|Rektascenzija||21 : 32.2 (u:m)
|Deklinacija||+48 : 26 (sto:m)
|Razdalja||0.825 (*1000 sv.l.)
|Vizual. magnituda||5.2 (mag)
|Zorni kot||32.0 (loc min)
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.
Woldemar Götz gives the Trumpler type of this cluster as III,2,m: Detached from the surrounding field but not concentrated toward its center, the stars are in a moderate range in brightness, and it is moderately rich (50--100 members). The Sky Catalog 2000 has III,2,p (i.e., "poor", less than 50 member stars).
While Kenneth Glyn Jones counts M39 as one of Messier's original discoveries, Burnham claims that its discovery "is often credited to Le Gentil in 1750, but P. Doig (1925) quotes a statement made by J.E. Gore that this cluster was noted by Aristotle as a cometary appearing object about 325 BC." Kenneth Glyn Jones rates the identification of Le Gentil's observation (done by Bigourdan) as "extremely doubtful".
This cluster is best observed with lowest powers.