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.
Core of Messier 22
|Observation data (w:J2000 epoch)|
|Right ascension||18h 36m 24.21s|
|Declination||-23° 54′ 12.2″|
|Distance||10.6 ± 1.0 kly (3.3 ± 0.3 kpc)|
|Apparent magnitude (V)||+6.17|
|Apparent dimensions (V)||32′.0|
|Mass||kg (105 to 106 M)|
|Radius||50 ± 5 ly|
|Estimated age||12 Gyr|
|Notable features||One of four globulars known|
to contain a planetary nebula.
|Other designations||NGC 6656, GCl 99|
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.  Then w:Halton Arp and w:William G. Melbourne continued studies in w:1959. 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.
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.
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.
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) and subsequently identified as a PN in 1989 by Gillett et al. The PN's central star is a blue star. The PN (designated w:GJJC1) is estimated to be a mere ~6,000 years old.
- "SIMBAD Astronomical Database". Results for NGC 6656. http://simbad.u-strasbg.fr/Simbad. Retrieved 2006-11-15.
- Monaco, L.; Pancino, E.; Ferraro, F. R.; Bellazzini, M. (2004). "Wide-field photometry of the Galactic globular cluster M22". Monthly Notices of the Royal Astronomical Society 349 (4): 1278–1290. doi:10.1111/j.1365-2966.2004.07599.x. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004MNRAS.349.1278M.
- distance × sin( diameter_angle / 2 ) = 50 ly. radius
- Gaudi, B. Scott (2002). "Interpreting the M22 Spike Events". The Astrophysical Journal 566 (1): 452–462. doi:10.1086/338041. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002ApJ...566..452G.
- Shapley, Harlow (1930). "The Mass-Spectrum Relation for Giant Stars in the Globular Cluster Messier 22". Harvard College Observatory Bulletin 874: 4–9. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1930BHarO.874....4S.
- Arp, H. C.; Melbourne, W. G. (1959). "Color-magnitude diagram for the globular cluster M22". The Astronomical Journal 64: 28. doi:10.1086/107848. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1959AJ.....64...28A.
- Hesser, J. E.; Hartwick, F. D. A.; McClure, R. D. (1977). "Cyanogen strengths and ultraviolet excesses of evolved stars in 17 globular clusters from DDO photometry". Astrophys. J., Suppl. Ser. 33: 471. doi:10.1086/190438. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1977ApJS...33..471H.
- I. Ivans, C. Sneden, G. Wallerstein, R. P. Kraft, J. E. Norris, J. P. Fulbright, and G. Gonzalez (2004). "On the Question of a Metallicity Spread in Globular Cluster M22 (NGC 6656)". Societa Astronomica Italiana 75: 286. http://sait.oat.ts.astro.it/MSAIt750204/PDF/2004MmSAI..75..286I.pdf#search=%22%22NGC%206656%22%20red%20giant%22.
- Gillett, F. C.; Neugebauer, G.; Emerson, J. P.; Rice, W. L. (w:January 15 w:1986). "IRAS 18333-2357 - an unusual source in M22". Astrophysical Journal, Part 1 (ISSN 0004-637X), w:NASA-supported research. 300: 722–728. doi:10.1086/163846. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1986ApJ...300..722G.
- Cohen, J. G.; Gillett, F. C. (w:November 15 w:1989). "The peculiar planetary nebula in M22". Astrophysical Journal, Part 1 (ISSN 0004-637X), Research supported by w:California Institute of Technology 346: 803–807. doi:10.1086/168061. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1989ApJ...346..803C.