Messier 18

M18 is a relatively dim open star cluster located in the constellation Sagittarius. The cluster lies at an approximate distance of 4,900 light-years, from Earth and has an apparent magnitude of 7.5. Its designation in the New General Catalogue is NGC 6613. It is also known as C 1817-171, MWSC 2892. Messier 18 may form a binary star cluster with the nearby NGC 6618, embedded within Messier 17 (the Omega Nebula). Scientists have speculated that the proximity of the two clusters may suggest that they had formed together. Messier 18 is one of Charles Messier’s original discoveries. Messier included the cluster in his catalogue of comet-like objects on June 3, 1764, and described it as a “cluster of small stars, a little below above nebula, No. 17, surrounded by slight nebulosity. We must give Messier great credit considering his observations were performed long before the nature of open clusters and stellar movement were understood. 

The small smattering of bright blue stars upper left of centre in this huge 615-megapixel ESO image is the perfect cosmic laboratory in which to study the life and death of stars. Known as Messier 18 this open star cluster contains stars that formed together from the same massive cloud of gas and dust. This image was captured by the OmegaCAM camera attached to the VLT Survey Telescope (VST) located at ESO’s Paranal Observatory in Chile.

The cluster is estimated to be about 32 million years old, which makes it quite young. 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. Most of the stars in M18 are quite small, but the cluster is also home to several bright blue, yellow and orange stars. It also contains a small amount of dust and nebulosity.

Messier 18 is located between two other prominent Messier objects, Messier 17 (the Omega) and Messier 24 (the Sagittarius Star Cloud) at right ascension 18h 19.9m, declination -17°08′. It is about 10 times more distant than the better known Messier 45 (the Pleiades), located in Taurus. M18 is probably the hardest of the 11 Messier objects located in this area of the sky to find.

The cluster lies against the backdrop of the galactic plane, which makes it even more challenging to locate and for astronomers to say with certainty how many stars it contains. The easiest way to find M18 is to first locate Messier 17 and then aim slightly south or to find Messier 24 and aim about two degrees north. Messier 18 can also be located using Kaus Borealis, Lambda Sagittarii, the top star of the Teapot asterism in Sagittarius. The cluster lies 8.5 degrees north and slightly west of the star. The best time of year to observe Messier 18 is in the summer when Sagittarius appears above the southern horizon in the evening. Credits: Messier Objects, NASA, Universe Today, Wikipedia.