astronomy

Polar-ring galaxy

Polar-ring galaxy 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 Polar-ring galaxy rather than just read about it. In short: A polar-ring galaxy is a type of galaxy with an outer ring of gas and stars that rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other.

Key takeaways

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

Reference excerpt

A polar-ring galaxy is a type of galaxy with an outer ring of gas and stars that rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that a material is tidally stripped from a passing galaxy to produce the polar ring. The other possibility is that a smaller galaxy collides orthogonally with the plane of rotation of the larger galaxy, with the smaller galaxy effectively forming the polar-ring structure. The best-known polar-ring galaxies are S0s (lenticular galaxies), but from the physical point of view they are part of a wider category of galaxies, including several ellipticals. The first four S0 galaxies that were identified as polar-ring galaxies were NGC 2685, NGC 4650A, A 0136 -0801, and ESO 415 -G26. While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified. Polar-ring S0 galaxies may be found around 0.5% of all nearby lenticular galaxies, and it is possible that 5% of lenticular galaxies may have had polar rings at some point during their lifetimes. The first polar-ring elliptical galaxies were identified in 1978. They were NGC 5128, NGC 5363, NGC 1947 and Cygnus A, while the polar-ring S0 galaxies NGC 2685 and NGC 4650A were at that time indicated as resulting from similar formation processes. Only some years later, when the first observations of the stellar and gas motion of polar-ring elliptical and S0 galaxies were possible with a better spectroscopic technology, the external origin of the gaseous rings was clarified. In addition to the best-known example, NGC 5128 (Cen A), a very regular polar ring elliptical, is NGC 5266.

Polar-ring Catalogue The Polar-Ring Catalogue, an atlas of 157 possible polar-ring galaxies, with 51 related objects. Four sections (A, B, C, and D) exist, including kinematically confirmed polar-ring galaxies (category A), good candidates based on their morphological appearance (category B), possible candidates (category C), and possibly related objects (category D). There are 6 galaxies in section A which are kinematically confirmed galaxies. There are 27 in section B, which are good candidates. In section C, there are 73 galaxies, which are possible candidates. There are 51 galaxies in section D, which are related objects, probably not classified as polar-ring galaxies.

SDSS-based Polar-ring Catalogue In 2011, a new catalogue of polar-ring galaxy candidates was published using images from the Sloan Digital Sky Survey, supplementing the PRC and significantly increasing the number of known candidate PRGs. The catalog is based on the results of the original Galaxy Zoo project. With 275 objects being included in the catalog, 70 objects have been identified as "best candidates and 53 galaxies are categorized as PRG-related objects, primarily consisting of galaxies with highly twisted discs and mergers. Furthermore, 37 galaxies with their assumed polar rings highly tilted towards the observer (appearing nearly directly in front). On average, the SPRC objects are dimmer and positioned at a greater distance compared to the galaxies from the PRC. However, the list does contain numerous new nearby potential PRGs. In 2019, 31 new polar-ring galaxy candidates were found with the use of the Sloan Digital Sky Survey.

Gallery

See also List of polar-ring galaxies Ring galaxy – Galaxy with an annular appearance

References

External links Astronomy Picture of the Day Polar Ring Galaxy NGC 4650A - May 10, 1999 Polar Ring Galaxy NGC 2685 - 2007 February 16 Polar Ring Galaxy NGC 660 - 2009 December 3 Internet Voters Get Two Galaxies in One from Hubble X marks the spot in dark matter web - Polar ring galaxies offer first-hand evidence of the existence of the cosmic web, New Scientist, 29 February 2008

Illustrations

Worked examples

Example 1 — a first encounter with Polar-ring galaxy

Start with the simplest possible case. Write down what Polar-ring galaxy 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 Polar-ring galaxy 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 Polar-ring galaxy 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 Polar-ring galaxy

In research: Polar-ring galaxy 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 Polar-ring galaxy 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: Polar-ring galaxy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Morphological types of galaxy, Polar-ring galaxies, so understanding it makes those chapters shorter.
In everyday life: Look for Polar-ring galaxy 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 Polar-ring galaxy in 20 minutes

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

Frequently asked questions

What is Polar-ring galaxy in simple terms?

Why does Polar-ring galaxy 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 Polar-ring galaxy?

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 Polar-ring galaxy.