NGC 6939 is a rich open star cluster in the northern constellation Cepheus that makes a rewarding target for wide-field and medium-focal-length astrophotography. Spanning roughly 15′ across the sky, it fits comfortably within a single frame while still resolving into a dense scattering of faint stars under adequate aperture. Its greatest photographic appeal comes from its proximity in the sky to the face-on spiral galaxy NGC 6946, allowing a striking star-cluster-and-galaxy pairing in one composition. Because the cluster’s stars are relatively subdued, longer integration times help draw out the fainter members and deepen the surrounding star field for a more three-dimensional result.
Names and Catalog numbers
- NGC 6939
- Mel 231 (Melotte 231)
Position and the cosmic neighborhood
Located in Cepheus at right ascension 20h 31m 40s and declination +60° 39′, NGC 6939 sits high in the northern sky and remains circumpolar for most mid-northern observers, giving imagers long, unbroken sessions across many months. Its most celebrated neighbor is the nearby galaxy NGC 6946, which lies close enough to share the same wide field of view and creates one of the sky’s finest galactic-plus-galaxy juxtapositions. With a physical diameter of about 26 light-years and a distance of roughly 6,100 light-years, the cluster is a modest but well-populated stellar grouping set against the rich background star fields typical of this part of the Milky Way.
Nice to Know
- NGC 6939 carries the alternate catalog designation Mel 231, from the Melotte catalog of star clusters.
- It shares its field of view with the spiral galaxy NGC 6946, making a cluster-and-galaxy pairing a popular imaging goal.
- The cluster has a radial velocity of -18.6 km/s, indicating it is moving toward us relative to the Sun.
- Its high northern declination in Cepheus keeps it well placed for northern astrophotographers throughout much of the year.
Brightness, distance and size
It lies about 6,100 light-years from Earth and spans roughly 26 light-years across. On the sky it appears about 15′ in size.
