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Messier 22
Messier 22
Messier 22
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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☉
Radius50 ± 5 ly[6]
VHB14.2
Metallicity[Fe/H] = –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]
See also: Globular cluster, List of globular clusters

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.[10] 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 1665[a] by Abraham Ihle[11][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.[12] Then Halton Arp and William G. Melbourne continued studies in 1959.[13] 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][14]

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; even so, it is the brightest globular cluster visible from mid-northern latitudes (such as Japan, Korea, Europe and most of North America).[15] 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][16] and its nature was found in 1989 by Gillett et al.[17] 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.[18] 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.[19] Interactions between stars and black holes could explain the unusually large core of the cluster.[19]

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  • location of M22 in Sagittarius
    location of M22 in Sagittarius
  • Wide field view of M22
    Wide field view of M22

See also

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Footnotes and references

[edit]
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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Messier 22

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from Grokipedia
Messier 22 (M22), also known as NGC 6656, is a —a tightly bound, spherical collection of stars—located in the constellation Sagittarius near the Milky Way's .
It lies approximately 10,000 light-years from , making it one of the closest globular clusters to our solar system.
Discovered on August 26, 1665, by German astronomer Abraham Ihle, it was cataloged by on June 5, 1764, as the 22nd entry in his famous comet-hunting list.
This ancient stellar system dates back 12 to 13 billion years, representing one of the earliest formations in the , and spans a of about 70 light-years while appearing roughly the size of the in the sky with an of 5.1, visible to the under clear, dark conditions. Messier 22 contains hundreds of thousands of stars, primarily low-mass, red giants and main-sequence dwarfs, densely packed in its core where stellar interactions are frequent.
It has a total mass equivalent to about 500,000 Suns and exhibits a concentration class of VII on the Shapley-Sawyer scale, indicating an intermediate concentration with a bright center.
The cluster orbits the roughly every 200 million years, influenced by the Milky Way's . Among its notable features, Messier 22 hosts one of only four known planetary nebulae within a , a rare ionized gas shell ejected from an .
Hubble Space Telescope observations have also revealed two stellar-mass black holes within the cluster, challenging earlier assumptions that such dense environments would eject them through dynamical interactions.
Additionally, it contains six planet-sized objects roaming freely without orbiting stars, likely remnants of disrupted planetary systems in this harsh stellar environment.
These discoveries highlight Messier 22's in studying the of ancient stellar populations and the dynamics of compact systems.

Discovery and Historical Context

Initial Discovery

Messier 22 was first discovered by German astronomer Johann Abraham Ihle on August 26, 1665, while observing Saturn. It was independently observed and cataloged by French astronomer on the night of June 5–6, 1764, during one of his routine sweeps of the sky in search of comets. At the time, Messier mistook the object for a due to the limitations of his 3.5-foot , which failed to resolve its stellar nature. In his original catalog entry, Messier described Messier 22 as a round nebula with uniform light, approximately 6 arcminutes in diameter. He noted its position as being slightly below the ecliptic, between the head and bow of Sagittarius, nearly in line with the stars Lambda (λ) and Mu (μ) Sagittarii, specifically 1° 33' east-southeast of Lambda Sagittarii and preceding it by 9 minutes 30 seconds of time. This precise positioning was crucial for future observers, as Messier aimed to document fixed celestial objects that could be confused with transient comets. Messier's work on such objects stemmed from his primary role as a comet hunter, a pursuit that defined his career and led to the creation of the Messier Catalog. Beginning in the 1750s, he systematically recorded diffuse, comet-like apparitions to avoid mistaking them for new comets, a task prompted by his successful recovery of Halley's Comet in 1758. The catalog, first published in 1771 with 45 entries and later expanded to 110, became a foundational resource for astronomers, though Messier himself viewed it as a practical aid rather than a comprehensive survey. Subsequently, on July 4, 1783, resolved the "nebula" into a cluster of stars, confirming its true nature as a .

Cataloging and Early Observations

Messier 22 was formally cataloged by French astronomer in his seminal 1781 catalogue of nebulae and star clusters, where it received the designation M22 as the twenty-second entry, based on its initial observation in 1764. This inclusion highlighted its appearance as a comet-like nebulous object, prompting further scrutiny amid the era's debates over the nature of such deep-sky phenomena. In the 1780s, British astronomer conducted pivotal observations of M22 using his advanced reflecting telescopes, marking the first resolution of the object into a distinct composed of numerous faint stars rather than a diffuse . On July 4, 1783, Herschel examined it with his 20-foot at a magnification of 200 and noted it as fully resolved into stars, describing the cluster's profound depth and estimating its structure extended to nearly the 344th order of minuteness. These findings, published in his sweeps of the heavens, distinguished M22 from true nebulae and contributed to the emerging understanding of as stellar aggregates. The object's cataloging continued to evolve in the through systematic surveys. In 1888, Danish-Irish astronomer John Louis Emil Dreyer incorporated M22 into the (NGC) as NGC 6656, drawing on prior positional data and descriptions to provide a standardized entry: a very bright, very large, round, very rich, and very much compressed with stars ranging from 11th to 15th magnitude. Earlier, William's son, , refined its position and characteristics during his extensive southern sky surveys, including observations from the between 1834 and 1838 using an 18-inch speculum . In his 1847 publication of these results, John described M22 as a "magnificent " approximately 7 arcminutes in diameter, gradually brighter toward the center without a distinct nucleus, with stars in two distinct magnitude classes—11th to 12th (some reddish) and 15th to 16th—resembling layered shells, and he emphasized its utility as a test for resolving power. These detailed accounts, accompanied by micrometric measures, enhanced the precision of its recorded and appearance for future astronomers.

Location and Observability

Coordinates and Distance

Messier 22, a in the constellation Sagittarius, lies near the in the . Its equatorial coordinates in the J2000 epoch are right ascension 18h 36m 23.94s and −23° 54′ 17.1″. In galactic coordinates, it is positioned at l = 9.89° and b = −7.55°. The distance to Messier 22 has been estimated at 3.303 ± 0.037 kpc (approximately 10,800 light-years), derived from a combination of Early Data Release 3 (EDR3) parallax measurements, (HST) data, and literature values. This determination incorporates kinematic modeling of the cluster's velocity dispersion and corrections for systematic parallax biases in EDR3.

Visibility from Earth

Messier 22 has an of 5.1, making it visible to the under dark, clear skies away from , though it appears as a faint, fuzzy patch requiring good conditions for unaided detection. For better resolution and to discern its globular structure, or a small are recommended, as they reveal a concentrated core surrounded by a halo of stars. From the , optimal visibility occurs in and , when the cluster rises in the evening and reaches its highest point near midnight, positioned low in the southern sky. In the , it is observable year-round, appearing higher overhead during winter months. It becomes circumpolar from high southern latitudes south of approximately 66°S. Its of 18h 36m and of −23° 54′ aid in locating it near prominent stars in Sagittarius, such as those in the asterism. Observation is challenged by its low southern declination, which keeps it near the horizon for northern observers, limiting viewing time and detail due to atmospheric . Additionally, interstellar dust and gas along the line of sight toward the in Sagittarius cause , reducing the cluster's apparent brightness by and absorbing light.

Physical Properties

Size, Mass, and Structure

Messier 22 exhibits an angular diameter of 32 arcminutes as observed from Earth. At its distance of approximately 10,600 light-years, this angular extent corresponds to a physical diameter of about 97 light-years. The cluster's total mass is estimated at approximately 416,000 solar masses. This mass is distributed with a projected half-light radius of 3.3 parsecs, marking the radius within which half of the cluster's light is contained. Key structural parameters include a core radius of 1.2 parsecs, defining the densely packed central region. The cluster has a concentration class VII, characteristic of an intermediate concentration with a moderately dense core and loose outer halo. These features are well-fitted by a King model, which describes the cluster's density profile through a balance of self-gravity and tidal influences from the .

Age, Metallicity, and Dynamics

Messier 22 is estimated to have an age of approximately 12 billion years, determined through isochrone fitting to Advanced Camera for Surveys photometry of its main-sequence turnoff and zero-age morphology. This method aligns the observed color-magnitude diagram with theoretical models, accounting for the cluster's and abundance, yielding a precise age consistent with other metal-poor globular clusters formed in the early . The cluster exhibits iron-poor with an overall [Fe/H] ≈ -1.8, reflecting a bimodal distribution indicative of multiple stellar populations, where metal-poor stars reach [Fe/H] ≈ -1.9 and metal-rich stars [Fe/H] ≈ -1.7, as derived from high-resolution of stars. This low iron content is typical of ancient globular clusters, with enhancements in alpha elements such as [Mg/Fe] ≈ +0.38 and [Ca/Fe] ≈ +0.22, attributed to enrichment from core-collapse supernovae in the early that preferentially produced these elements relative to iron-peak . Dynamically, Messier 22 displays a central line-of-sight velocity dispersion of approximately 7.8 km/s, indicative of its internal gravitational equilibrium and mass distribution, measured via surveys of member stars. Orbiting the , the cluster follows an eccentric path with eccentricity ≈ 0.5 and an apogalacticon of about 9.5 kpc, derived from DR2 proper motions and space integration in a potential model. These parameters position Messier 22 in the inner halo, subject to moderate tidal influences that shape its long-term evolution.

Stellar Content

Variable Stars and Pulsars

Messier 22 hosts a rich population of variable stars, with over 140 known examples cataloged in the field of the cluster, the majority of which are confirmed or likely members. The dominant type is RR Lyrae stars, particularly the ab-type (fundamental-mode pulsators), which exhibit pulsation periods typically ranging from 0.5 to 0.6 days. These stars serve as standard candles for distance calibration to the cluster, leveraging their well-established period-luminosity-metallicity relationship derived from broadband photometry. The cluster also contains 4 millisecond pulsars, discovered through sensitive radio surveys conducted primarily with the and Parkes radio telescope starting in the 1990s. These pulsars have short spin periods of approximately 10–30 ms, characteristic of recycled neutron stars that have gained angular momentum via accretion from companion stars in binary systems. A representative example is PSR J1836–2354A, a binary millisecond pulsar with a spin period of 20.7 ms, identified via multiband observations that confirm its cluster membership. Binary systems are prevalent among the variable stars in Messier 22, including several eclipsing binaries (EW-type) and other close pairs detected through photometric monitoring. These binaries contribute to evolutionary models of the cluster by revealing the binary fraction, processes, and dynamical interactions in the dense environment, helping to explain the observed stellar populations and anomalies.

Blue Stragglers and Other Anomalies

Messier 22 contains a population of approximately 90 stars (BSSs), identified through high-resolution imaging within a radius of 625 arcseconds from the cluster center. These stars are characterized by their positions in the color-magnitude diagram, appearing brighter and bluer than the main-sequence turnoff, which suggests they have masses around 1.06 M⊙ and ages ranging from 0.5 to 7 Gyr. BSSs in this cluster are thought to form primarily through the coalescence of binary companions or in binary systems, processes that rejuvenate the stars and make them appear younger than the cluster's ancient population. The spatial distribution of BSSs in Messier 22 exhibits a bimodal radial profile, with a peak density in (r ≤ 104 arcseconds) and another rising trend in the outer halo before flattening, indicating ongoing dynamical interactions such as that segregate these more massive objects toward the center. This distribution aligns with models of intermediate dynamical age for the cluster, where BSSs behave as a distinct family of stars influenced by relaxation processes. Mass segregation is a prominent dynamical feature in Messier 22, with heavier stars—including red giants and BSSs—concentrated in the dense core, while lighter main-sequence stars are more prevalent in the extended halo. Hubble Space Telescope observations of the spatially resolved mass function reveal a significant enhancement of stars with masses 0.5–0.8 M⊙ in the inner regions compared to the outskirts, up to five core radii. This segregation arises from two-body relaxation, as confirmed by numerical modeling of the cluster's evolution, which shows the mass function becoming steeper with distance from the center. In addition to dynamical anomalies, Messier 22 displays chemical signatures of multiple stellar populations, including sodium-oxygen anticorrelations observed via high-resolution of stars. These anticorrelations manifest in both the metal-poor ([Fe/H] ≈ -1.82) and metal-rich ([Fe/H] ≈ -1.67) subpopulations, with sodium abundances ranging from [Na/Fe] ≈ -0.3 to +0.6 and oxygen from [O/Fe] ≈ -0.1 to +0.6, though the metal-rich group shows systematically higher sodium at fixed oxygen levels. abundance variations are also present, particularly in the metal-rich subpopulation, with enhancements up to ΔY ≈ 0.07 inferred from photometric analysis of the bump magnitudes, while the metal-poor group shows no significant helium spread. These features, spanning five distinct subpopulations based on CN-CH photometry, highlight the cluster's complex formation involving from earlier generations of stars.

Scientific Significance

Key Research Findings

In the 1990s, observations of Messier 22 (M22) marked a major breakthrough by resolving the extreme core crowding that had previously hindered ground-based studies, enabling high-resolution photometry of individual stars in the dense central region. These early HST images, taken with the Wide Field Planetary Camera 2, revealed detailed stellar fields, including a significant number of variable stars such as RR Lyrae types, and facilitated the construction of deep luminosity functions to probe the cluster's low-mass stellar content and evolutionary stage. A pivotal study in 2009 using high-resolution of stars in M22 uncovered evidence for multiple stellar populations, distinguished by variations in light elements and heavy metal abundances, which indicated an extended episode of spanning several hundred million years. This finding positioned M22 as a key example of globular clusters with internal chemical inhomogeneities, prompting further investigations into pollution from stars or early supernovae. The 2022 release of Data Release 3 provided unprecedented astrometric precision for M22, refining proper motions and membership probabilities for tens of thousands of stars across the cluster and its surrounding field. This dataset has allowed for accurate determination of the cluster's systemic motion, internal velocity dispersion, and the identification of likely members down to faint magnitudes, significantly improving models of dynamical evolution and mass loss.

Astrophysical Implications

Messier 22, with an estimated age of approximately 12 billion years, acts as a preserved relic of the early , illuminating the galaxy's formative stages through its stellar composition. Its intermediate , averaging [Fe/H] ≈ -1.7, combined with an intrinsic spread of about 0.2–0.3 dex, reflects the initial chemical enrichment from the first generations of massive stars, offering a snapshot of nucleosynthetic processes in the primordial . This spread, unusual among globular clusters, mirrors conditions in the 's earliest massive star clusters and supports models of heterogeneous enrichment during galaxy assembly. The cluster's dense core facilitates dynamical interactions that shape its , providing a natural laboratory for studying binary formation and in high-density environments. Recycled pulsars discovered in Messier 22, such as the two identified through observations, demonstrate the efficiency of stellar encounters in recycling neutron stars via from companions, validating theoretical predictions for binary hardening and retention in globular clusters. Similarly, the population of blue stragglers in the cluster, whose radial distribution indicates a bimodal structure consistent with dynamical segregation, informs estimates of stellar merger rates; these stars, formed likely through binary mergers or collisions, highlight how core density drives anomalous and segregation over billions of years. Observations of RR Lyrae variables in Messier 22 contribute to calibrating the by refining the period-luminosity-metallicity relation for these standard candles. With over 30 known RR Lyrae stars exhibiting calibrated broadband photometry, the cluster's well-determined distance of about 10,000 light-years anchors absolute magnitudes that extend to Population II systems in nearby galaxies, such as the , thereby reducing uncertainties in extragalactic distance measurements.

References

  1. https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-22/
  2. https://www.sci.news/astronomy/hubble-space-telescope-globular-cluster-messier22-02670.html
  3. https://www.nrao.edu/pr/2012/m22/
  4. https://esahubble.org/images/potw1514a/
  5. http://www.messier.seds.org/m/m022.html
  6. http://www.messier.seds.org/xtra/history/m-cat71.html
  7. https://www.astronomy.com/science/the-obsessive-comet-hunter/
  8. https://archive.org/details/messier-catalogue-des-nebuleuses-et-des-amas-detoiles...
  9. https://www.messier-objects.com/messier-catalogue/
  10. https://gardenastronomer.com/2025/07/27/messier-22-ngc-6656-a-cosmic-heavyweight-just-off-the-teapot/
  11. https://archive.org/details/newgeneralcatalo00dreyrich
  12. http://www.docdb.net/show_object.php?id=ngc_6656
  13. https://ui.adsabs.harvard.edu/abs/2021MNRAS.505.5957B/abstract
  14. https://simbad.cds.unistra.fr/simbad/sim-id?Ident=Messier+22
  15. https://arxiv.org/abs/2105.09526
  16. https://simbad.cds.unistra.fr/simbad/sim-basic?Ident=Messier+22
  17. https://physics.mcmaster.ca/~harris/mwgc.dat
  18. https://people.smp.uq.edu.au/HolgerBaumgardt/globular/
  19. https://arxiv.org/abs/1804.08359
  20. https://iopscience.iop.org/article/10.1088/0004-637X/775/2/134
  21. https://arxiv.org/abs/0909.5265
  22. https://academic.oup.com/mnras/article/482/4/5138/5173087
  23. https://ui.adsabs.harvard.edu/abs/2013AJ....146..119K/abstract
  24. https://www3.mpifr-bonn.mpg.de/staff/pfreire/GCpsr.html
  25. https://ui.adsabs.harvard.edu/abs/2019MNRAS.486.3992A/abstract
  26. https://www.astro.utoronto.ca/~cclement/cat/C1833m239
  27. https://academic.oup.com/mnras/article/482/4/4874/5212534
  28. https://arxiv.org/abs/astro-ph/0207267
  29. https://www.aanda.org/articles/aa/full_html/2011/08/aa16546-11/aa16546-11.html
  30. https://arxiv.org/abs/2001.00679
  31. https://iopscience.iop.org/article/10.1088/0004-637X/705/2/1481
  32. https://academic.oup.com/mnras/article/526/3/4107/7276628
  33. https://doi.org/10.3847/2041-8213/ad60c8
  34. https://arxiv.org/abs/1101.1467
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