# Messier Index/M22

Messier 22

Core of Messier 22
Observation data (w:J2000 epoch)
Class VII
Constellation Sagittarius
Right ascension 18h 36m 24.21s[1]
Declination -23° 54′ 12.2″[1]
Distance 10.6 ± 1.0 kly (3.3 ± 0.3 kpc[2])
Apparent magnitude (V) +6.17[1]
Apparent dimensions (V) 32′.0
Physical characteristics
Mass kg (105 to 106 M${\displaystyle {\odot }}$)
VHB 14.2
Estimated age 12 Gyr[4]
Notable features One of four globulars known
to contain a planetary nebula.
Other designations NGC 6656, GCl 99[1]

Messier 22 (also known as M22 or NGC 6656) is an elliptical w:globular cluster in the w:constellation Sagittarius near the w:Galactic bulge region. It is one of the brightest globulars that is visible in the night sky.

w:Globular Cluster Messier 22 - Courtesy Hunter Wilson

## History

M22 was one of the first globulars to be discovered in w:1665 by w:Abraham Ihle[2] and it was included in Charles Messier's catalog of w:comet-like objects on June 5, w:1764.

It was one of the first globular clusters to be carefully studied first by w:Harlow Shapley in w:1930. He discovered roughly 70,000 stars and found it had a dense core. [5] Then w:Halton Arp and w:William G. Melbourne continued studies in w:1959.[6] Because of the large color spread of its w:red giant branch (RGB) sequence, which is similar to that observed in w:Omega Centauri, it became the object of intense scrutiny starting in w:1977 with w:James E. Hesser et al.[7][2]

## Characteristics

M22 is one of the nearer globular clusters to w:Earth at a distance of about 10,600 w:light-years away. It spans 32' on the sky which translates to a spatial diameter of 99 ± 9 light-years. 32 w:variable stars have been recorded in M22. It is projected in front of the galactic bulge and is therefore useful for its w:microlensing effect on the background stars in the bulge.[4]

Despite its relative proximity to us, this w:metal-poor cluster's light is limited by w:dust extinction, giving it an apparent magnitude of 5.5 making it the brightest globular cluster in the norther hemisphere.[8]

### Planetary Nebula

M22 is very unusual in that it is one of only four globulars (the others being M15, w:NGC 6441 and w:Palomar 6) that are known to contain a w:planetary nebula (PN). It was discovered using the w:IRAS satellite by w:Fred Gillett et al.,in 1986 as a pointlike source (w:IRAS 18333-2357)[9] and subsequently identified as a PN in 1989 by Gillett et al.[10] The PN's central star is a blue star. The PN (designated w:GJJC1) is estimated to be a mere ~6,000 years old.[2]