Sigma Librae

σ Librae

Template:Location mark Location of σ Librae (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Libra
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	3.20 - 3.46
Characteristics
Evolutionary stage	Asymptotic giant branch
Spectral type	M2.5III
U−B Template:Engvar	+1.94
B−V Template:Engvar	+1.70
Variable type	SRb
Astrometry
Radial velocity (Rv)	−4.2 km/s
Proper motion (μ)	RA: Template:Val mas/yr Dec.: Template:Val mas/yr
Parallax (π)	12.539±0.3 mas
Distance	Template:Val ly (Template:Val pc)
Absolute magnitude (MV)	−1.5
Details
Mass	Template:Val Template:Solar mass
Radius	Template:Val Template:Solar radius
Luminosity	1,800Template:Efn Template:Solar luminosity
Surface gravity (log g)	Template:Val cgs
Temperature	3,596 K
Metallicity [Fe/H]	0.00 dex
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
Brachium, Template:Odlist, Template:Odlist
Database references
SIMBAD	data

Sigma Librae is a star in the constellation of Libra. It has the proper name Brachium, Template:Nobr 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 Template:Convert from the Sun, with a 2% margin of error. At that distance, the visual magnitude is diminished by Template:Val 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 Template:Nobr or Template:Nobr. 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, (), 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 (, Template:Langx).

Properties

File:SigmaLibLightCurve.png

A light curve for Sigma Librae, plotted from Hipparcos data

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 Template:Convert. Template:Nobr has around 20% the mass of the Sun.