M39, Cygnus, SV78S f3.8, MX7C, 8min

Info courtesy of SEDS

Discovered by Charles Messier in 1764.

M39 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.

M39's brightest star is of magnitude 6.83 visually, and of spectral type A0. All stars were found to be main sequence stars in the Color-Magnitude Diagram (CMD), or Hertzsprung-Russell Diagram (HRD), with the brightest stars apparently just before the point of evolution toward the red giant phase. The Sky Catalogue 2000.0 gives an estimated age of 270 million years for this cluster - this is between the two determinations cited by Kenneth Glyn Jones of 300 million years by Lohmann and 230 million years by Van Hoerner. M39 is approaching us at 28 km/s; its proper motion was given as 0.024" per year in the direction of position angle 222 deg (by E.G. Ebbighausen 1940, according to Burnham).