The Fireworks Galaxy
NGC 6946 is a face-on intermediate spiral galaxy with a small bright nucleus, whose location in the sky straddles the boundary between the northern constellations of Cepheus and Cygnus. Its distance from Earth is about 22.5 million light-years or 6.8 megaparsecs, similar to the distance of M101 (NGC 5457) in the constellation Ursa Major. Both were once considered to be part of the Local Group. but are now known to be among the dozen bright spiral galaxies near the Milky Way but beyond the confines of the Local Group. NGC 6946 lies within the Virgo Supercluster.
Discovered by William Herschel on 9 September 1798, this well-studied galaxy has a diameter of approximately 40,000 light-years, about one-third of the Milky Way’s size, and it contains roughly half the number of stars as the Milky Way. The galaxy is heavily obscured by interstellar matter as it lies quite close to the galactic plane of the Milky Way. Due to its prodigious star formation it has been classified as an active starburst galaxy.
Various unusual celestial objects have been observed within NGC 6964. This includes the so-called ‘Red Ellipse’ along one of the northern arms that looks like a super-bubble or very large supernova remnant, and which may have been formed by an open cluster containing massive stars. There are also two regions of unusual dark lanes of nebulosity, while within the spiral arms several regions appear devoid of stars and gaseous hydrogen, some spanning up to two kiloparsecs across. A third peculiar object, discovered in 1967, is now known as “Hodge’s Complex”. This was once thought to be a young supergiant cluster, but in 2017 it was conjectured to be an interacting dwarf galaxy superimposed on NGC 6964.
Ten supernovae have been observed in NGC 6946 in the last century: SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et, SN 2008S, and SN 2017eaw. For this reason NGC 6946 in 2005 was dubbed the Fireworks Galaxy, a name becoming increasingly popular. NGC 6946 has an unusually high rate of supernovae production compared to our Milky Way galaxy, whose rate averages just one supernova event per century. This is the more remarkable as our Galaxy comprises twice as many stars.