Key facts
- Right ascension
- 18 19 58.0
- Declination
- –17 ° 06 ′ 06 ″
- Distance
- 4.23 kly (1.296 kpc )
- Apparent dimensions v
- 9.8 ′
- 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
via Wikipedia infobox
Article
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.