Messier 18
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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.<ref name=Adam2018/> From the perspective of Earth, M18 is situated between the Omega Nebula (M17) and the Small Sagittarius Star Cloud (M24).<ref name=Frommert2007/>
This is a sparse<ref name=McSwain2005/> cluster with a linear diameter of 8.04 pc,<ref name=Joshi2016/> a tidal radius of 7.3 pc,<ref name=Piskunov2008/> and is centrally concentrated with core radius of 0.012 pc.<ref name=Silva2012/> It has a Trumpler class of Template:Nowrap.<ref name=Lindoff1971/> The cluster is 33<ref name=Piskunov2008/> million years old with an estimated mass of Template:Val.<ref name=Joshi2016/> It has one definite Be star and 29 B-type stars in total.<ref name=McSwain2005/> There are three supergiant stars, all of class A or earlier.<ref name=Eggenberger2002/> 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.<ref name=Lindoff1971/>
Messier 18 is 1,296 kpc<ref name=kharchenko2005/> from the Earth and 6,830 kpc from the Galactic Center.<ref name=Joshi2016/> 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.<ref name=Wu2009/>
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. <ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
GalleryEdit
- Messier18.jpg
M18 photo from the 2MASS project
- M18map.png
Map showing location of M18 (Roberto Mura)
- The star cluster Messier 18 and its surroundings.jpg
Star cluster Messier 18 (upper left) and its surroundings, taken by the OmegaCAM<ref name=eso2016/>
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