New photographic photometry is given for the open cluster NGC 6939. Using earlier proper motion measurements, a colour–magnitude diagram is obtained for those stars believed to be cluster members. The cluster appears to have an age of about 109 years, and has a low density region on the main sequence in agreement with a theoretically predicted gap. The red giant branch shows one very marked concentration which is tentatively identified with the horizontal branch found in globular clusters. There are a few anomalous bright blue stars, similar to those in other old clusters, well above the main sequence turn-off.
- 1 - natural sciences ; 2 - physics & astronomy ; 3 - astronomy & astrophysics
- 1 - Physical Sciences ; 2 - Earth and Planetary Sciences ; 3 - Space and Planetary Science
- 1 - Physical Sciences ; 2 - Physics and Astronomy ; 3 - Astronomy and Astrophysics