astronomy

NGC 21

NGC 21 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 21 rather than just read about it. In short: NGC 21 (also known as NGC 29) is a spiral galaxy in the Andromeda constellation. It was discovered by German-British astronomer William Herschel on 26 November 1790.

Key takeaways

NGC 21 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 21 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 21 from memory before moving on to harder problems.

Reference excerpt

NGC 21 (also known as NGC 29) is a spiral galaxy in the Andromeda constellation. It was discovered by German-British astronomer William Herschel on 26 November 1790. Lewis Swift observed it again in 1885, leading to its double listing in the New General Catalogue.

NGC 7831 Group According to A.M. Garcia, NGC 21 is a member of the NGC 7831 group (also known as LGG 1), which contains at least 18 galaxies, including NGC 13, NGC 19, NGC 20, NGC 39, NGC 43, NGC 7805, NGC 7806, NGC 7819, and NGC 7836.

See also NGC List of NGC objects (1–1000) List of NGC objects Galaxy

References

External links Media related to NGC 21 at Wikimedia Commons Wikisky image of NGC 29

Illustrations

NGC 21: NGC 21 in Infrared — NGC 21 in Infrared

Worked examples

Example 1 — a first encounter with NGC 21

Start with the simplest possible case. Write down what NGC 21 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 21 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 21 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 21

In research: NGC 21 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 21 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 21 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Andromeda (constellation), Astronomical objects discovered in 1790, Discoveries by William Herschel, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 21 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 21 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 21 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 21 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 21 in simple terms?

NGC 21 (also known as NGC 29) is a spiral galaxy in the Andromeda constellation. It was discovered by German-British astronomer William Herschel on 26 November 1790.

Why does NGC 21 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 21?

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 21.