astronomy

Robert's Quartet

Robert's Quartet 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 Robert's Quartet rather than just read about it. In short: Robert's Quartet is a compact interacting galaxy group located approximately 160 million light-years (49 Mpc) away in the southern constellation Phoenix. The four member galaxies occupy a radius of roughly 75,000 light-years.

Key takeaways

Robert's Quartet belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Robert's Quartet to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Robert's Quartet from memory before moving on to harder problems.

Reference excerpt

Robert's Quartet is a compact interacting galaxy group located approximately 160 million light-years (49 Mpc) away in the southern constellation Phoenix. The four member galaxies occupy a radius of roughly 75,000 light-years. This makes the entire group smaller across than the disk of the Milky Way. The system has a combined visual magnitude of 13.0. Extreme physical proximity causes severe gravitational disruptions among the members, driving structural distortion and starting a starburst formation within the individual galaxies.

Morphology and Interactions The system consists of four primary galaxies undergoing tidal disruption. NGC 87 is an irregular, gas-rich galaxy containing at least 56 distinct H II regions scattered across its disk. Adjacent to it sits NGC 88, a barred spiral galaxy encased in a diffuse outer gas envelope that has been stripped away by the gravitational pull of its neighbors. NGC 89 operates as an active galaxy, powered by a Seyfert 2 active galactic nucleus. Classified as a barred spiral, it displays heavily distorted spiral arms. The largest component of the group is NGC 92, an unbarred spiral containing over 200 massive H II regions spanning 500–1,500 light-years in diameter. Gravitational shearing forces have drawn out a tail of dust and stars extending over 100,000 light-years away from NGC 92 into the surrounding intergalactic medium.

Discovery and Cataloging English astronomer John Herschel discovered the four galaxies on September 30, 1834, during a southern sky survey conducted from the Cape of Good Hope. Astronomers Halton Arp and Barry F. Madore cataloged the system as AM 0018-485 in 1987 in A Catalogue of Southern Peculiar Galaxies and Associations. They named the group "Robert's Quartet" after Robert Freedman, the astronomer who calculated and updated the positional data for the catalog's peculiar galaxy systems.

See also Wild's Triplet Zwicky's Triplet Stephan's Quintet Seyfert's Sextet Copeland Septet

References

External links ESO: Cosmic Portrait of a Perturbed Family "What's In A Name?"

Robert's Quartet on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

Illustrations

Worked examples

Example 1 — a first encounter with Robert's Quartet

Start with the simplest possible case. Write down what Robert's Quartet 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 Robert's Quartet 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 Robert's Quartet 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 Robert's Quartet

In research: Robert's Quartet 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 Robert's Quartet 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: Robert's Quartet is common in secondary-school and first-year university syllabi. It links to neighbouring topics Galaxy clusters, Phoenix (constellation), Robert's Quartet, so understanding it makes those chapters shorter.
In everyday life: Look for Robert's Quartet 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 Robert's Quartet in 20 minutes

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

Frequently asked questions

What is Robert's Quartet in simple terms?

Why does Robert's Quartet 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 Robert's Quartet?

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 Robert's Quartet.