Alpha Librae

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Alpha Librae
Diagram showing star positions and boundaries of the constellation Libra and its surroundings
Cercle rouge 100%.svg

Location of α Librae (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Libra
α1 Lib
Right ascension 14h 50m 41.18097s[1]
Declination –15° 59′ 50.0482″[1]
Apparent magnitude (V) +5.153[2]
α2 Lib
Right ascension 14h 50m 52.71309s[1]
Declination –16° 02′ 30.3955″[1]
Apparent magnitude (V) +2.741[2]
α1 Lib
Spectral type F3 V[3]
U−B color index –0.02[4]
B−V color index +0.39[4]
α2 Lib
Spectral type kA2hA5mA4 IV-V[5]
U−B color index +0.10[4]
B−V color index +0.15[4]
α1 Lib
Radial velocity (Rv) 23.8[6] km/s
Proper motion (μ) RA: –136.27[1] mas/yr
Dec.: –59.04[1] mas/yr
Parallax (π) 43.52 ± 0.43[1] mas
Distance 74.9 ± 0.7 ly
(23.0 ± 0.2 pc)
Absolute magnitude (MV) +3.35[7]
α2 Lib
Proper motion (μ) RA: –105.68[1] mas/yr
Dec.: –68.40[1] mas/yr
Parallax (π) 43.03 ± 0.19[1] mas
Distance 75.8 ± 0.3 ly
(23.2 ± 0.1 pc)
Period (P) 70.34 days
Semi-major axis (a) 0.51 au
Eccentricity (e) 0.41
Semi-amplitude (K2)
43.1 km/s
α1 Lib
Mass 1.97 + 1.60[6] M
Surface gravity (log g) 4.25[5] cgs
Temperature 6,653[7] K
Metallicity [Fe/H] –0.07[7] dex
Rotational velocity (v sin i) 5.95[8] km/s
Age 1.1+0.6
[7] Gyr
α2 Lib
Mass 1.4–1.5/0.5–0.6[9] M
Surface gravity (log g) 3.91[5] cgs
Temperature 8128[5] K
Other designations
α Librae, α Lib, alf Lib, Kiffa Australis, Lanx australis, Zubenelgenubi.
α1 Lib: 8 Librae, BD–15 3965, FK5 1387, HD 130819, HIP 72603, HR 5530, SAO 158836.[10]
α2 Lib: 9 Librae, BD–15 3966, FK5 548, HD 130841, HIP 72622, HR 5531, SAO 158840.[11]
Database references
α1 Lib
α2 Lib

Alpha Librae (α Librae, abbreviated Alpha Lib, α Lib), is a double star and despite its 'alpha' designation the second-brightest star in the constellation of Libra. The two components are designated α¹ Librae and α² Librae. The system bore the traditional name of Zubenelgenubi, though the International Astronomical Union now regards that name as only applying to α² Librae.[12]

Alpha² Librae is 0.33 degree from the ecliptic so it can be easily occulted by the Moon and (very rarely) by planets. The next occultation by a planet will be by Mercury on 10 November 2052.[13]


α Librae (Latinised to Alpha Librae) is the system's Bayer designation.

Zubenelgenubi /ˌzbənɛləˈnbi/, also rendered Zuben Elgenubi, derives from the Arabic ّالزُبَانَى الجَنُوبِي al-zubānā al-janūbiyy "the southern claw", which was coined before Libra was recognized as a constellation distinct from Scorpius. The alternative name Kiffa Australis (Elkhiffa Australis) is a partial Latin translation of the Arabic al-kiffah al-janūbiyy "southern pan (of the scales)". Another name used in older astronomy texts, equivalent to "southern pan", was Lanx Australis.[14]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[15] to catalogue and standardize proper names for stars. The WGSN approved the name Zubenelgenubi for α² Librae on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[12]


Alpha Librae is about 77 light-years (24 parsecs) from the Sun. The two brightest components of Alpha Librae form a double star moving together through space as common proper motion companions. The brightest member, α2 Librae, is itself a spectroscopic binary system. The second member, α1 Librae, is separated from the primary system by around 5400 AU. It too is a spectroscopic binary with an orbital period of 5,870 days and an angular separation of 0.383 arcseconds; equal to about 10 AU. The system may have a fifth component, the star KU Librae at a separation of 2.6°, thus forming a hierarchical quintuple star system. KU Lib shares a similar motion through space to the Alpha Librae system, but is separated from the other stars by about a parsec. It is sufficiently close to be gravitationally bound to the other members,[9] but has a substantially different metallicity.[16]

The two brightest members of Alpha Librae are separated in the sky by an angular distance of 231" (3'51"). The position angle of the companion is 314 degrees. The brighter of the two is a white star of spectral type A3, with an apparent brightness of 2.8. Its companion is a type F4 star of apparent brightness 5.2. They are probably members of the Castor Moving Group of stars that have a similar motion through space and share a common origin some 200 million years ago.[9]

See also[edit]


  1. ^ a b c d e f g h i j van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b Gutierrez-Moreno, Adelina; et al. (1966), "A System of photometric standards", Publ. Dept. Astron. Univ. Chile, Publicaciones Universidad de Chile, Department de Astronomy, 1: 1–17, Bibcode:1966PDAUC...1....1G 
  3. ^ Houk, N.; Smith-Moore, M. (1988). "Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4, Declinations -26°.0 to -12°.0". Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4. Bibcode:1988MSS...C04....0H. 
  4. ^ a b c d Mermilliod, J.-C. (1986). "Compilation of Eggen's UBV data, transformed to UBV (unpublished)". Catalogue of Eggen's UBV data. Bibcode:1986EgUBV........0M. 
  5. ^ a b c d Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770Freely accessible, Bibcode:2006AJ....132..161G, doi:10.1086/504637 
  6. ^ a b c Fuhrmann, K.; Chini, R.; Barr, A.; Buda, L.-S.; Kaderhandt, L.; Pozo, F.; Ramolla, M. (2014). "On the bright A-type star Alpha Librae A". Monthly Notices of the Royal Astronomical Society. 437 (3): 2303. Bibcode:2014MNRAS.437.2303F. doi:10.1093/mnras/stt2046. 
  7. ^ a b c d Holmberg, J.; Nordström, B.; Andersen, J. (July 2009), "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics", Astronomy and Astrophysics, 501 (3): 941–947, arXiv:0811.3982Freely accessible, Bibcode:2009A&A...501..941H, doi:10.1051/0004-6361/200811191 
  8. ^ Martínez-Arnáiz, R.; et al. (September 2010), "Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter", Astronomy and Astrophysics, 520: A79, arXiv:1002.4391Freely accessible, Bibcode:2010A&A...520A..79M, doi:10.1051/0004-6361/200913725 
  9. ^ a b c Caballero, J. A. (May 2010), "Reaching the boundary between stellar kinematic groups and very wide binaries. II. α Librae + KU Librae: a common proper motion system in Castor separated by 1.0 pc", Astronomy and Astrophysics, 514: A98, arXiv:1001.5432Freely accessible, Bibcode:2010A&A...514A..98C, doi:10.1051/0004-6361/200913986 
  10. ^ "HD 130819 -- Star in Double System", SIMBAD Astronomical Object Database, Centre de Données astronomiques de Strasbourg 
  11. ^ "HD 130841 -- Variable Star", SIMBAD Astronomical Object Database, Centre de Données astronomiques de Strasbourg, retrieved 2007-01-23 
  12. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  13. ^ Peuschel, Marco (2003). "Astronomische Ereignisse der besonderen Art". Retrieved July 10, 2005.
  14. ^ Philippe La Hire, "Tabulae Astronomicae" (1727), see star table, page 13.
  15. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016. 
  16. ^ Fuhrmann, K.; Chini, R. (2015). "Multiplicity among F-type stars. II". The Astrophysical Journal. 809: 107. Bibcode:2015ApJ...809..107F. doi:10.1088/0004-637X/809/1/107. 

External links[edit]

Coordinates: Sky map 14h 50m 41.26s, −15° 59′ 49.5″