astronomy

NGC 7421

NGC 7421 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 7421 rather than just read about it. In short: NGC 7421 is a barred spiral galaxy in the southern constellation of Grus. It was discovered by English astronomer John Herschel on August 30, 1834.

Key takeaways

NGC 7421 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 7421 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 7421 from memory before moving on to harder problems.

Reference excerpt

NGC 7421 is a barred spiral galaxy in the southern constellation of Grus. It was discovered by English astronomer John Herschel on August 30, 1834. In Danish astronomer J. L. E. Dreyer's New General Catalogue of Nebulae and Clusters of Stars it was described as: considerably bright, large, very little extended, gradually pretty much brighter middle, and partially resolved. NGC 7421 is located at an estimated distance of 81.6 million light-years (25.01 Mpc) from the Sun. It is a member of the IC 1459 galaxy group. In the Third Reference Catalogue of Bright Galaxies, NGC 7421 was assigned a morphological classification of SB(rs)bc, which indicates a barred spiral galaxy (SB) with a transitional ring structure (rs) and moderately wound spiral arms (bc). The galactic plane is inclined at an angle of 36.2° to the plane of the sky, with the major axis aligned along a position angle of 80.6°. It displays an asymmetric morphology, which shows up in a lopsided optical appearance and in the distribution of CO and neutral hydrogen atoms. New stars are forming at a rate of 0.274±0.041 M☉·yr−1. The gas fraction and star formation rate is significantly lower than normal, suggesting an interaction with the external environment. The western boundary of this galaxy resembles a bow shock that suggests an interaction with the intracluster medium. Radio mapping of neutral hydrogen by the ATCA displays an extended wake to the north and south of the galaxy, supporting this hypothesis. A past tidal interaction may be needed to explain the asymmetry of this galaxy. A candidate galaxy is NGC 7418, which is located at an angular separation of ~20′. However, there are no tidal tails visible from such an encounter. The total mass of the neutral hydrogen in this galaxy is 1.5×109 M☉.

Supernova Two supernovae have been observed in NGC 7421:

AT 2018bbl (Type II, mag. 17.4) was discovered by the Backyard Observatory Supernova Search (BOSS) on 24 April 2018. SN 2023abdg (Type II, mag. 14.753) was discovered by ATLAS on December 12, 2023.

See also NGC 4654, a galaxy in Virgo with a similar appearance

References

Illustrations

Worked examples

Example 1 — a first encounter with NGC 7421

Start with the simplest possible case. Write down what NGC 7421 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 7421 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 7421 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 7421

In research: NGC 7421 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 7421 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 7421 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1834, Barred spiral galaxies, Discoveries by John Herschel, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 7421 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 7421 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 7421 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 7421 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 7421 in simple terms?

NGC 7421 is a barred spiral galaxy in the southern constellation of Grus. It was discovered by English astronomer John Herschel on August 30, 1834.

Why does NGC 7421 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 7421?

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 7421.