astronomy

Sigma Coronae Borealis

Sigma Coronae Borealis 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 Sigma Coronae Borealis rather than just read about it. In short: Sigma Coronae Borealis is a multiple star system in the northern constellation of Corona Borealis. Its name is a Bayer designation that is Latinized from σ Coronae Borealis and abbreviated Sigma CrB or σ CrB.

Key takeaways

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

Reference excerpt

Sigma Coronae Borealis is a multiple star system in the northern constellation of Corona Borealis. Its name is a Bayer designation that is Latinized from σ Coronae Borealis and abbreviated Sigma CrB or σ CrB. This is a quintuple star system containing three sunlike main-sequence stars and two other low-mass stars. The combined visual magnitude is 5.3 and the system lies 74 light years from Earth. σ CrB A is the variable star TZ Coronae Borealis.

System components

The brightest components of Sigma Coronae Borealis form a visual binary with an angular separation of 7 arcsecond first resolved in the 19th century, and are designated σ Corona Borealis A and B. More recently, the designations σ2 and σ1 Corona Borealis have come into use. Somewhat confusingly, the brighter component A is referred to as σ2 because it has the higher right ascension. A third component, while being separated by 635″ (translating to a minimum distance of 14,000 au), has a similar parallax and proper motion to the brighter stars and is physically associated. It is known in the Washington Double Star Catalog (WDS), a compilation of observations of double stars, as component E, but it is usually called Sigma Coronae Borealis C. Sigma1 Corona Borealis is a G-type main-sequence star like the Sun, and has similar parameters: a mass roughly equal to that of the Sun, and an effective temperature of 5950 K. A visual orbit has been calculated, with a period of about 730 years and a high eccentricity of 0.72. Sigma2 Corona Borealis itself is a close binary. Here, the two stars are extremely close and orbit fairly quickly, every 1.14 days. This tiny separation of only 0.0279 au has allowed the two stars to exert tidal forces on each other, leading to synchronization of their rotation. They have also been classified as RS Canum Venaticorum variables (RS CVn)—young, active stars that show variability in their apparent magnitude due to starspots on their surfaces. Despite Sigma2 Corona Borealis's two stars being separated only by about the diameter of each star, they were resolved using the CHARA optical interferometer at the Mount Wilson Observatory. As of 2006, it is the shortest-period binary ever to be resolved. The primary is 13.7% more massive than the Sun, while the secondary is 9.0% more massive than the Sun, and both are 24.4% wider than the Sun. σ Coronae Borealis C, also known as HIP 79551, appears as a red dwarf with a spectral type of M2.5V. It too is a binary star, with a companion in a 52-year orbit. The companion has a mass of 0.10 M☉ and has been detected through astrometry.

Optical companions The Washington Double Star Catalog (WDS), a compilation of observations of double stars, lists several components to the main system. Two of those are listed in the WDS as components C, and D. As of 1984, component C was separated from the primary by 18″ along a position angle of 103° and as of 1996, component D was separated from the primary by 88″ along a position angle of 82°. However, both of them have different proper motions through space and are not related, just optical alignments.

Variability

The spectroscopic binary σ2 CrB is an RS Canum Venaticorum variable. It varies in brightness by 0.05 magnitudes every 1.139789 days, the same as the orbital period. The brightness changes are caused by variations in surface brightness on the stars, effectively giant sunspots. Variable star designations are not given to stars with Bayer designations, but in this case only one component of σ Coronae Borealis is identified as variable, so it has the designation TZ Coronae Borealis.

See also Xi Ursae Majoris, another quintuple containing an RS CVn binary

References

External links Kaler, James B. "Sigma Coronae Borealis". Stars. University of Illinois. Retrieved 2026-07-19.

Illustrations

Worked examples

Example 1 — a first encounter with Sigma Coronae Borealis

Start with the simplest possible case. Write down what Sigma Coronae Borealis 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 Sigma Coronae Borealis 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 Sigma Coronae Borealis 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 Sigma Coronae Borealis

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

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

Frequently asked questions

What is Sigma Coronae Borealis in simple terms?

Why does Sigma Coronae Borealis 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 Sigma Coronae Borealis?

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 Sigma Coronae Borealis.