Messier 22

Template:Short description Template:Infobox globular cluster

Messier 22 or M22, also known as NGC 6656 or the Great Sagittarius Cluster, is an elliptical globular cluster of stars in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars visible in the night sky. The brightest stars are 11th magnitude, with hundreds of stars bright enough to resolve with an 8" telescope.<ref>Burnham's Celestial Handbook vol.3, Robert Burnham (ed.), 1978, Dover (publisher), at pages 1594–1599; this statement applies in a dark, clear, night sky.</ref> It is just south of the sun's position in mid-December, and northeast of Lambda Sagittarii (Kaus Borealis), the northernmost star of the "Teapot" asterism.

M22 was one of the first globulars to be discovered, in 1665Template:Efn by Abraham Ihle<ref>Kirch, Gottfried (1682) Annus II. Ephemeridum Motuum Coelestium Ad Annum Aerae Christianae M. DC. LXXXII. … [Second year. Ephemerides of the celestial motions for the year of the Christian era 1682.] Leipzig, (Germany): Heirs of Friedrich Lanckisch. (in Latin) 54 pages. The pages of this book are not numbered. However, in the Appendix, section III. Stella nebulosa prope pedem borealem Ganymedis observata, Lipsia, die 1. Sept. 1681. (III. Nebula near the northern foot of Ganymede observed, Leipzig, 1. September 1681.), first paragraph, Kirch enumerated recently discovered nebulae: " […] & tertia in Sagittaris, quam Dn. Joh. Abrah. Ihle Anno 1665. deprehendit; […] " ([…] and the third [nebula] in Sagittarius, which Mr. Johann Abraham Ihle discovered in the year 1665; […]) Downloadable at: Digitale Sammlungen der Universitäts- und Landesbibliothek Sachsen-Anhalt (Digital collections of the university- and state library of Sachsen-Anhalt)</ref><ref name="Monacoetal2004" /> and it was included in Charles Messier's catalog of comet-like objects in 1764.Template:Efn It was one of the first globular clusters to be carefully studied – first by Harlow Shapley in 1930. He placed within it roughly 70,000 stars and found it had a dense core.<ref name="Shapley1930"> Template:Cite journal</ref> Then Halton Arp and William G. Melbourne continued studies in 1959.<ref name="Arpetal1959"> Template:Cite journal</ref> Due to the large color spread of its red giant branch (RGB) sequence, akin to that in Omega Centauri, it became the object of intense scrutiny starting in 1977 with James E. Hesser et al.<ref name="Monacoetal2004" /><ref name="Hesseretal1977"> Template:Cite journal</ref>

M22 is one of the nearer globular clusters to Earth – at about 10,600 light-years away. It spans 32′ on the sky which means its diameter (width across) is 99 ± 9 light-years, given its estimated distance. 32 variable stars have been recorded in M22. It is in front of part of the galactic bulge and is therefore useful for its microlensing effect on those background stars.<ref name="Gaudi2002" />

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; even so, it is the brightest globular cluster visible from mid-northern latitudes (such as Japan, Korea, Europe and most of North America).<ref name="ivansetal2004"> Template:Cite journal</ref> From those latitudes due to its declination of nearly 24° south of the (celestial) equator, its daily path is low in the southern sky. It thus appears less impressive to people in the temperate northern hemisphere than counterparts fairly near in angle (best viewed in the Summer night sky) such as M13 and M5.

M22 is one of only four globulars of our galaxyTemplate:Efn known to contain a planetary nebula (an expanding, glowing gas swell from a massive star, often a red giant). It was an object first noted of interest using the IRAS satellite by Fred Gillett and his associates in 1986, as a pointlike light sourceTemplate:Efn<ref name="Gillettetal1986"> Template:Cite journal</ref> and its nature was found in 1989 by Gillett et al.<ref name="Gillettetal1989"> Template:Cite journal</ref> The planetary nebula's central star is a blue star. The nebula, designated GJJC1, is likely about only 6,000 years old.<ref name="Monacoetal2004" />

Two black holes of between 10 and 20 solar masses (Template:Solar mass) each were unearthed with the Very Large Array radio telescope in New Mexico and corroborated by the Chandra X-ray telescope in 2012.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These imply that gravitational ejection of black holes from clusters is not as efficient as was previously thought, and leads to estimates of a total 5 to 100 black holes within M22.<ref name="strader2012">Template:Cite journal</ref> Interactions between stars and black holes could explain the unusually large core of the cluster.<ref name="strader2012" />