NGC 6939 is an open star cluster in the northern constellation Cepheus that presents a rich, well-populated field ideal for wide-field and medium-focal-length astrophotography. Spanning about 15′ across the sky, it fits comfortably within the frame of most telescopes paired with a modern APS-C or full-frame sensor, leaving room to balance the composition. Its moderate surface brightness rewards longer integration times under dark skies, where the subtle colors of its many member stars begin to emerge in RGB or broadband data. Because it sits near the photogenic face-on spiral galaxy NGC 6946, imagers often frame the pair together for a striking cluster-and-galaxy portrait.
Names, key data and data sources
All values come from the astronomical database SIMBAD (CDS Strasbourg). Tap a row to see where the value comes from.
Catalogue numbersNGC 6939 · Mel 231 (Melotte 231)Primary designation: NGC 6939SIMBAD
TypeOpen ClusterSIMBAD
ConstellationCepheus (Cep)computed
Position (J2000)RA 20h 31m 40s · Dec +60° 39′ 11″SIMBAD
Galactic coordinatesl = 95.9° · b = 12.3°Position relative to the Milky Way plane: 12° north of itcomputed
Apparent size15′SIMBAD
Orientation0°Position angle of the major axis, measured from north through eastSIMBAD
Distance5,870 light-years1.80 kpcSIMBAD
Diameter25.3 light-yearscomputed
Blueshiftz = −0.000062−19 km/s: approaching usSIMBAD
Sources and methods (10)
- Catalogue numbers: All identifiers under which SIMBAD lists the object; order according to the Astrocamp catalogue rule.
- Type: Entry in SIMBAD, object type “OpC”. Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Constellation: Computed from the position (IAU constellation boundaries). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Position (J2000): Entry in SIMBAD, quality E (SIMBAD scale A–E). Publication: 2021A&A...647A..19T (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Galactic coordinates: Converted from the position (IAU 1958 galactic system). Publication: 2021A&A...647A..19T (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Apparent size: One catalogue value, measured in visible light, quality D (SIMBAD scale A–E). Publication: 2020A&A...633A..99C (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Orientation: One catalogue value, measured in visible light, quality D (SIMBAD scale A–E). Publication: 2020A&A...633A..99C (NASA ADS). Value since Oct 20, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Distance: Median of all 7 measurements listed in SIMBAD (methods: paral 3, ST-L 1), middle half 5,800 light-years – 5,930 light-years, full range 4,530 light-years – 6,100 light-years, rounded according to the scatter. Value since Oct 20, 2026 · last confirmed with SIMBAD Oct 20, 2026
- Diameter: Computed from the major axis and the distance; takes the weaker rating of the two values.
- Blueshift: One catalogue value, error ± 0.1 km/s, quality A (SIMBAD scale A–E). Publication: 2021A&A...647A..19T (NASA ADS). Value since Jul 4, 2026 · last confirmed with SIMBAD Oct 20, 2026
Checked against SIMBAD on Oct 20, 2026 · Open NGC 6939 in SIMBAD
Position and the cosmic neighborhood
Perched high in the far-northern sky within Cepheus, NGC 6939 is a near-circumpolar target for mid-northern latitudes, making it accessible across much of the year and well suited to extended multi-night imaging projects. It lies close to the border with Cygnus, in a region threaded by the faint glow of the Milky Way, which lends dense star fields to the surrounding background. Its most celebrated companion is the nearby spiral galaxy NGC 6946, the famous Fireworks Galaxy, which shares the same field of view at modest focal lengths. This chance alignment of a Galactic star cluster with a distant island universe makes the area a favorite for astrophotographers seeking depth and contrast in a single frame.
Click marks an object (its path tonight) · click again for its constellation, once more to zoom to its field · click a frame to see it, again to zoom · its name shows details · click outside zooms out · drag to pan
Nice to Know
- NGC 6939 lies at a distance of around 5,870 light-years, a value derived from the median of 7 published measurements, giving imagers a reliable sense of scale for this cluster.
- The cluster measures about 25.3 light-years across, a compact true size that translates to its roughly 15′ apparent span on the sky.
- It carries two catalog designations, NGC 6939 and Melotte 231, the latter recalling early photographic surveys of star clusters.
- Its celebrated proximity to the Fireworks Galaxy (NGC 6946) makes it a classic two-for-one composition, pairing a Galactic cluster with a distant spiral in one exposure.
