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>{{#invoke:citation/CS1|citation |CitationClass=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>{{#invoke:citation/CS1|citation |CitationClass=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 in the 20th and early 21st century: SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et,<ref name="Li2005"/> SN 2008S, and SN 2017eaw.<ref name="Bishop-2017eaw"/><ref name=telegram1/><ref name=telegram2/><ref name=list/> 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.

On 27 September 2004, the Type II supernova SN 2004et was observed at magnitude 15.2 and rose to a maximum visual magnitude of 12.7. 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" />

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>

In May 2017, supernova SN 2017eaw 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>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/>

As of 2017, more supernovae had been seen in NGC 6946 than in any other galaxy,<ref name="Bishop-2017eaw">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> a record that has since been surpassed by NGC 3690.<ref name="MostProlific">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Supernovae in NGC 6946<ref name="Bishop-2017eaw"/>

Supernova	apmag	type
2017eaw	12.6	IIP
2008S	16.5	IIn-pec/LBV
2004et	12.3	II
2002hh	15.0	II
1980K	11.4	IIL
1969P	13.9	?
1968D	13.5	II
1948B	14.9	II
1939C	13.0	?
1917A	14.6	II

Gallery

• NGC 6946 (Kitt Peak National Observatory) (2023-115).png

  This image from the Kitt Peak National Observatory contextualizes the locations of supernovae 2004et and 2017eaw within the galaxy.

• NBO 2017eaw.png

  Images of 2017eaw in NGC 6946, taken between May and November 2017, as the supernova was slowly fading

• Giant star N6946-BH1 before and after it vanished out of sight by imploding to form a black hole.jpg

  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

• Dusty supernovae (MIRI, annotated) (dustysupernovae2).jpg

  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 Giant Star Becomes A Black Hole Right Before Our Eyes!
• Template:WikiSky
• List of observations and images of supernova SN 2017eaw