Sigma Librae


Sigma Librae is a star in the constellation of Libra. It has the proper name Brachium, 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 from the Sun, with a 2% margin of error. At that distance, the visual magnitude is diminished by from extinction caused by intervening gas and dust.

Nomenclature

σ Librae is the star's current Bayer designation. It can be abbreviated as or. 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 on July 31, 1930.
It bore the traditional Latin names Brachium and Cornu, and the non-unique minor Arabic names Zuben el Genubi ; Zuben Hakrabi, and Ankaa. In 2016, the IAU organized a Working Group on Star Names 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, meaning Executions , 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.
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. has around 20% the mass of the Sun.