NGC 6946

Template:Short description {{ safesubst:#invoke:Unsubst||date=__DATE__|$B= Template:Ambox }} Template:Use dmy dates Template:Infobox Galaxy

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,<ref name=Eldridgedistance /> similar to the distance of M101 (NGC 5457) in the constellation Ursa Major.<ref name="CAG1994"/> Both were once considered to be part of the Local Group,<ref name="SEDS2013"/> but are now known to be among the dozen bright spiral galaxies near the Milky Way but beyond the confines of the Local Group.<ref name="Efremov2017"/> NGC 6946 lies within the Virgo Supercluster.<ref>Template:Cite web</ref>

The galaxy was discovered by William Herschel on 9 September 1798. Based on an estimation by the Third Reference Catalogue of Bright Galaxies (RC3) in 1991, the galaxy has a D₂₅ B-band isophotal diameter of Template:Convert.<ref name="ned" /><ref name=RC3 /> It is heavily obscured by interstellar matter due to its location close to the galactic plane of the Milky Way.<ref>Template:Cite web</ref> Due to its prodigious star formation it has been classified as an active starburst galaxy.<ref name="Efremov2017"/> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

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.<ref name="Efremov2017"/>{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= Template:Fix }} A third peculiar object, discovered in 1967, is now known as "Hodge's Complex".<ref name="Hodge1967"/> 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.<ref name="Efremov2017"/>

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".<ref name=gemini/><ref name=fireworks/> This is about ten times the rate observed in our Milky Way galaxy,<ref name=gemini2005/> even though the Milky Way has twice as many stars as NGC 6946.

• SN 1917A (type unknown, mag. 14) was discovered by American optician George Willis Ritchey on 19 July 1917.<ref>

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• SN 1939C (type unknown, mag. 14.4) was discovered by Fritz Zwicky on 17 July 1939.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
• SN 1948B ([[Type II supernova|TypeTemplate:NbspII]], mag. 14.9) was discovered by Nicholas U. Mayall on 6 July 1948.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
• SN 1968D (Type II, mag. 13.5) was discovered by Paul Wild and Justus R. Dunlap on 29 February 1968.<ref name=IAUC2057>Template:Cite journal</ref><ref>Template:Cite web</ref> A prediscovery photograph was also found which showed the supernova on 6 February with a magnitude of 13.5.Template:R
• SN 1969P (type unknown, mag. 13.9) was discovered by Leonida Rosino on 11 December 1969.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
• SN 1980K ([[Type II supernova#Light curves for Type II-L and Type II-P supernovae|TypeTemplate:NbspII-L]], mag. 13) was discovered by Paul Wild on 28 October 1980.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
• SN 2002hh (Type II, mag. 16.5) was discovered by LOTOSS (Lick Observatory and Tenagra Observatory Supernova Searches) on 31 October 2002.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
• SN 2004et ([[Type II supernova#Light curves for Type II-L and Type II-P supernovae|TypeTemplate:NbspII-P]], mag. 12.8) was discovered by S. Moretti on 27 September 2004.<ref>Template:Cite journal</ref> 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 15Template:Solar mass in an interacting binary system shared with a blue supergiant.<ref name="Li2005" />
• SN 2008S ([[Type II supernova#Type IIn supernovae|TypeTemplate:NbspIIn-pec]]/LBV, mag. 17.6) was discovered by Ron Arbour on 1 February 2008.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
• SN 2017eaw (Type II-P, mag. 12.8) was discovered by Patrick Wiggins on 14 May 2017.<ref name="Bishop-2017eaw"/><ref name=telegram1/> 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.<ref name=telegram2/><ref>Template:Cite journal</ref> The progenitor was determined to have been a red supergiant, with a mass of around 15Template:Solar mass.<ref>Template:Cite journal</ref><ref name=KF18/>

During 2009, a bright star within NGC 6946 flared up over several months to become over one million times as bright as the Sun. Shortly thereafter it faded rapidly. Observations with the Hubble Space Telescope suggest that the star did not survive, although there remains some infrared emission from its position. This is thought to come from debris falling onto a black hole that formed when the star died. This potential black hole-forming star is designated N6946-BH1.<ref name="Adams2016"/> The progenitor is believed to have been a yellow hypergiant star.<ref>Template:Cite journal</ref>

As of 2017, more supernovae had been seen in NGC 6946 than in any other galaxy,<ref name="Bishop-2017eaw">Template:Cite web</ref> a record that has since been surpassed by NGC 3690.<ref name="MostProlific">Template:Cite web</ref>

Supernovae in NGC 6946

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

Nova images

• This image from the Kitt Peak National Observatory contextualizes the locations of supernovae 2004et and 2017eaw within the galaxy.
  This image from the Kitt Peak National Observatory contextualizes the locations of supernovae 2004et and 2017eaw within the galaxy.
• Images of 2017eaw in NGC 6946, taken between May and November 2017, as the supernova was slowly fading
  Images of 2017eaw in NGC 6946, taken between May and November 2017, as the supernova was slowly fading
• Pair of visible-light and near-infrared photos from the Hubble Space Telescope showing the giant star N6946-BH1 before and after it vanished out of sight by imploding to form a black hole
  Pair of visible-light and near-infrared photos from the Hubble Space Telescope showing the giant star N6946-BH1 before and after it vanished out of sight by imploding to form a black hole
• NASA/ESA/CSA James Webb Space Telescope image of NGC 6946, revealing large amounts of dust from the two supernovae SN 2004et and SN 2017eaw
  NASA/ESA/CSA James Webb Space Telescope image of NGC 6946, revealing large amounts of dust from the two supernovae SN 2004et and SN 2017eaw

See also

• IC 342 – similar galaxy heavily obscured by Milky Way stars and dust.
• List of galaxies

External links

• SEDS: Spiral Galaxy NGC 6946
• Atlas of the Universe
• N6946-BH1 Template:Webarchive Giant Star Becomes A Black Hole Right Before Our Eyes!
• Template:WikiSky
• List of observations and images of supernova SN 2017eaw
• A science-fiction story by astronomer Andrew Fraknoi about NGC 6946