Fireworks Galaxy NGC 6946 + Cluster NGC 6939

Object Description

NGC 6946
NGC 6946: Fireworks Galaxy in Cepheus; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 450+120+120+120 min LRGB; Bernese Highlands; © 2017 Radek Chromik

The open star cluster NGC 6939 was discovered on 9 September 1798 by Wilhelm Herschel. The star cluster has an apparent diameter of about 10 arc minutes and is of the Trumpler type I1m, which means the following: I = The star cluster stands out from the surrounding stars and shows a clear central concentration. 1 = Most of the stars in the cluster are about the same magnitude. m = medium star richness (50-100 stars). Measured distances are in the range of 1800 pc to 1870 pc (about 6000 light-years) and it is moving in our direction at a speed of 19 km/s. [145, 196]

Just 38 arc minutes to the southeast is the beautiful spiral galaxy NGC 6946. It was discovered by Wilhelm Herschel that same night. At that time it was still believed that these "spiral nebulae" were located in our galaxy and could represent planetary systems in the making. [196]

NGC 6939, NGC 6946
NGC 6939, NGC 6946: Offener Sternhaufen NGC 6939 mit Galaxie NGC 6946. Ausschnitt aus dem Digitized Sky Survey (DSS) [147]

In Halton Arp's 1966 Atlas of Peculiar Galaxies, NGC 6946 is listed as Arp 29 for an example of a galaxy with an enlarged arm (Arp 25...30). [199]

NGC 6946 is of the morphological type SAB(rs)cd and is located at a distance of 5.5 Mpc (about 18 million light-years) using the T-RGB (Tip of the Red Giant Branch) method. Spectroscopically measured speeds range from 31 km/s to 80 km/s. Distances determined from this range from 4.1 Mpc to 6.7 Mpc. The galaxy lies roughly in the plane of our Milky Way and is obscured by dust in our galaxy. Short-wave radiation is partially absorbed, which is why the galaxy appears reddened. From there, our Milky Way would appear as an edge-on galaxy. NGC 6946 is a Seyfert 2 galaxy with a bright core and an active rate of star formation, and has a supernovae rate about five times higher per century than our own galaxy. This makes it a «Starburst» galaxy. [145, 196]

Revised+Historic NGC/IC, Version 22/9, © Dr. Wolfgang Steinicke [277]
NameRADecTypebMagvMagDimDreyer DescriptionIdentification, Remarks
NGC 693920 31 30.0+60 39 42OCL (I1m)7.810Cl, pL, eRi, pCM, st 11…16OCL 217
NGC 694620 34 52.1+60 09 12Gx (SBc)9.68.811.5 × 9.8vF, vL, vg, vsbM, rrUGC 11597, MCG 10-29-6, CGCG 304-6, Arp 29, IRAS 20338+5958

Finder Chart

The unequal pair can be found in the constellation Cepheus, approximately on the line connecting the two stars η Cephei (3.4 mag) and 33 Cygni (4.3 mag), so that the outermost 4° Telrad circle lies on the star η Cephei.

Chart Fireworks Galaxy NGC 6946 + Cluster NGC 6939
Fireworks Galaxy NGC 6946 + Cluster NGC 6939 in constellation Cepheus. Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

350 mm aperture: The open star cluster NGC 6939 can be easily resolved into individual stars. Despite its relatively small size, it is still quite spacious, in any case it fits almost seamlessly into the Milky Way.

The galaxy NGC 6946 can be found not far from the above star cluster. The search is extremely easy and convenient with the help of NGC 6939. The faint spiral arms are still clearly visible in the galaxy itself. [192]

14" PWO-Dobson, F:4.6 / TV-Nagler 13mm, 123x, 0.67°
Eduard von Bergen

More Objects Nearby (±15°)

References

145SIMBAD astronomical database; simbad.u-strasbg.fr/simbad
147Aladin Lite; aladin.u-strasbg.fr/AladinLite (2020-12-23)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
192Deep-Sky Guide; fernrohr.ch/1d_deep-sky-guide.html (2020-12-25)
196Celestial Atlas by Curtney Seligman; cseligman.com/text/atlas.htm (2020-12-28)
199«Atlas Of Peculiar Galaxies», Halton Arp, 1966; Astrophysical Journal Supplement, vol. 14, p.1 (1966); DOI:10.1086/190147; Bibcode:1966ApJS...14....1A
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)