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{{Infobox open cluster
| name = Messier 18
| image =
| caption = Star cluster Messier 18 (upper left) and its surroundings, taken by the OmegaCAM.[1]
| epoch = J2000.0
| class = II,3,p,n
| ra = {{RA|18|19|58.0}}[2]
| dec = {{Dec|–17|06|06}}[2]
| dist_ly = {{convert|1.296|kpc|kly|order=flip|abbr=on|lk=on}}[4]
| dist_pc =
| appmag_v = 7.5[5]
| size_v = {{Val|9.8|ul=arcminute}}[6]
| mass_kg =
| mass_msol = {{Val|186|161|86}}[7]
| radius_ly = {{Convert|8.04|pc|ly|order=flip|disp=out}}[7]
| v_hb =
| age = {{Val|33|e=6|u=years}}[9]
| notes =
| names = Messier 18, NGC 6613, OCl 40
}}Messier 18 or M18, also designated NGC 6613, 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.[11] From the perspective of Earth, M18 is situated between the Omega Nebula (M17) and the Sagittarius Star Cloud (M24).[5]This is a sparse[13] cluster with a linear diameter of 8.04 pc,[7] a tidal radius of 7.3 pc,[9] and is centrally concentrated with core radius of 0.012 pc.[16] It has a Trumpler class of {{nowrap|II 3 p}}.[6] The cluster is 33[9] million years old with an estimated mass of {{Val|188|ul=solar mass}}.[7] It has one definite Be star and 29 B-type stars in total.[13] There are three supergiant stars, all of class A or earlier.[21] 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.[6] Messier 18 is 1,296 pc[4] from the Earth and 6,830 pc from the Galactic Center.[7] 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.[2] GalleryReferences1. ^1 {{citation | title=Imaging the Messier Objects Remotely from Your Laptop | series=The Patrick Moore Practical Astronomy Series | first1=Len | last1=Adam | year=2018 | postscript=. | publisher=Springer | isbn=978-3319653853 | pages=117–120 | url=https://books.google.com/books?id=7nNUDwAAQBAJ&pg=PA117 | bibcode=2018imor.book.....A }}
2. ^1 2 {{citation | first1=Hartmut | last1=Frommert | first2=Christine | last2=Kronberg | title=Messier 18 | date=August 21, 2007 | publisher=Students for the Exploration and Development of Space (SEDS) | url=http://www.messier.seds.org/m/m018.html | accessdate=2018-11-11 | postscript=. }}
3. ^1 2 3 {{citation | title=The open clusters NGC 6613 (M 18) and NGC 6716 | last1=Lindoff | first1=U. | journal=Astronomy and Astrophysics | volume=15 | pages=439–449 | date=December 1971 | bibcode=1971A&A....15..439L | postscript=. }}
4. ^1 {{citation | title=Stellar Lab in Sagittarius | url=http://www.eso.org/public/news/eso1628/ | website=www.eso.org | postscript=. | accessdate=11 August 2016 }}
5. ^1 2 {{Citation | last1=Kharchenko | first1=N. V. | last2=Piskunov | first2=A. E. | last3=Röser | first3=S. | last4=Schilbach | first4=E. | last5=Scholz | first5=R.-D. | display-authors=1 | title=Astrophysical parameters of Galactic open clusters | journal=Astronomy and Astrophysics | volume=438 | issue=3 | pages=1163–1173 | year=2005 | bibcode=2005A&A...438.1163K | arxiv=astro-ph/0501674 | doi=10.1051/0004-6361:20042523 | postscript=. }}
6. ^1 2 3 {{Citation | display-authors=1 | last1=Wu | first1 = Zhen-Yu | last2=Zhou | first2 = Xu | last3=Ma | first3 = Jun | last4=Du | first4 = Cui-Hua | title=The orbits of open clusters in the Galaxy | journal=Monthly Notices of the Royal Astronomical Society | volume=399 | issue=4 | pages=2146–2164 | date=November 2009 | arxiv=0909.3737 | bibcode=2009MNRAS.399.2146W | doi=10.1111/j.1365-2966.2009.15416.x | postscript=. }}
7. ^1 2 3 {{citation | display-authors=1 | last1=Piskunov | first1=A. E. | last2=Schilbach | first2=E. | last3=Kharchenko | first3=N. V. | last4=Röser | first4=S. | last5=Scholz | first5=R.-D. | title=Tidal radii and masses of open clusters | journal=Astronomy and Astrophysics | postscript=. | volume=477 | issue=1 | date=January 2008 | pages=165–172 | doi=10.1051/0004-6361:20078525 | bibcode=2008A&A...477..165P }}
8. ^1 2 3 4 5 {{citation | display-authors=1 | title=Study of open clusters within 1.8 kpc and understanding the Galactic structure | last1=Joshi | first1=Y. C. | last2=Dambis | first2=A. K. | last3=Pandey | first3=A. K. | last4=Joshi | first4=S. | journal=Astronomy & Astrophysics | postscript=. | volume=593 | id=A116 | pages=13 | date=October 2016 | doi=10.1051/0004-6361/201628944 | bibcode=2016A&A...593A.116J | arxiv=1606.06425 }}
9. ^1 {{citation | last1=Santos-Silva | first1=T. | last2=Gregorio-Hetem | first2=J. | title=Characterisation of young stellar clusters | journal=Astronomy & Astrophysics | date=November 6, 2012 | volume=547 | pages=A107 | doi=10.1051/0004-6361/201219695 | postscript=. | arxiv=1209.1585 | bibcode=2012A&A...547A.107S }}
10. ^1 2 {{Citation | last1=McSwain | first1=M. Virginia | last2=Gies | first2=Douglas R. | title=The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters | journal=The Astrophysical Journal Supplement Series | date=November 2005 | volume=161 | issue=1 | pages=118–146 | arxiv=astro-ph/0505032 | bibcode=2005ApJS..161..118M | doi=10.1086/432757 | postscript=. }}
11. ^1 {{citation | last1=Eggenberger | first1=P. | last2=Meynet | first2=G. | last3=Maeder | first3=A. | display-authors=1 | title=The blue to red supergiant ratio in young clusters at various metallicities | journal=Astronomy and Astrophysics | volume=386 | issue=2 | pages=576–582 | date=May 2002 | doi=10.1051/0004-6361:20020262 | postscript=. | bibcode=2002A&A...386..576E | arxiv=astro-ph/0202478 }}
}}External links{{commons|Messier 18|Messier 18}}{{Messier objects}}{{Sky|18|19|54|-|17|07|60|4900}} 6 : Open clusters|Carina–Sagittarius Arm|Sagittarius (constellation)|Messier objects|NGC objects|Astronomical objects discovered in 1764 |