astronomy

NGC 4639

NGC 4639 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 4639 rather than just read about it. In short: NGC 4639 is a barred spiral galaxy located in the equatorial constellation of Virgo. It was discovered by German-born astronomer William Herschel on 12 April 1784.

Key takeaways

NGC 4639 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 4639 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 4639 from memory before moving on to harder problems.

Reference excerpt

NGC 4639 is a barred spiral galaxy located in the equatorial constellation of Virgo. It was discovered by German-born astronomer William Herschel on 12 April 1784. John L. E. Dreyer described it as "pretty bright, small, extended, mottled but not resolved, 12th magnitude star 1 arcmin to southeast". This is a relatively nearby galaxy, lying approximately 72 million light-years away from the Milky Way. It is a companion to NGC 4654, and the two appear to have interacted roughly 500 million years ago. NGC 4639 is a member of the Virgo Cluster. The morphological classification of this galaxy is SAB(rs)bc, indicating a spiral galaxy with a weak bar (SAB), an incomplete ring around the bar (rs), and moderate to loosely-wound spiral arms (bc). NGC 4639 has a mildly active galactic nucleus of the Seyfert type 1; one of the weakest known. The compact central source has been detected by its X-ray emission, and is variable on timescales of months to years. There is a supermassive black hole at the core with an estimated mass of 7.9×106 M☉.

Supernovae Two supernovae have been observed in NGC 4639:

SN 1990N (Type Ia, mag. 15.5) was discovered by E. Thouvenot on 22 June 1990, two weeks before reaching peak brightness. It was positioned 63.2″ east and 1.8″ south of the galaxy core. The brightness and proximity of this supernova event has allowed it to be used as a standard candle. SN 2018imf (Type IIP, mag. 15.8) was discovered by Kōichi Itagaki on 14 November 2018.

Notes

References

External links Media related to NGC 4639 at Wikimedia Commons

Illustrations

Worked examples

Example 1 — a first encounter with NGC 4639

Start with the simplest possible case. Write down what NGC 4639 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 4639 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 4639 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 4639

In research: NGC 4639 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 4639 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 4639 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Barred spiral galaxies, NGC objects, Principal Galaxies Catalogue objects, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 4639 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 4639 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 4639 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 4639 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 4639 in simple terms?

NGC 4639 is a barred spiral galaxy located in the equatorial constellation of Virgo. It was discovered by German-born astronomer William Herschel on 12 April 1784.

Why does NGC 4639 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 4639?

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 4639.