M39 - Open Cluster in Cygnus
Messier 39 (M39, NGC 7092) is a very large but very loose open cluster, situated some 9 degrees east and a bit north of Deneb (Alpha Cygni).
Its distance is only about 800 light years, and it is of intermediate age (estimates between 230 and 300 million years). 30 stars are proven members and contained in a volume of about 7 light years diameter. Its apparent visual brightness of 4.6 magnitudes (e.g., Sky Catalogue 2000.0, Uranometria 2000) corresponds to an absolute magnitude of -2.5, or an intrinsic luminosity of 830 suns. Kenneth Glyn Jones gives its apparent visual brightness as 5.2 mag only, while Don Machholz has estimated it at mag 5.4, in agreement with estimates quoted by Mallas/Kreimer, who also mention D.F. Gray's estimate of a total visual brightness of 6.0 magnitudes.