NGC 6939 in Cepheus
August 2025 - Nebula and Cluster of the Month
August brings with it the welcome return of dark skies, and a plethora of clusters and nebulae become available for observation. This month, we’re going to be looking straight up. In mid-Britain, this open cluster transits at midnight on the 17th August, reaching an altitude of 83°.
It is NGC 6939 and it lies in Cepheus, very close to its border with Cygnus. Discovered (predictably) by William Herschel on the night of 9 September 1798, it became the final entry in his sixth class – Very compressed and rich clusters of stars
, as 42 H.VI. On that night, he described it as A beautiful compressed cluster of small stars, extremely rich, of an irregular form. The preceding part of it is round, and branching out on the following side, both towards the north and towards the south, 8 or 9’ in diameter
.
The description may seem a little obscure, but comparing it with my observation, I think I know what he meant. When the object entered the NGC in 1888, the description became Cluster, pretty large, extremely rich, pretty compressed towards the middle, stars magnitudes 11—16
. This is a pretty good description of the object’s appearance in a moderate telescope.
NGC 6939 is located about 4000 light-years (1200 parsecs) away, close to the edge of the galactic disc. The stars within open clusters are loosely bound gravitationally, but these gravitational bonds are disrupted and eventually broken by tidal forces within the Galaxy. Clusters therefore ‘dissolve’ over time, their constituent members being pulled away from the cluster to lead more solitary lives. It is quite likely, for example, that the Sun was once a member of a now-dissolved open cluster. NGC 6939, being well away from the strongest tidal forces, has so far escaped this fate and has survived to an unusually old age (see also the article on NGC 188 for May, 2025). Estimates of the cluster’s age vary between 1 billion and 1.6 billion years, depending on the method used1,2, compared with about 6.2 billion years for NGC 188.
In addition to its age, NGC 6939’s chemical composition, specifically its metallicity, has also been studied extensively. The [Fe/H] values of its component stars are lower than those found in stars in the solar neighbourhood. This has been used as evidence to support the theory that the Milky Way has a metallicity gradient, in which the metallicity of stars decreases with distance from the core3.
NGC 6939 is thus an important object in the field of astrophysics, and has served as an experimental test-bed for several theories.
It forms a triangle with the stars η and θ Cephei, being 2° southwest of η and 2.3° south of θ. On a good night and with sufficient magnification, it makes a very fine sight. I observed it with my 12” (300mm) Newtonian in July 2016 and found it to be small, rich and compressed. I described the brightest stars to be of about 12th magnitude, forming two straight lines at right angles to each other. These, I think, may be the branches described by Herschel. The cluster is well-detached from the background stars and stands out immediately the field is found.
More recently, I took a picture of the cluster, a short exposure that gives a reasonably good impression of how the object appears through a moderate telescope.
Drifting away from nebulae and clusters for a moment, it would be remiss of me not to mention the very fine galaxy, NGC 6946, which lies 39’ to the southeast of NGC 6939. This is a large, open-armed, face-on spiral galaxy. Its magnitude is given as 8.8, but this is misleading. It has a low surface brightness and can be quite tricky to see. I found when I observed it that there was something quite ‘planetary nebulary’ about it. I was pleased, therefore, when I found out that William Herschel (who discovered it, of course) had placed in his fourth class – planetary nebulae.
| Object | RA | Dec | Type | Magnitude |
|---|---|---|---|---|
| NGC 6939 | 20h 31m 31s | +60° 40’ | Open cluster | 7.8 |
References:
- Bragaglia, A., Tosi, M., Marconi, G., & Carretta, E. (2001). NGC 6939: An old open cluster in the outskirts of the Galactic disc. Astronomy & Astrophysics, 371, 527–535.
- Rosvick, J. M., & Balam, D. D. (2002). A CCD study of NGC 6939. The Astronomical Journal, 124(4), 2093.
- Dias, W. S., Alessi, B. S., Moitinho, A., & Lépine, J. R. D. (2002). New catalogue of optically visible open clusters and candidates. Astronomy & Astrophysics, 389, 871–873.
