Sigma Librae

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σ Librae
Libra constellation map.svg
Red circle.svg
Location of σ Librae (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Libra
Right ascension  15h 04m 04.21608s[1]
Declination –25° 16′ 55.0606″[1]
Apparent magnitude (V) 3.20 - 3.46[2]
Spectral type M2.5III[3]
U−B color index +1.94[4]
B−V color index +1.70[4]
Variable type SRb[2]
Radial velocity (Rv)–4.2[5] km/s
Proper motion (μ) RA: –71.16[1] mas/yr
Dec.: −43.34[1] mas/yr
Parallax (π)11.31 ± 0.25[1] mas
Distance288 ± 6 ly
(88 ± 2 pc)
Absolute magnitude (MV)–1.5[6]
Mass2.2[7] M
Radius108[8] R
Luminosity1,820[7] L
Surface gravity (log g)0.9 ± 0.3[9] cgs
Temperature3,596[7] K
Metallicity [Fe/H]0.00[9] dex
Other designations
Brachium, 20 Librae, CD–24 11834, FK5 556, HD 133216, HIP 73714, HR 5603, SAO 183139.,[10] WDS 15041-2517[11]
Database references

Sigma Librae (σ Librae, abbreviated Sigma Lib, σ Lib) is a binary star[11] in the constellation of Libra. The apparent visual magnitude is +3.29,[4] making it visible to the naked eye. Based upon parallax measurements, this system is at a distance of roughly 288 light-years (88 parsecs) from the Sun, with a 2% margin of error.[1] At that distance, the visual magnitude is diminished by 0.20 ± 0.17 from extinction caused by intervening gas and dust.[9]

The two components are designated Sigma Librae A (officially named Brachium /ˈbrkiəm/,[12] the traditional name for the systm)[13] and B.


σ Librae (Latinised to Sigma Librae) is the system's current Bayer designation (the star originally bore the designation Gamma Scorpii) [14] 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.[15]) 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, and adopted by the International Astronomical Union (IAU).[16]

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).[citation needed] In 2016, the IAU organized a Working Group on Star Names (WGSN)[17] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[18] 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.[13]

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.[19] Consequently, the Chinese name for σ Librae is 折威七 (Shé Wēi qī, English: the Seventh Star of Executions).[20]


The primary, Sigma Librae A, has a spectral class M2.5 III,[3] which places it in the red giant stage of its evolution. This is a semi-regular variable star with a single pulsation period of 20 days.[6] 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.[21] 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 along concentric shells surrounding an inert core of carbon and oxygen.[22]

The companion, Sigma Librae B, is of the 16th magnitude and over an arc minute away.[11]


  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357.
  2. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (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. 1. Bibcode:2009yCat....102025S.
  3. ^ a b Keenan, Philip C; McNeil, Raymond C (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. ^ a b c Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode:1978A&AS...34....1N.
  5. ^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35), Bibcode:1999VeARI..35....1W.
  6. ^ a b Yeşilyaprak, C.; Aslan, Z. (December 2004), "Period-luminosity relation for M-type semiregular variables from Hipparcos parallaxes", Monthly Notices of the Royal Astronomical Society, 355 (2): 601–607, Bibcode:2004MNRAS.355..601Y, doi:10.1111/j.1365-2966.2004.08344.x.
  7. ^ a b c Tsuji, T (2008). "Cool luminous stars: The hybrid nature of their infrared spectra". Astronomy and Astrophysics. 489 (3): 1271. arXiv:0807.4387. Bibcode:2008A&A...489.1271T. doi:10.1051/0004-6361:200809869.
  8. ^ Cruzalèbes, P; Jorissen, A; Rabbia, Y; Sacuto, S; Chiavassa, A; Pasquato, E; Plez, B; Eriksson, K; Spang, A; Chesneau, O (2013). "Fundamental parameters of 16 late-type stars derived from their angular diameter measured with VLTI/AMBER". Monthly Notices of the Royal Astronomical Society. 434: 437. arXiv:1306.3288. Bibcode:2013MNRAS.434..437C. doi:10.1093/mnras/stt1037.
  9. ^ a b c Dehaes, S.; et al. (September 2011), "Structure of the outer layers of cool standard stars", Astronomy & Astrophysics, 533: A107, arXiv:0905.1240, Bibcode:2011A&A...533A.107D, doi:10.1051/0004-6361/200912442
  10. ^ "sig Lib -- Semi-regular pulsating Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-14.
  11. ^ a b c "Washington Double Star Catalog". United States Naval Observatory. Archived from the original on 14 February 2011. Retrieved 2 January 2018. Cite uses deprecated parameter |deadurl= (help)
  12. ^ "brachium". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)
  13. ^ a b "Naming Stars". Retrieved 16 December 2017.
  14. ^ Burnham, Robert (1978), Burnham's celestial handbook: an observer's guide to the universe beyond the Solar System, Dover Books on Astronomy, 2 (2nd ed.), Courier Dover Publications, p. 1108, ISBN 0486235688.
  15. ^ "Report of Commissions: Ephemerides", Transactions of the International Astronomical Union, 4: 20, 1932.
  16. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  17. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  18. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  19. ^ (in Chinese) 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.
  20. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 28 日
  21. ^ Ruban, E. V.; Arkharov, A. A. (December 2010), "Microvariability and fast variability of stars. II. The semiregular red giants L2 Pup, 2 Cen, η Gem, and σ Lib", Astrophysics, 53 (4): 523–535, Bibcode:2010Ap.....53..523R, doi:10.1007/s10511-010-9144-7.
  22. ^ Lebzelter, T.; Hron, J. (January 2008), "BRITE stars on the AGB", Communications in Asteroseismology, 152: 178–181, Bibcode:2008CoAst.152..178L, doi:10.1553/cia152s178

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