astronomy

NGC 6946

NGC 6946 is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand NGC 6946 rather than just read about it. In short: 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 (NGC 5457) in the constellation Ursa Major.

Key takeaways

NGC 6946 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NGC 6946 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 6946 from memory before moving on to harder problems.

Reference excerpt

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 (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. 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 26.77 kiloparsecs (87,300 light-years). 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 (type unknown, mag. 14) was discovered by American optician George Willis Ritchey on 19 July 1917. SN 1939C (type unknown, mag. 14.4) was discovered by Fritz Zwicky on 17 July 1939. SN 1948B (Type II, mag. 14.9) was discovered by Nicholas U. Mayall on 6 July 1948. SN 1968D (Type II, mag. 13.5) 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 (type unknown, mag. 13.9) was discovered by Leonida Rosino on 11 December 1969. SN 1980K (Type II-L, mag. 13) was discovered by Paul Wild on 28 October 1980. SN 2002hh (Type II, mag. 16.5) was discovered by LOTOSS (Lick Observatory and Tenagra Observatory Supernova Searches) on 31 October 2002. SN 2004et (Type II-P, mag. 12.8) 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 15M☉ in an interacting binary system shared with a blue supergiant. SN 2008S (Type IIn-pec/LBV, mag. 17.6) was discovered by Ron Arbour on 1 February 2008. SN 2017eaw (Type II-P, mag. 12.8) 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 15M☉. 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. The progenitor is believed to have been a yellow hypergiant star. 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.

Nova images

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

References

External links

SEDS: Spiral Galaxy NGC 6946 Atlas of the Universe N6946-BH1 Archived 29 May 2017 at the Wayback Machine Giant Star Becomes A Black Hole Right Before Our Eyes!

NGC 6946 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images List of observations and images of supernova SN 2017eaw A science-fiction story by astronomer Andrew Fraknoi about NGC 6946

Illustrations

Worked examples

Example 1 — a first encounter with NGC 6946

Start with the simplest possible case. Write down what NGC 6946 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to NGC 6946 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about NGC 6946 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of NGC 6946

In research: NGC 6946 appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses NGC 6946 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: NGC 6946 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Arp objects, Astronomical objects discovered in 1798, Caldwell objects, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 6946 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study NGC 6946 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 6946 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain NGC 6946 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 6946 in simple terms?

Why does NGC 6946 matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study NGC 6946?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on NGC 6946.