NGC 6939: Open Cluster in Cepheus
July 2013 :
Our September “Sky Object of the Month” takes us to the southwest corner of Cepheus and the 8th magnitude open cluster NGC 6939. Discovered by William Herschel in 1798, it contains some 80-100 stars occupying an area 7 arcminutes across.
My first encounter with NGC 6939 was with a 3-inch f/10 reflector at 30X back in September of 1979. I entered the following description in my logbook. “Nice; intriguing object. Surprisingly easy – a dull glow w/slight condensation; almost stellar.” Earlier this summer, I revisited NGC 6939, this time with a 10-inch f/5 reflector and a magnification of 78X. As might be expected, the boost in aperture and magnification garnered more detail. My logbook entry reads, “Seems fan or V-shaped with perhaps a dozen stars mags 11-13, plus a glow from fainter members.”
The finder chart shows the location of NGC 6939 about 2 degrees southwest of the magnitude 3.4 star eta (η) Cephei – and a similar distance south of magnitude 4.2 theta (θ) Cephei. Note the presence of the galaxy NGC 6946 just 40 arcminutes southeast of NGC 6939. Both can be picked up in the same low-power field. Be forewarned, however, that NGC 6946 is no piece of cake. We’ll take a closer look at this elusive spiral next month.
