NGC 6946
NGC 6946, sometimes referred to as the Fireworks Galaxy, is a grand design, 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 25.2 million light-years or 7.72 megaparsecs, similar to the distance of M101 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.
The galaxy was discovered by William Herschel on 9 September 1798. Based on an estimation by the Third Reference Catalogue of Bright Galaxies in 1991, the galaxy has a D25 B-band isophotal diameter of. It is heavily obscured by interstellar matter due to its location close to the galactic plane of the Milky Way. Due to its prodigious star formation it has been classified as an active starburst galaxy. NGC 6946 has also been classified as a double-barred spiral galaxy, with the inner, smaller bar presumably responsible for funneling gas into its center.
Various unusual celestial objects have been observed within NGC 6946. 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 6946.
Supernovae
Ten supernovae have been observed in NGC 6946 since 1917. For this reason, NGC 6946 has sometimes been referred to as the "Fireworks Galaxy". This is about ten times the rate observed in our Milky Way galaxy, even though the Milky Way has twice as many stars as NGC 6946.- SN 1917A was discovered by American optician George Willis Ritchey on 19 July 1917.
- SN 1939C was discovered by Fritz Zwicky on 17 July 1939.
- SN 1948B was discovered by Nicholas U. Mayall on 6 July 1948.
- SN 1968D was discovered by Paul Wild and Justus R. Dunlap on 29 February 1968. A prediscovery photograph was also found which showed the supernova on 6 February with a magnitude of 13.5.
- SN 1969P was discovered by Leonida Rosino on 11 December 1969.
- SN 1980K was discovered by Paul Wild on 28 October 1980.
- SN 2002hh was discovered by LOTOSS on 31 October 2002.
- SN 2004et was discovered by S. Moretti on 27 September 2004. Images taken during the preceding days revealed that the supernova explosion occurred on 22 September. The progenitor of the supernova was identified on earlier images –– only the seventh time that such an event was directly identified with its host star. The red supergiant progenitor had an initial mass of about 15 in an interacting binary system shared with a blue supergiant.
- SN 2008S was discovered by Ron Arbour on 1 February 2008.
- SN 2017eaw was discovered by Patrick Wiggins on 14 May 2017. It was detected in the northwest region of the galaxy, and light curves obtained over the next 600 days showed that it was a Type II-P. The progenitor was determined to have been a red supergiant, with a mass of around 15.
As of 2017, more supernovae had been seen in NGC 6946 than in any other galaxy, a record that has since been surpassed by NGC 3690.
| Designation | Discovery magnitude | Type |
| 1917A | 14 | II |
| 1939C | 14.4 | ? |
| 1948B | 14.9 | II |
| 1968D | 13.5 | II |
| 1969P | 13.9 | ? |
| 1980K | 13 | II-L |
| 2002hh | 16.5 | II |
| 2004et | 12.8 | II |
| 2008S | 17.6 | IIn-pec/LBV |
| 2017eaw | 12.8 | II-P |