Sigma Librae (σ Librae, abbreviated Sigma Lib, σ Lib) is a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Libra Libra generally refers to: * Libra (constellation), a constellation * Libra (astrology), an astrological sign based on the star constellation Libra may also refer to: Arts and entertainment * ''Libra'' (novel), a 1988 novel by Don DeLillo Musi ...

. The apparent visual magnitude is +3.29, making it visible to the naked eye. Based upon

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

measurements, this system is at a distance of roughly 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. The two components are designated Sigma Librae A (officially named Brachium , the traditional name for the system) and B.

Nomenclature

''σ Librae'' ( Latinised to ''Sigma Librae'') is the system's current

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...

(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.) The designations of the two components as ''Sigma Librae A'' and ''B'' derives from the convention used by the Washington Multiplicity Catalog (WMC) for

multiple star systems A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking, ...

, and adopted by the International Astronomical Union (IAU). 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 Alpha Phoenicis (α Phoenicis, abbreviated Alpha Phe or α Phe), formally named Ankaa (distinguish Ancha, with the same pronunciation) is the brightest star in the constellation of Phoenix (constellation), Phoenix. Nomenclature ''Alp ...

). In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. It approved the name ''Brachium'' for the primary component Sigma Librae A 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 Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of va ...

, (), meaning ''

Executions Capital punishment, also known as the death penalty, is the state-sanctioned practice of deliberately killing a person as a punishment for an actual or supposed crime, usually following an authorized, rule-governed process to conclude that t ...

(asterism)'', refers to an asterism consisting of σ Librae, 50 Hydrae, 3 Librae, 4 Librae and

12 Librae 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1. I ...

. Consequently, the Chinese name for σ Librae is (, en, the Seventh Star of Executions).AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 28 日
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Properties

The primary, Sigma Librae A, has a spectral class M2.5 III, which places it in the

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

stage of its evolution. This is a semi-regular 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 The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen. The companion, Sigma Librae B, is of the 16th magnitude and over an arc minute away.

References

External links

* {{DEFAULTSORT:Sigma Librae Libra (constellation) Librae, Sigma M-type giants Brachium Librae, 20 Semiregular variable stars 073714 133216 5603 Durchmusterung objects