Discovered by Charles Messier in 1764.

M18 is best observed in small telescopes, which show over a dozen of fairly bright stars (the Sky Catalog 2000 lists it with 20 members). It is about 0.2 degrees in diameter, thus appears loose and poor, its Trumpler type is given as II,3,p,n by all sources. Its distance is about 4,900 light years, according to Kenneth Glyn Jones and Burnham, but the sources disagree: Mallas gives 6,000, the Sky Catalog 2000 has 3,900 light years. Adopting our value of 4,900 light years, its linear diameter should be about 17 light years.

As the hottest stars in M18 are of spectral type B3, this cluster is quite young; its age is estimated at 32 million years. As seen in the color image in our collection of more images of M18, this cluster contains bright blue as well as bright yellow or orange stars.

M18 is situated between the Omega Nebula M17 and the star cloud M24. This region around M17, M18 and M24 was photographed e.g. with the UK Schmidt Telescope.

Open cluster M18 is one of the original discoveries of Charles Messier, who cataloged it on June 3, 1764.