astronomy

Sigma Librae

Sigma Librae 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 Librae rather than just read about it. In short: Sigma Librae is a star in the constellation of Libra. It has the proper name Brachium, Sigma Librae is its Bayer designation.

Key takeaways

Sigma Librae 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 Librae to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sigma Librae from memory before moving on to harder problems.

Reference excerpt

Sigma Librae is a star in the constellation of Libra. It has the proper name Brachium, Sigma Librae is its Bayer designation. The apparent visual magnitude is +3.29, making it visible to the naked eye. Based upon parallax measurements, this star is at a distance of roughly 260 light-years (80 parsecs) from the Sun, with a 2% margin of error. At that distance, the visual magnitude is diminished by 0.20±0.17 from extinction caused by intervening gas and dust.

Nomenclature σ Librae (Latinised to Sigma Librae) is the star's current Bayer designation. It can be abbreviated as Sigma Lib or σ Lib. The star originally bore the designation Gamma Scorpii and did not receive its current designation until the new designation was agreed upon by Commission 3 of the International Astronomical Union (IAU) on July 31, 1930. It bore the traditional Latin names Brachium (arm) and Cornu (horn), and the non-unique minor Arabic names Zuben el Genubi (southern claw) (shared with Alpha Librae); Zuben Hakrabi (shared with Gamma Librae and Eta Librae, also rendered as Zuban Alakrab), and Ankaa (shared with Alpha Phoenicis). In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. It approved the name Brachium on 5 September 2017. Ankaa had previously been approved as the name for Alpha Phoenicis on 29 July 2016. Both are now so included in the List of IAU-approved Star Names. In Chinese, 折威 (Shé Wēi), meaning Executions (asterism), refers to an asterism consisting of σ Librae, 50 Hydrae, 3 Librae, 4 Librae and 12 Librae. Consequently, the Chinese name for σ Librae is 折威七 (Shé Wēi qī, English: the Seventh Star of Executions). Sigma Librae was the brightest star in the obsolete constellation Turdus Solitarius, and was designated γ Turdii Solitarii by Le Monnier due to its former alias of γ Sco.

Properties

The spectrum of this star matches a spectral class of M2.5 III, which places it in the red giant stage of its evolution. This is a semiregular variable star with a single pulsation period of 20 days. It shows small amplitude variations in magnitude of 0.10–0.15 on time scales as brief as 15–20 minutes, with cycles of repetition over intervals of 2.5–3.0 hours. This form of variability indicates that the star is on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium within concentric shells surrounding an inert core of carbon and oxygen. Sigma Librae has a companion, Sigma Librae B, a 16th magnitude star separated by 102.8". It has similar proper motions with the primary and thus may form a binary star system, but this has not been confirmed yet. If a binary system, the physical separation is at least 102,800 au (1.626 ly; 0.498 pc). σ Lib B has around 20% the mass of the Sun.

Notes

References

External links Kaler, James B., "ZUBENHAKRABI (Sigma Librae = Gamma Scorpii)", Stars, University of Illinois, retrieved 2012-03-22

Illustrations

Worked examples

Example 1 — a first encounter with Sigma Librae

Start with the simplest possible case. Write down what Sigma Librae 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 Librae 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 Librae 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 Librae

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

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

Frequently asked questions

What is Sigma Librae in simple terms?

Sigma Librae is a star in the constellation of Libra. It has the proper name Brachium, Sigma Librae is its Bayer designation.

Why does Sigma Librae 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 Librae?

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 Librae.