astronomy

R Doradus

R Doradus 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 R Doradus rather than just read about it. In short: R Doradus (HD 29712 or P Doradus) is a red giant variable star in the far-southern constellation Dorado, close to the border with Reticulum. Its distance from Earth is 178 light-years (55 parsecs).

Key takeaways

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

Reference excerpt

R Doradus (HD 29712 or P Doradus) is a red giant variable star in the far-southern constellation Dorado, close to the border with Reticulum. Its distance from Earth is 178 light-years (55 parsecs). Having a uniform disk diameter of 57±5 mas, it is thought to be the extrasolar star with the largest apparent size as viewed from Earth.

Variability

The visible magnitude of R Doradus varies between 4.8 and 6.3, which means it is usually visible to the naked eye, but in the infrared it is one of the brightest stars in the sky. With a near-infrared J band magnitude of −2.65, only Betelgeuse and Antares at −2.9 and −2.73 (respectively) are brighter. In the infrared K band, it is sometimes the brightest star in the sky, although usually Betelgeuse is brighter. It is classified as a semiregular variable star of type SRb, indicating giants with slow poorly defined variations, often alternating between periodic and irregular brightness changes. Some studies show it alternating between periods of about 175 and 332 days, and a period of 117.3 days has also been identified. It has been likened to a Mira variable when its variations are relatively regular, although its amplitude of only 1.5 magnitudes is smaller than Mira variables. The star was discovered to be variable in 1874 by Benjamin Gould, and received the variable-star designation R Doradus.

Angular diameter The angular diameter of R Doradus is easily measured using interferometry. Its uniform disc diameter, the diameter when interpreted as a disc of uniform brightness, when viewed at 1.25 μm is 57±5 mas. When viewed at 2.3 μm and interpreted as a limb-darkened disc, the diameter is 51.18±2.24 mas. The angular diameter of R Doradus is larger than any other measured star other than the Sun. The angular diameter of the next-largest star, Betelgeuse, is around 45 mas.

Properties The Hipparcos parallax of R Doradus is 18.31±0.99 mas, corresponding to a distance of 55±3 pc. The bolometric luminosity of R Doradus, derived from its bolometric flux at a distance of 55 pc, is 4,350±520 L☉. The measured angular diameter, again assuming a distance of 55 pc gives a radius of 298±21 R☉. The angular diameter and bolometric flux of R Doradus derive a cool surface effective temperature of 2,710±170 K. Comparison of its properties with theoretical evolutionary tracks gives an age of between 6 and 14 billion years. R Doradus has lost part of its mass during its evolution, and currently has a mass of either 0.7 or 1.0 M☉. Its initial mass would be either 1 or 1.25 M☉. Because of the enlarged surface and low mass, R Doradus has a surface gravity of only 0.026% that of Earth. It is on the asymptotic giant branch having exhausted helium at its core. The radius of 298 R☉ means that the diameter of R Doradus is 415 million km (2.77 AU). If placed at the centre of the Solar System, the perihelion of Mars would be within the star. R Doradus has a projected equatorial rotation velocity of 1±0.1 km/s. It is calculated to take 57.5 years to rotate once on its axis. While such value is very slow if compared to stars like the Sun, it is more than 100 times faster for a typical AGB star, suggesting R Doradus may have a close companion modifying its rotation. Using ALMA facilities, researchers at Chalmers University, in July to August 2023, were able to record the movement of hot gas bubbles on the surface of the star. Such bubbles, witness of the convective activity linked to deep nuclear fusion, would have a life of about a month, and a size more than 75 times that of the Sun.

See also List of stars with resolved images

Notes

References

External links Swinburne Astronomy Online; information about R Doradus Variáveis Binoculares The 3μ spectrum of R Doradus observed with the ISO-SWS

Illustrations

Worked examples

Example 1 — a first encounter with R Doradus

Start with the simplest possible case. Write down what R Doradus 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 R Doradus 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 R Doradus 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 R Doradus

In research: R Doradus 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 R Doradus 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: R Doradus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bayer objects, Bright Star Catalogue objects, Dorado, so understanding it makes those chapters shorter.
In everyday life: Look for R Doradus 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 R Doradus in 20 minutes

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

Frequently asked questions

What is R Doradus in simple terms?

R Doradus (HD 29712 or P Doradus) is a red giant variable star in the far-southern constellation Dorado, close to the border with Reticulum. Its distance from Earth is 178 light-years (55 parsecs).

Why does R Doradus 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 R Doradus?

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 R Doradus.