Borealis (λ Sag - mag. +2.8).
The globular is best seen from southern and equatorial regions during the months of June, July and August. From northern temperate locations it never rises particularly high above the southern horizon.
M22 is a cluster for all types of optical instruments. Through 7x50 or 10x50 binoculars, it appears as a diffuse halo of light that's obviously non-stellar but no discernible details are visible. A small 80mm (3.1-inch) telescope displays a mottled fuzzy ball that hints at resolution. At high magnifications a 150mm (6-inch) scope will easily resolve many outer stars, the brightest of which shine at mag. +11. The cluster's core appears slightly brighter with subtle changes visible across the surface. M22 is noticeably elliptical in shape and when viewed through large scopes, such as a 300mm (12-inch) instrument, it looks spectacular with thousands of stars visible across the complete surface. The core is not particularly dense. Due to its southerly declination, M22 never rises high in the sky for Northern Hemisphere observers and therefore appears less impressive than other globulars such as M13 and M5.
It's estimated that M22 contains about 80,000 stars spread across a spatial diameter of 86 light-years. Of these, 32 variable stars have been identified. The age of the cluster is 12 billion years and it's unusual in that it's one of only four globulars known to contain a planetary nebula (along with M15, NGC 6441 and Palomar 6).
M22 Data Table
|Object Type||Globular cluster|
|RA (J2000)||18h 36m 24s|
|DEC (J2000)||-23d 54m 12s|
|Apparent Size (arc mins)||32 x 32|
|Age (years)||12 Billion|
|Number of Stars||80,000|
|Notable Feature||One of four globulars known to contain a planetary nebula|