Messier 39 is a large but loosely-structured open cluster, situated about 9° northeast of Deneb in the constellation Cygnus.
The discovery of M 39 is often credited to Le Gentil in 1750; Messier added it to his catalogue in 1764. However, this cluster may have been noted by Aristotle around 325 BC as a "cometary-appearing" object.
Messier 39 is a 5th-magnitude cluster, best observed with low powers because of its large angular size: 32', bigger than the Moon. Under good conditions, it can just be glimpsed with the unaided eye. It is well-resolved in binoculars, where its shape stands out: an equilateral triangle with 30' sides and bright stars at each corner. Two stars of 7th magnitude mark the SE and SW corners, with a 9th magnitude star at the northern corner. About 25 fainter stars lie within, many grouped in pairs. Though loose, the cluster's bright stars stand out well against the rich Milky Way background of faint stars.
The cluster only is 800 light years away, and contains 30 proven members in a volume about 7.5 light years in diameter. Its an absolute magnitude of -2.5 corresponds to a luminosity of just 830 suns. M 39's brightest star is of spectral type A0. All of its members are main sequence stars, the brightest apparently just before the point of evolution into the red giant phase. The cluster has an estimated age of 270 million years, and is approaching us at 28 km/sec.