astronomy

NGC 4921

NGC 4921 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 4921 rather than just read about it. In short: NGC 4921 is a large barred spiral galaxy in the Coma Cluster, located in the constellation of Coma Berenices. It was discovered by German-British astronomer William Herschel on 11 April 1785.

Key takeaways

NGC 4921 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 4921 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGC 4921 from memory before moving on to harder problems.

Reference excerpt

NGC 4921 is a large barred spiral galaxy in the Coma Cluster, located in the constellation of Coma Berenices. It was discovered by German-British astronomer William Herschel on 11 April 1785. It is about 320 million light-years from Earth. The galaxy has a nucleus with a bar structure that is surrounded by a distinct ring of dust that contains recently formed, hot blue stars. The outer part consists of unusually smooth, poorly distinguished spiral arms. In 1976, the Canadian astronomer Sidney Van den Bergh categorized this galaxy as "anemic" because of the low rate at which stars are being formed. He noted that it has "an unusually low surface brightness and exhibits remarkably diffuse spiral arms". Nonetheless, it is the brightest spiral galaxy in the Coma Cluster. This galaxy is located near the center of the cluster and has a high relative velocity (7,560 km/s) compared to the mean cluster velocity. When examined at the 21 cm wavelength Hydrogen line, NGC 4921 was found to be strongly H I deficient, indicating that it is low in hydrogen. The distribution of hydrogen has also been deeply perturbed toward the SE spiral arm and is less extended than the optical disk of the galaxy. This may have been caused by interaction with the intergalactic medium, which is stripping off the gas via ram pressure. Some filaments of gas that have been stripped away are falling back to the galaxy, making NGC 4921 possibly the first observational evidence of this fallback.

Supernova One supernova has been observed in NGC 4921. On May 4, 1959, Milton Humason discovered SN 1959B (type unknown, mag. 18.5) in this galaxy using a Schmidt telescope at the Palomar Observatory. It appeared "quite far from the center" of the galaxy. The light curve proved similar to supernova SN 1987A in the Large Magellanic Cloud, and it displayed "unusual photometric behavior".

See also List of NGC objects (4001–5000)

References

External links Media related to NGC 4921 at Wikimedia Commons

Illustrations

Worked examples

Example 1 — a first encounter with NGC 4921

Start with the simplest possible case. Write down what NGC 4921 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 4921 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 4921 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 4921

In research: NGC 4921 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 4921 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 4921 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomical objects discovered in 1785, Barred spiral galaxies, Coma Berenices, so understanding it makes those chapters shorter.
In everyday life: Look for NGC 4921 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 4921 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGC 4921 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 4921 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGC 4921 in simple terms?

NGC 4921 is a large barred spiral galaxy in the Coma Cluster, located in the constellation of Coma Berenices. It was discovered by German-British astronomer William Herschel on 11 April 1785.

Why does NGC 4921 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 4921?

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 4921.