# Messier 22

Messier 22
Core of Messier 22
Observation data (J2000 epoch)
ClassVII[1]
ConstellationSagittarius
Right ascension18h 36m 23.94s[2]
Declination–23° 54′ 17.1″[2]
Distance10.6 ± 1.0 kly (3 ± 0.3 kpc)[3]
Apparent magnitude (V)+5.1[4]
Apparent dimensions (V)32 arcmins
Physical characteristics
Mass2.9×105[5] M
VHB14.2
Metallicity${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.49[7] dex
Estimated age12 Gyr[8]
Notable featuresOne of four globulars known to contain a planetary nebula.
Other designationsNGC 6656, GCl 99[9]

Messier 22 or M22, also known as NGC 6656, 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.[10] It is just south of the sun's position in mid-December, and northwest of Lambda Sagittarii (Kaus Borealis), the northernmost star of the "Teapot" asterism.

M22 was one of the first globulars to be discovered, in 1665[a] by Abraham Ihle[3] and it was included in Charles Messier's catalog of comet-like objects in 1764.[b] 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.[11] Then Halton Arp and William G. Melbourne continued studies in 1959.[12] 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.[3][13]

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.[8]

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 making it the brightest globular cluster visible from mid-northern latitudes (such as Japan, Korea, Europe and most of North America).[14] 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 galaxy[c] 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 source[d][15] and its nature was found in 1989 by Gillett et al.[16] The planetary nebula's central star is a blue star. The nebula, designated GJJC1, is likely about only 6,000 years old.[3]

Two black holes of between 10 and 20 solar masses (M) each were unearthed with the Very Large Array radio telescope in New Mexico and corroborated by the Chandra X-ray telescope in 2012.[17] 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.[18] Interactions between stars and black holes could explain the unusually large core of the cluster.[18]

## Footnotes and references

### Footnotes

1. ^ on August 26
2. ^ on June 5
3. ^ the others being M15, NGC 6441 and Palomar 6
4. ^ IRAS 18333-2357

### References

1. ^ Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters". Harvard College Observatory Bulletin. 849 (849): 11–14. Bibcode:1927BHarO.849...11S.
2. ^ a b Goldsbury, Ryan; et al. (December 2010). "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters". The Astronomical Journal. 140 (6): 1830–1837. arXiv:1008.2755. Bibcode:2010AJ....140.1830G. doi:10.1088/0004-6256/140/6/1830.
3. ^ a b c d 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. arXiv:astro-ph/0401392. Bibcode:2004MNRAS.349.1278M. doi:10.1111/j.1365-2966.2004.07599.x.
4. ^ "Galactic Globular Clusters Database (M22)". Archived from the original on 2009-06-03.
5. ^ Marks, Michael; Kroupa, Pavel (August 2010). "Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way". Monthly Notices of the Royal Astronomical Society. 406 (3): 2000–2012. arXiv:1004.2255. Bibcode:2010MNRAS.406.2000M. doi:10.1111/j.1365-2966.2010.16813.x. Mass is from MPD on Table 1.
6. ^ From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 50 ly
7. ^ Forbes, Duncan A.; Bridges, Terry (May 2010). "Accreted versus in situ Milky Way globular clusters". Monthly Notices of the Royal Astronomical Society. 404 (3): 1203–1214. arXiv:1001.4289. Bibcode:2010MNRAS.404.1203F. doi:10.1111/j.1365-2966.2010.16373.x.
8. ^ a b Gaudi, B. Scott (2002). "Interpreting the M22 Spike Events". Astrophysical Journal. 566 (1): 452–462. arXiv:astro-ph/0108301. Bibcode:2002ApJ...566..452G. doi:10.1086/338041.
9. ^ "M 22". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-15.
10. ^ 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.
11. ^ Shapley, Harlow (1930). "The Mass-Spectrum Relation for Giant Stars in the Globular Cluster Messier 22". Harvard College Observatory Bulletin. 874: 4–9. Bibcode:1930BHarO.874....4S.
12. ^ Arp, H. C.; Melbourne, W. G. (1959). "Color-magnitude diagram for the globular cluster M22". Astronomical Journal. 64: 28. Bibcode:1959AJ.....64...28A. doi:10.1086/107848.
13. ^ 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". Astrophysical Journal Supplement Series. 33: 471. Bibcode:1977ApJS...33..471H. doi:10.1086/190438.
14. ^ Ivans, I.; Sneden, C.; Wallerstein, G.; Kraft, R. P.; Norris, J. E.; Fulbright, J. P. & Gonzalez, G. (2004). "On the Question of a Metallicity Spread in Globular Cluster M22 (NGC 6656)". Memorie della Società Astronomica Italiana. 75: 286. Bibcode:2004MmSAI..75..286I.
15. ^ Gillett, F. C.; Neugebauer, G.; Emerson, J. P.; Rice, W. L. (1986). "IRAS 18333-2357 – an unusual source in M22". Astrophysical Journal. 300: 722–728. Bibcode:1986ApJ...300..722G. doi:10.1086/163846.
16. ^ Cohen, J. G.; Gillett, F. C. (1989). "The peculiar planetary nebula in M22" (PDF). Astrophysical Journal. 346: 803–807. Bibcode:1989ApJ...346..803C. doi:10.1086/168061.
17. ^ Gary, Stuart (4 October 2012). "Astronomers discover twin black holes". ABC Science News. Australian Broadcasting Corporation. Retrieved 5 October 2012.
18. ^ a b Strader, J.; Chomiuk, L.; MacCarone, T. J.; Miller-Jones, J. C. A.; Seth, A. C. (2012). "Two stellar-mass black holes in the globular cluster M22". Nature. 490 (7418): 71–73. arXiv:1210.0901. Bibcode:2012Natur.490...71S. doi:10.1038/nature11490. PMID 23038466.