NGC 6946, sometimes referred to as the Fireworks Galaxy, is a face-on
intermediate spiral galaxy
An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. It is designated as SAB in the galaxy morphological classification system devised by Gerard de Vaucouleur ...
with a small bright nucleus, whose location in the sky straddles the boundary between the
northern
Northern may refer to the following:
Geography
* North, a point in direction
* Northern Europe, the northern part or region of Europe
* Northern Highland, a region of Wisconsin, United States
* Northern Province, Sri Lanka
* Northern Range, a ra ...
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
s of
Cepheus and
Cygnus. Its distance from Earth is about 25.2 million
light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s or 7.72
megaparsecs
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...
,
similar to the distance of
M101 (NGC 5457) in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
Ursa Major
Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...
.
Both were once considered to be part of the
Local Group
The Local Group is the galaxy group that includes the Milky Way.
It has a total diameter of roughly , and a total mass of the order of .
It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...
,
but are now known to be among the dozen bright spiral galaxies near the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
but beyond the confines of the Local Group.
NGC 6946 lies within the
Virgo Supercluster
The Virgo Supercluster (Virgo SC) or the Local Supercluster (LSC or LS) is a mass concentration of galaxies containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At least ...
.
The galaxy was discovered by
William Herschel
Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...
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 .
It is heavily obscured by
interstellar matter
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar s ...
due to its location close to the
galactic plane
The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usuall ...
of the Milky Way. Due to its prodigious star formation it has been classified as an active
starburst galaxy
A starburst galaxy is one undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formatio ...
.
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
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
, 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
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...
cluster, but in 2017 it was conjectured to be an interacting dwarf galaxy superimposed on NGC 6946.
Supernovae
Ten
supernovae
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a ...
have been observed in NGC 6946 in the 20th and early 21st century:
SN 1917A
SN 1917A is a supernova event in the Fireworks Galaxy (NGC 6946), positioned west and south of the galactic core. Discovered by American optician George Willis Ritchey on 19 July 1917, it reached a peak visual magnitude of 13.6. Based on a poor q ...
,
SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et,
SN 2008S, and SN 2017eaw.
[ 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
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
galaxy,[ 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
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
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 15 in an interacting binary system shared with a blue supergiant.
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
N6946-BH1 is a disappearing giant star formerly seen in the galaxy NGC 6946, on the northern border of the constellation of Cygnus (constellation), Cygnus. The star, either a red supergiant or a yellow hypergiant, was 25 times the mas ...
. The progenitor is believed to have been a yellow hypergiant star.
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. The progenitor was determined to have been a red supergiant
Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Anta ...
, with a mass of around 15.[
More supernovae have been seen in NGC 6946 than in any other ]galaxy
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
.
See also
*IC 342
IC 342 (also known as Caldwell 5) is an intermediate spiral galaxy in the constellation Camelopardalis, located relatively close to the Milky Way. Despite its size and actual brightness, its location behind dusty areas near the galactic equator m ...
- similar galaxy heavily obscured by Milky Way stars and dust.
*List of galaxies
The following is a list of notable galaxies.
There are about 51 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in the Local Supercluster, and an estimated 100 billion in all of the obse ...
References
External links
SEDS: Spiral Galaxy NGC 6946
N6946-BH1
Giant Star Becomes A Black Hole Right Before Our Eyes!
*
{{DEFAULTSORT:Ngc 6946
Cepheus (constellation)
Cygnus (constellation)
Intermediate spiral galaxies
6946
11597
65001
029
012b
17980909
Virgo Supercluster