The two components are designated Sigma Librae A (officially named Brachium/ˈbreɪkiəm/,[14] the traditional name for the system)[15] and B.
Nomenclature
σ Librae (
Latinised to Sigma Librae) is the system's current
Bayer designation (the star originally bore the designation Gamma Scorpii[16] 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.[17]) 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).[18]
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)[19] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire
multiple systems.[20] 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.[15]
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.[7] 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.[23] 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.[24]
The companion, Sigma Librae B, is of the 16th magnitude and over an
arc minute away.[13]
Notes
^Luminosity calculated from Sigma Librae's
absolute bolometric magnitude, with respect to the Sun's absolute bolometric magnitude of 4.74: 100.4 • (4.74−(−3.4)) = 1,803.02
L☉.
^
abSamus, 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.
^
abcNicolet, 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.
^Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1,
Bibcode:
1999VeARI..35....1W.
^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].