Degree of difficulty 2 (of 5)

Minimum aperture Naked eye

Designation NGC 7092

Type Open cluster

Class III2m

Distance 1010 ly (K2005) 1010 ly (Hipparcos, 1999)

Size 9 ly

Constellation Cygnus

R.A. 21h 32.2min

Decl. +48° 27′

Magnitude 4.6

Surface brightness –

Apparent diameter 30′

Discoverer Messier, 1764

History Some sources state that Aristotle had already described this region of the night sky as nebulous in 325 BC. However, without an exact position and with the rich Milky Way all around M 39, we may seriously doubt that he referred specifically to this cluster. Nor was M 39 discovered, as often stated, by le Gentil. In his own words, he wrote about his observation: “At the tip of the tail of Cygnus. It appears to be of a nature different from all nebulae observed so far and to the Milky Way, across which it lies, making a right angle with it. It is a large cloud, wider on one end than the other, the smaller end facing southeast. The cloud is about 6° away from the tail of the Swan, it appears opaque and very dark; it can be seen without a telescope.” This may be the first mentioning of the longish dark nebula directly beside M 39, the “Dark Cigar” B 168, one of the most obvious dark nebulae of the Milky Way. Consequently, it was Charles Messier who finally discovered M 39 on the 24th of October 1764. He wrote: “Cluster of stars near the tail of the Swan, 1° diameter.” Later observers weren't very impressed with Messier's find: John Herschel called it “very coarsely scattered,” Smyth noted “rather splashy field of stars” and Lord Rosse observed “little concentration.”

Astrophysics According to McNamara and Sanders, the galactic open cluster M 39 has only 30 real member stars in the brightness range of 6.8 to 10.0 magnitude, which stand in front of a rich Milky Way back-ground.