Messier 22

Lore & Background

M22 was one of the first globular clusters to be discovered, in 1665 by Abraham Ihle, and was included in Charles Messier's catalog of comet-like objects in 1764. It was one of the first globular clusters to be carefully studied, first by Harlow Shapley in 1930, who placed roughly 70,000 stars within it and found it had a dense core. Halton Arp and William G. Melbourne continued studies in 1959. Due to the large color spread of its red giant branch sequence, akin to that in Omega Centauri, it became the object of intense scrutiny starting in 1977 with James E. Hesser et al.

M22 is in front of part of the galactic bulge and is useful for its microlensing effect on background stars. Despite its relative proximity, its light is limited by dust extinction, giving it an apparent magnitude of 5.5; even so, it is the brightest globular cluster visible from mid-northern latitudes. From those latitudes, due to its declination of nearly 24° south of the celestial equator, its daily path is low in the southern sky, appearing less impressive than counterparts such as M13 and M5.

M22 is one of only four globulars of our galaxy known to contain a planetary nebula, designated GJJC1, likely about 6,000 years old. Its central star is a blue star. The nebula was first noted as a pointlike light source using the IRAS satellite by Fred Gillett and his associates in 1986, and its nature was found in 1989 by Gillett et al. Two black holes of between 10 and 20 solar masses each were unearthed with the Very Large Array radio telescope and corroborated by the Chandra X-ray telescope in 2012, implying that gravitational ejection of black holes from clusters is not as efficient as previously thought, leading to estimates of 5 to 100 black holes within M22.

Reader's Guide

M22 holds significance as one of the nearest globular clusters to Earth, at about 10,600 light-years, and as one of the brightest visible from mid-northern latitudes. Its discovery in 1665 by Abraham Ihle marks it as one of the first globulars known, and its inclusion in Messier's 1764 catalog underscores its historical importance in early deep-sky observation. The cluster's dense core and large color spread in its red giant branch sequence, studied by Harlow Shapley in 1930 and later by Halton Arp and William G. Melbourne in 1959, made it a focus of intense scrutiny from 1977 onward. Its location in front of the galactic bulge allows it to be used for microlensing studies of background stars. The discovery of a planetary nebula within M22 in 1986–1989, one of only four known in galactic globulars, adds to its uniqueness. The 2012 detection of two black holes of 10–20 solar masses each, using the Very Large Array and Chandra X-ray telescope, challenged previous assumptions about black hole ejection efficiency and suggests a total of 5 to 100 black holes within the cluster, potentially explaining its unusually large core through interactions between stars and black holes.

Did You Know?

• M22 is one of only four globular clusters in our galaxy known to contain a planetary nebula.
• The cluster is about 10,600 light-years away and spans 99 ± 9 light-years in diameter.
• It was discovered in 1665 by Abraham Ihle, making it one of the first globulars known.