NGC 6939 is an open cluster featuring a rich concentration of stars, offering an attractive target for night sky photography. The cluster lies close to the border of Cepheus and Cygnus, not far from the well-known Fireworks Galaxy (NGC 6946). Its moderate apparent magnitude makes it ideal for imaging even with small telescopes or binoculars. Revealing subtle dust and nebulosity in wider fields, this cluster provides interesting practice for revealing fainter features in astrophotography.
Names and Catalog numbers
- NGC: NGC 6939
- Melotte: Melotte 231
Position and the cosmic neighborhood
NGC 6939 sits in the rich Milky Way fields at the southwest corner of Cepheus, near the border with Cygnus. Its close proximity to the spiral galaxy NGC 6946—often called the Fireworks Galaxy—makes this pairing a favorite for wide-field astrophotography. The cluster lies about two degrees southwest of the bright star Eta Cephei and forms a picturesque duo with NGC 6946 just two-thirds of a degree apart, allowing both objects to be framed together in larger fields of view.
Nice to Know
- NGC 6939 is more than one billion years old, making it relatively mature for an open star cluster.
- The area around the cluster is noted for faint dusty nebulosity, which can surprise astrophotographers who use sensitive equipment and long exposure times.
- Until now, over 500 stars have been detected as cluster members, some of which are variable stars, including eclipsing binaries of different types.
- When imaged in longer exposures, NGC 6939 gives astrophotographers the chance to detect subtle background dust alongside the cluster’s scattered stars.
Brightnes, distance and size
NGC 6939 shines at an apparent magnitude of 7.8, which means it can be detected with binoculars or small telescopes under dark skies. The distance to the cluster is approximately 3,860 light-years. Its physical extent covers about 7 to 12 arcminutes in the sky—about a quarter to a third of the diameter of the full moon—and the true span of the cluster in space is estimated to be around several light-years across. Its compactness and moderate brightness make it a satisfying subject for astrophotography, especially when capturing detailed star fields or wide shots with NGC 6946 in the same frame.
