M 39 is a rather unassuming open cluster about 101 thousand lights years away and estimated to be about 9 light years across. The vast majority of brighter stars are Type A, Dwarf stars, something similar to Sirius, in their main sequence stage (burning hydrogen at their cores), whilst the brightest star is a Type B with a magnitude of about 6. This understanding has lead to an age estimate for M 39 of about 240 to 280 million years; a long time, but as things go in the universe, M 39 is almost a baby, especially if you start comparing it with globular clustsers.
M 39 is one of the closest and smallest Messier clusters which helps explain its rather loose and angular visual appearance and even in a small aperture viewfinder, it is obvious that it is composed of stars, making it a wonder why Messier placed M 39 in his list of pseudo-comets in 1764. Other astronomers have been equally less-impressed. Herschel noted that it was 'coarsely scattered', Rosse of 'little concentration' and Smyth observed that it was a 'little splashy'.
It is said that M 39 can be seen with the naked eye from a sufficiently dark site, appearing as a brighter spot within the rich star field of the Milky Way but in the city it is invisible and with no clear, nearby stars to guide you, it can be quite a challenge to find. Nevertheless, contrary to Herschel et al, I think it is well worth the effort.
In the viewfinder, M 39 looks like a triangle filled-in and surrounded by a relatively rich field of sparklers, making it a perfect observation for owners of low f/ratio scopes or low magnification eyepieces of about 32mm. I have no idea what stars should be included in the cluster, nor, let it be said, whether the sketch I have included is M 39 or one of the other rich clusters found in and around Cygnus. I have read that about 20% of the stars brighter than 10th magnitude do not even belong to the cluster, so probably what I have included in the sketch will be a tad misleading. If anyone could guide me here, I would be most grateful.
A useful tip for this given cluster is to sit with her for a while and let the different star colours become evermore apparent. I have tried to include the differing colours seen, but obviously this will be quite a subjective experience.