| NGC 6939 | |
|---|---|
 NGC 6939 | |
| Observation data (J2000epoch) | |
| Right ascension | 20h 31m 30s[1] |
| Declination | +60° 39′ 42″[1] |
| Distance | 3,860ly (1,185pc[2]) |
| Apparent magnitude (V) | 7.8[1] |
| Apparent dimensions (V) | 7' |
| Physical characteristics | |
| Estimated age | 2.2 billions years[2] |
| Other designations | Melotte 231 |
| Associations | |
| Constellation | Cepheus |
| See also:Open cluster,List of open clusters | |
NGC 6939 is anopen cluster in the constellationCepheus. It was discovered byWilliam Herschel in 1798. The cluster lies 2/3° northwest from the spiral galaxyNGC 6946. The cluster lies approximately 4,000 light years away and it is over a billion years old.
NGC 6939 is located near the border of the constellationsCepheus andCygnus, at the southwest corner of Cepheus. The open cluster is located two degrees southwest ofEta Cephei and 2/3° northwest from the spiral galaxyNGC 6946, which has visual magnitude 8.7. They appear as two patches of haze with 10x50binoculars. NGC 6939 can be seen and glimpsed with 7x35 binoculars where as 25x200 binoculars are required to start resolve the cluster.[3] The cluster can be glimpsed with 4 inchestelescope and is resolved at x72 magnification. NGC 6939 is included in theHerschel 400 Catalogue.[4]
NGC 6939 is an old open cluster, located 400 parsec above the galactic plane and 8.400 parsec away from the galactic centre.[5] With the use ofphotometric studies, the age of the cluster was estimated to be between 1,0 and 1,3 billion years!, using as sample 638 stars within the field. Themetallicity of the cluster is slightly subsolar[6] (-0.19±0.09).[7]
Some of its members arevariable stars: a study in 1998 led to the discovery of six variable stars among thered giants of the cluster, with two of which are Algol type and V466 Cephei appears to be a W Ursae Majoris type. A further study in 2004 identified further 10 variables near the cluster, five of which were W Ursae Majoris type and one RR Lyrae.[8]
