astronomy

NGC 4691

NGC 4691 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 4691 rather than just read about it. In short: NGC 4691 is a barred spiral galaxy in the constellation Virgo. The galaxy lies about 70 million light years away from Earth, which means, given its apparent dimensions, that NGC 4691 is approximately 60,000 light years across.

Key takeaways

NGC 4691 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 4691 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 4691 from memory before moving on to harder problems.

Reference excerpt

NGC 4691 is a barred spiral galaxy in the constellation Virgo. The galaxy lies about 70 million light years away from Earth, which means, given its apparent dimensions, that NGC 4691 is approximately 60,000 light years across. It was discovered by German-British astronomer William Herschel on 17 April 1784.

Characteristics The galaxy has a prominent bar with some bright knots. From the bar emerge two diffuse low-surface-brightness spiral arms which are devoid of knots. The arms form a nearly complete outer ring. Dust is visible across the inner regions of the galaxy. The dust lanes are generally perpendicular to the major axis of the bar. The galaxy is seen face-on. The total molecular gas of the galaxy based on CO emission is estimated to be about 9×108 M☉. Two large molecular clouds are visible along the bar in each side of the nucleus and one more is seen perpendicularly to the bar, located within the inner Lindblad resonance. The nucleus of the galaxy has been categorised as active, however there is no evidence of it being active in optical wavelengths, but it is a star forming region (HII region) instead. A broad H-alpha component has been observed in the central region of the galaxy that is blueshifted by about 500 km/s in respect to the galaxy, indicating it is an outflow that pushes gas away from the plane of the galaxy. It is possible the outflow was created by supernova remnants in the circumnuclear star forming regions.

Supernova One supernova has been observed in NGC 4691. SN 1997X was discovered by Masakatsu Aoki on 1 February 1997 at an apparent magnitude of 13.6. It got as bright as magnitude 13.5, making it the brightest supernova of 1997. It has been identified spectrographically as a type Ic supernova about one week post maximum.

Nearby galaxies NGC 4691 forms a pair with NGC 4684, which lies 19 arcminutes away. However the difference in redshift is about 470 km/s, indicating that the pairing is likely a projection effect. NGC 4691 is a member of the NGC 4753 Group, along with NGC 4629, NGC 4753, NGC 4771, NGC 4772, NGC 4845, and NGC 4904. It is part of a Virgo II Groups, a chain of groups extending from the Virgo Cluster.

Gallery

References

External links

NGC 4691 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images NGC 4691 on SIMBAD

Illustrations

Worked examples

Example 1 — a first encounter with NGC 4691

Start with the simplest possible case. Write down what NGC 4691 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 4691 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 4691 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 4691

In research: NGC 4691 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 4691 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 4691 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1784, Barred lenticular galaxies, Barred spiral galaxies, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 4691 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study NGC 4691 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 4691 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 4691 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 4691 in simple terms?

Why does NGC 4691 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 4691?

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 4691.