Sigma Librae

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title: "Sigma Librae" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["libra-(constellation)", "bayer-objects", "m-type-giants", "stars-with-proper-names", "flamsteed-objects", "semiregular-variable-stars", "hipparcos-objects", "henry-draper-catalogue-objects", "bright-star-catalogue-objects", "durchmusterung-objects"] description: "Star in the constellation Libra" topic_path: "science/astronomy" source: "https://en.wikipedia.org/wiki/Sigma_Librae" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Star in the constellation Libra ::

| name = σ Librae | image= |image=Libra constellation map.svg|alt=|float=center|width=260 |label=|position=right |mark=Red circle.svg|mark_width=10|mark_link=σ Lib |x=556|y=698 |caption=Location of σ Librae (circled) | epoch = J2000.0 | equinox = J2000.0 (ICRS) | constell = Libra | ra = | dec = | appmag_v = 3.20 - 3.46 | type = Asymptotic giant branch | class = M2.5III | b-v = +1.70 | u-b = +1.94 | r-i = | variable = SRb | radial_v = −4.2 | prop_mo_ra = | prop_mo_dec = | pm_footnote = | parallax = 12.539 | p_error = 0.3 | parallax_footnote = | dist_pc = | dist_ly = | dist_footnote = | absmag_v = −1.5 | mass = | radius = | luminosity = 1,800 | temperature = 3,596 | gravity = | metal_fe = 0.00 | names = Brachium, , |Simbad=sig+Lib 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 260 ly 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 (Latinised to Sigma 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 (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, ).

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

::figure[src="https://upload.wikimedia.org/wikipedia/commons/2/21/SigmaLibLightCurve.png" caption="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 102800 au. has around 20% the mass of the Sun.

Notes

References

| title=The HIPPARCOS and TYCHO catalogues | author=EAS | journal=Astrometric and Photometric Star Catalogues Derived from the ESA Hipparcos Space Astrometry Mission | publication-place=Noordwijk, Netherlands | publisher=ESA Publications Division | year=1997 | series=ESA SP Series | volume=1200 | isbn=9290923997 | bibcode=1997HIP...C......0E | url=https://cdsarc.cds.unistra.fr/viz-bin/ftp-index?/ftp/cats/more/HIP/cdroms/cats | access-date=15 October 2022}}

References

1. {{OED. brachium
2. "IAU Working Group on Star Names (WGSN)".
3. "Naming Stars". IAU.org.
4. {{in lang. zh ''中國星座神話'', written by 陳久金. Published by 台灣書房出版有限公司, 2005, {{ISBN. 978-986-7332-25-7.
5. {{in lang. zh [http://aeea.nmns.edu.tw/2006/0606/ap060628.html AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 28 日] {{Webarchive. link. (2011-07-16)
6. http://www.ianridpath.com/startales/solitaire.html Ian Ridpath's Star Tales - Turdus Solitarius]
7. {{Cite DR3. 6227443304915069056
8. {{cite constellation. HR 5603
9. (January 2022). "Stellar and substellar companions from Gaia EDR3. Proper-motion anomaly and resolved common proper-motion pairs". Astronomy and Astrophysics.
10. (2008). "Cool luminous stars: The hybrid nature of their infrared spectra". Astronomy and Astrophysics.
11. (2013). "Fundamental parameters of 16 late-type stars derived from their angular diameter measured with VLTI/AMBER". Monthly Notices of the Royal Astronomical Society.
12. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S.
13. (1999). "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions". Astronomisches Rechen-Institut Heidelberg.
14. (1978). "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System". Astronomy and Astrophysics Supplement Series.
15. (December 2004). "Period-luminosity relation for M-type semiregular variables from Hipparcos parallaxes". [[Monthly Notices of the Royal Astronomical Society]].
16. (December 2010). "Microvariability and fast variability of stars. II. The semiregular red giants L2 Pup, 2 Cen, η Gem, and σ Lib". Astrophysics.
17. "sig Lib -- Semi-regular pulsating Star". Centre de Données astronomiques de Strasbourg.
18. (1978). "Burnham's celestial handbook: an observer's guide to the universe beyond the Solar System". Courier Dover Publications.
19. (1932). "Report of Commissions: Ephemerides". Transactions of the International Astronomical Union.
20. (January 2008). "BRITE stars on the AGB". Communications in Asteroseismology.
21. (September 2011). "Structure of the outer layers of cool standard stars". Astronomy & Astrophysics.
22. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series.
23. "Washington Double Star Catalog". United States Naval Observatory.
24. (2021-03-01). "Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3". The Astronomical Journal.
25. (2015). "Exploring masses and CNO surface abundances of red giant stars". Monthly Notices of the Royal Astronomical Society.

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