astronomy

NGC 391

NGC 391 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 391 rather than just read about it. In short: NGC 391 is an unbarred lenticular galaxy located in the constellation Cetus. Its velocity with respect to the cosmic microwave background is 5,022±23 km/s, which corresponds to a Hubble distance of 241.6 ± 17.0 Mly (74.07 ± 5.20 Mpc).

Key takeaways

NGC 391 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 391 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 391 from memory before moving on to harder problems.

Reference excerpt

NGC 391 is an unbarred lenticular galaxy located in the constellation Cetus. Its velocity with respect to the cosmic microwave background is 5,022±23 km/s, which corresponds to a Hubble distance of 241.6 ± 17.0 Mly (74.07 ± 5.20 Mpc). Additionally, one non-redshift measurement gives a farther distance of 272 Mly (83.3 Mpc). It was discovered by American astronomer George Bond on January 8, 1853. It was described by Dreyer as "faint, small, mottled but not resolved (Auwers 9)."

Supernova One supernova has been observed in NGC 391: SN 2025rat (Type Ia, mag. 19.9454) was discovered by Zwicky Transient Facility on 11 July 2025.

Illustrations

Worked examples

Example 1 — a first encounter with NGC 391

Start with the simplest possible case. Write down what NGC 391 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 391 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 391 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 391

In research: NGC 391 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 391 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 391 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1853, Cetus (constellation), Lenticular galaxy stubs, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 391 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 391 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 391 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 391 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 391 in simple terms?

Why does NGC 391 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 391?

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 391.