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Messier 18
Open cluster Messier 18 in Sagittarius
Observation data (J2000.0 epoch)
Right ascension	18 19 58.0
Declination	–17° 06′ 06″
Distance	4.23 kly (1.296 kpc)
Apparent magnitude (V)	7.5
Apparent dimensions (V)	9.8′
Physical characteristics
Mass	186+161 −86 M☉
Radius	26.2 light-years
Estimated age	33×10 years
Other designations	Messier 18, NGC 6613, Cr 376, OCl 40
Associations
See also: Open cluster, List of open clusters

Messier 18 or M18, also designated NGC 6613 and sometimes known as the Black Swan Cluster, is an open cluster of stars in the constellation Sagittarius. It was discovered by Charles Messier in 1764 and included in his list of comet-like objects. From the perspective of Earth, M18 is situated between the Omega Nebula (M17) and the Small Sagittarius Star Cloud (M24).

This is a sparse cluster with a linear diameter of 8.04 pc, a tidal radius of 7.3 pc, and is centrally concentrated with core radius of 0.012 pc. It has a Trumpler class of II 3 p. The cluster is 33 million years old with an estimated mass of 188 M_☉. It has one definite Be star and 29 B-type stars in total. There are three supergiant stars, all of class A or earlier. The brightest component (lucida), designated HD 168352, is a B-type giant star with a class of B2 III and a visual magnitude of 8.65.

Messier 18 is 1,296 kpc from the Earth and 6,830 kpc from the Galactic Center. It is orbiting the Milky Way core with a period of 186.5 million years and an eccentricity of 0.02. This carries it to as close as 6.5 kpc to, and as far as 6.8 kpc from the galactic core. It passes vertically through the galactic plane once every 27.4 million years, ranging no more than 80 pc above or below.

As of January 2022, Messier 18 is one of the few remaining objects within the Messier Catalog to not have been photographed by the Hubble Space Telescope.

Gallery

• M18 photo from the 2MASS project
• Map showing location of M18 (Roberto Mura)
• Star cluster Messier 18 (upper left) and its surroundings, taken by the OmegaCAM

See also

• List of Messier objects

References

1. ^ Wu, Zhen-Yu; et al. (November 2009), "The orbits of open clusters in the Galaxy", Monthly Notices of the Royal Astronomical Society, 399 (4): 2146–2164, arXiv:0909.3737, Bibcode:2009MNRAS.399.2146W, doi:10.1111/j.1365-2966.2009.15416.x, S2CID 6066790.
2. ^ Kharchenko, N. V.; et al. (2005), "Astrophysical parameters of Galactic open clusters", Astronomy and Astrophysics, 438 (3): 1163–1173, arXiv:astro-ph/0501674, Bibcode:2005A&A...438.1163K, doi:10.1051/0004-6361:20042523, S2CID 9079873.
3. ^ Frommert, Hartmut; Kronberg, Christine (August 21, 2007), Messier 18, Students for the Exploration and Development of Space (SEDS), retrieved 2018-11-11.
4. ^ Lindoff, U. (December 1971), "The open clusters NGC 6613 (M 18) and NGC 6716", Astronomy and Astrophysics, 15: 439–449, Bibcode:1971A&A....15..439L.
5. ^ Joshi, Y. C.; et al. (October 2016), "Study of open clusters within 1.8 kpc and understanding the Galactic structure", Astronomy & Astrophysics, 593: 13, arXiv:1606.06425, Bibcode:2016A&A...593A.116J, doi:10.1051/0004-6361/201628944, S2CID 118610932, A116.
6. ^ Piskunov, A. E.; et al. (January 2008), "Tidal radii and masses of open clusters", Astronomy and Astrophysics, 477 (1): 165–172, Bibcode:2008A&A...477..165P, doi:10.1051/0004-6361:20078525.
7. "M 18". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved November 11, 2018.
8. Adam, Len (2018), Imaging the Messier Objects Remotely from Your Laptop, The Patrick Moore Practical Astronomy Series, Springer, pp. 117–120, Bibcode:2018imor.book.....A, ISBN 978-3319653853.
9. ^ McSwain, M. Virginia; Gies, Douglas R. (November 2005), "The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters", The Astrophysical Journal Supplement Series, 161 (1): 118–146, arXiv:astro-ph/0505032, Bibcode:2005ApJS..161..118M, doi:10.1086/432757, S2CID 14498164.
10. Santos-Silva, T.; Gregorio-Hetem, J. (November 6, 2012), "Characterisation of young stellar clusters", Astronomy & Astrophysics, 547: A107, arXiv:1209.1585, Bibcode:2012A&A...547A.107S, doi:10.1051/0004-6361/201219695, S2CID 119213441.
11. Eggenberger, P.; et al. (May 2002), "The blue to red supergiant ratio in young clusters at various metallicities", Astronomy and Astrophysics, 386 (2): 576–582, arXiv:astro-ph/0202478, Bibcode:2002A&A...386..576E, doi:10.1051/0004-6361:20020262, S2CID 14373322.
12. "Explore - the Night Sky | Hubble's Messier Catalog". 28 August 2017.
13. "Stellar Lab in Sagittarius", www.eso.org, retrieved 11 August 2016.

External links

• Messier 18, SEDS Messier pages
• Messier 18 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

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• Outer space

v t e Messier objects
List	M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21 M22 M23 M24 M25 M26 M27 M28 M29 M30 M31 M32 M33 M34 M35 M36 M37 M38 M39 M40 M41 M42 M43 M44 M45 M46 M47 M48 M49 M50 M51 M52 M53 M54 M55 M56 M57 M58 M59 M60 M61 M62 M63 M64 M65 M66 M67 M68 M69 M70 M71 M72 M73 M74 M75 M76 M77 M78 M79 M80 M81 M82 M83 M84 M85 M86 M87 M88 M89 M90 M91 M92 M93 M94 M95 M96 M97 M98 M99 M100 M101 M102 M103 Added M104 M105 M106 M107 M108 M109 M110
See also	Caldwell catalogue Catalogue of Nebulae and Clusters of Stars Herschel 400 Catalogue Index Catalogue New General Catalogue Revised New General Catalogue
Category Commons Portal

• Open clusters
• Carina–Sagittarius Arm
• Sagittarius (constellation)
• Messier objects
• NGC objects
• Astronomical objects discovered in 1764
• Discoveries by Charles Messier