astronomy

RR Lyrae

RR Lyrae 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 RR Lyrae rather than just read about it. In short: RR Lyrae is a variable star in the Lyra constellation, figuring in its west near to Cygnus. As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astronomers.

Key takeaways

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

Reference excerpt

RR Lyrae is a variable star in the Lyra constellation, figuring in its west near to Cygnus. As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astronomers. RR Lyrae variables serve as important standard candles that are used to measure astronomical distances. The period of pulsation of an RR Lyrae variable depends on its mass, luminosity and temperature, while the difference between the measured luminosity and the actual luminosity allows its distance to be determined via the inverse-square law. Hence, understanding the period-luminosity relation for a local set of such stars allows the distance of more distant stars of this type to be determined.

History The variable nature of RR Lyrae was discovered by the Scottish astronomer Williamina Fleming at Harvard Observatory in 1901. The distance of RR Lyrae remained uncertain until 2002 when the Hubble Space Telescope's fine guidance sensor was used to determine the distance of RR Lyrae within a 5% margin of error, yielding a value of 262 parsecs (855 light-years). When combined with measurements from the Hipparcos satellite and other sources, the result is a distance estimate of 258 pc (841 ly).

Variable star class

This type of low-mass star has consumed the hydrogen at its core, evolved away from the main sequence, and passed through the red giant stage. Energy is now being produced by the thermonuclear fusion of helium at its core, and the star has entered an evolutionary stage called the horizontal branch (HB). The effective temperature of an HB star's outer envelope will gradually increase over time. When its resulting stellar classification enters a range known as the instability strip—typically at stellar class A—the outer envelope can begin to pulsate. RR Lyrae shows just such a regular pattern of pulsation, which is causing its apparent magnitude to vary between 7.06 and 8.12 over a short cycle lasting 0.567 days (13 hours, 36 minutes). Each radial pulsation causes the radius of the star to vary between 5.1 and 5.6 times the Sun's radius. This star belongs to a subset of RR Lyrae-type variables that show a characteristic behavior called the Blazhko effect, named after Russian astronomer Sergey Blazhko. This effect is observed as a periodic modulation of a variable star's pulsation strength or phase; sometimes both. It causes the light curve of RR Lyrae to change from cycle to cycle. In 2014, Time-series photometric observations demonstrated the physical origin of this effect.

Other stellar classifications As with other RR Lyrae-type variables, RR Lyrae itself has a low abundance of elements other than hydrogen and helium – what astronomers term its metallicity: It belongs to the Population II category of stars that formed during the early period of the Universe when there was a lower abundance of metals in star-forming regions. The trajectory of this star is carrying it along an orbit that is close to the plane of the Milky Way, taking it no more than 680 ly (210 pc) above or below this plane. The Blazhko period for RR Lyrae is 39.1 ± 0.3 days. The orbit has a high eccentricity, bringing RR Lyrae as close as 6.80 kly (2.08 kpc) to the Galactic Center at periapsis, and taking it as far as 59.9 kly (18.4 kpc) at apapsis.

References

External links Kaler, James B., "RR LYR (RR Lyrae)", Stars, University of Illinois, retrieved 2012-01-08 Kaler, James B. (2002), The hundred greatest stars, Copernicus Series, Springer, p. 163, ISBN 0-387-95436-8 image RR Lyrae

Illustrations

Worked examples

Example 1 — a first encounter with RR Lyrae

Start with the simplest possible case. Write down what RR Lyrae 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 RR Lyrae 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 RR Lyrae 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 RR Lyrae

In research: RR Lyrae 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 RR Lyrae 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: RR Lyrae is common in secondary-school and first-year university syllabi. It links to neighbouring topics A-type giants, Astronomical objects discovered in 1901, Discoveries by Williamina Fleming, so understanding it makes those chapters shorter.
In everyday life: Look for RR Lyrae 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 RR Lyrae in 20 minutes

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

Frequently asked questions

What is RR Lyrae in simple terms?

Why does RR Lyrae 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 RR Lyrae?

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 RR Lyrae.