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 8.04pc, a tidal radius of 7.3pc, and is centrally concentrated with core radius of 0.012pc. It has a Trumpler class of . The cluster is 33million years old with an estimated mass of . 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 HD168352, is a B-type giant star with a class of B2III and a visual magnitude of 8.65.

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

See also

• List of Messier objects

References

External links

• Messier 18, SEDS Messier pages