M39 is a very loose naked eye open cluster of at least 30 young stars, located in a dense portion of the Milky Way in Cygnus. The cluster is positioned in a beautiful rich region of the sky, with many other open clusters, thousands of stars, regions of dark nebulae, wispy emission and reflection nebulae visible nearby.
The discovery of M39 is an interesting story. It's often credited as one of Charles Messier's original discoveries, which he made on October 24, 1764. However, the discovery has also been credited to French astronomer Guillaume Le Gentil in 1750 and even suggested that Aristotle observed it as far back as 325 BC.
The cluster is relatively easy to find. It's located in the northeastern part of Cygnus, not far from the constellation's brightest star, Deneb (α Cyg - mag. +1.25). This blue-white luminous supergiant forms the faintest corner of the well-known and much observed "Summer Triangle". The other two stars that make up the triangle are Vega (α Lyr - mag. +0.03) and Altair (α Aqr - mag. +0.77). Once you have located Deneb, move approx. 4.5 degrees east-southeast to ξ Cyg (mag. +3.72) and then hop another 5.5 degrees northeast to ρ Cyg (mag. +3.98). Positioned 3 degrees directly north of this star is M39. The cluster is best seen from northern latitudes during July, August and September.
Of all of the open clusters in Cygnus, M39 is one of the brightest. With an apparent mag. of +5.5, it's faintly visible to the naked eye and an easy binocular object. M39 is also one of the constellations largest clusters and is spread over an apparent diameter of 31 arc minutes, which corresponds to about the same size as that of the full Moon.
Through 7x50 or 10x50 binoculars, M39 appears as a large, loose, misty triangular shaped object that easily stands out from the background stars. The cluster is probably best seen and at its most impressive, when viewed through a small or medium sized telescope at low powers. For example, a 80mm (3.1 inch) refractor reveals many bright stars in an almost "W" type form, with the brightest member being of mag. +6.8. Dotted in and around the main shape are numerous fainter stars. Despite its looseness, the cluster is impressive when observed at lower powers. Larger scopes reveal at least 30 stars in total, although at higher magnifications, it overspills the eyepiece field of view and some appeal is lost.
M39 is best viewed from northern temperate latitudes and the tropics during the warm summer months, where it appears high in the sky. The cluster is only 800 light-years from Earth and is estimated to be 250 million years old. What is perhaps surprising is that M39, is one of only two Messier objects located in the rich constellation of Cygnus, the other being M29.
M39 Data Table
|Object Type||Open cluster|
|RA (J2000)||21h 31m 48s|
|DEC (J2000)||+48d 26m 55s|
|Apparent Size (arc mins)||31 x 31|
|Age (years)||250 Million|
|Number of Stars||>30|
|Other Name||Collinder 438|