astronomy

NGC 7331

NGC 7331 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 7331 rather than just read about it. In short: NGC 7331, also known as Caldwell 30, is an unbarred spiral galaxy about 13.427 megaparsecs (43.79 million light-years) away in the constellation Pegasus. It was discovered by William Herschel on 6 September 1784.

Key takeaways

NGC 7331 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 7331 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 7331 from memory before moving on to harder problems.

Reference excerpt

NGC 7331, also known as Caldwell 30, is an unbarred spiral galaxy about 13.427 megaparsecs (43.79 million light-years) away in the constellation Pegasus. It was discovered by William Herschel on 6 September 1784. The galaxy appears similar almost in size and structure to the Milky Way, and is sometimes referred to as "the Milky Way's twin". However, discoveries in the 2000s regarding the structure of the Milky Way may call this similarity into doubt, particularly because the latter is now believed to be a barred spiral, compared to the unbarred status of NGC 7331. In spiral galaxies the central bulge typically co-rotates with the disk but the bulge in the galaxy NGC 7331 is rotating in the opposite direction to the rest of the disk. In both visible light and infrared photos of the NGC 7331, the core of the galaxy appears to be slightly off-center, with one side of the disk appearing to extend further away from the core than the opposite side.

Galaxy Groups NGC 7331 is the brightest galaxy in the field of a visual grouping known as the NGC 7331 Group of galaxies. In fact, the other members of the group, NGC 7335, NGC 7336, NGC 7337 and NGC 7340, lie far in the background at distances of approximately 300–350 million light years. All of the members of the NGC 7331 Group, along with NGC 7325, NGC 7326, NGC 7327, NGC 7333, NGC 7338, are listed together as Holm 795 in Erik Holmberg's A Study of Double and Multiple Galaxies Together with Inquiries into some General Metagalactic Problems, published in 1937.

Supernovae

Four supernovae have been observed in NGC 7331:

SN 1959D (Type II-L, mag. 13.4) was discovered by Milton Humason and Howard S. Gates in a survey at Palomar Observatory on 28 June 1959. SN 2013bu (Type II, mag. 16.6) was discovered by Kōichi Itagaki on 21 April 2013. SN 2014C was discovered by the Lick Observatory Supernova Search (LOSS) on 5 January 2014. The supernova underwent an unusual "metamorphosis" from a hydrogen-poor Type Ib to a hydrogen-rich Type IIn over the course of a year. SN 2025rbs (Type Ia, mag. 17.07) was discovered by GOTO on 14 July 2025. It got as bright as magnitude 11.9, making it the brightest supernova of 2025. By the end of August 2025, the supernova was still around magnitude 14.4. In addition to the confirmed supernovae, a 1903 photographic plate from Yerkes Observatory shows a magnitude 16.6 candidate transient that may have also been a supernova.

See also M94 – another galaxy with a prominent starburst ring NGC 1512 – another galaxy with a prominent starburst ring Flocculent spiral galaxy List of NGC objects (7001–7840) List of NGC objects

Notes

References

External links Media related to NGC 7331 at Wikimedia Commons Calar Alto Observatory – NGC 7331 APOD (2004-07-01) – "A Galaxy So Inclined" SST – "Morphology of Our Galaxy's 'Twin'" NGC 7331 at the astro-photography site of Mr. T. Yoshida NGC7331 at W. Kloehr Astrophotography

NGC 7331 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images SEDS – NGC 7331

Illustrations

Worked examples

Example 1 — a first encounter with NGC 7331

Start with the simplest possible case. Write down what NGC 7331 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 7331 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 7331 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 7331

In research: NGC 7331 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 7331 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 7331 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1784, Caldwell objects, IRAS catalogue objects, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 7331 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 7331 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 7331 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 7331 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 7331 in simple terms?

Why does NGC 7331 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 7331?

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 7331.