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Alpha Librae

Coordinates: Sky map 14h 50m 41.26s, −15° 59′ 49.5″
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α Librae
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)−23.47[6] km/s
Proper motion (μ) RA: −136.27[1] mas/yr
Dec.: −59.04[1] mas/yr
Parallax (π)43.52 ± 0.43 mas[1]
Distance74.9 ± 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 mas[1]
Distance75.8 ± 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
Mass1.4–1.5/0.5–0.6[8] M
Radius1.409[9][a] R
Surface gravity (log g)4.25[5] cgs
Temperature6,653[7] K
Metallicity [Fe/H]−0.07[7] dex
Rotational velocity (v sin i)5.95[10] km/s
[7] Gyr
α2 Lib
Mass1.95 + 1.79[11] M
Radius2.8[9][b] R
Surface gravity (log g)3.91[5] cgs
Temperature8128[5] K
Metallicity [Fe/H]−0.24[5] dex
Other designations
Zubenelgenubi, Kiffa Australis, Lanx Australis, α Lib.
α1 Lib: 8 Librae, BD−15 3965, FK5 1387, HD 130819, HIP 72603, HR 5530, SAO 158836.[12]
α2 Lib: 9 Librae, BD−15 3966, FK5 548, HD 130841, HIP 72622, HR 5531, SAO 158840.[13]
Database references
α1 Lib
α2 Lib

Alpha Librae (α Librae, abbreviated Alpha Lib, α Lib) is a double star and, despite its 'alpha' designation, it is the second-brightest star system (or star) in the constellation of Libra. The two components are designated α1 Librae and α2 Librae. The system bore the traditional name of Zubenelgenubi /zˌbɛnɛlɪˈnbi/,[14] though the International Astronomical Union now regards that name as only applying to α2 Librae.[15]

Alpha2 Librae is 0.33 degrees north of the ecliptic so it can be occulted by the Moon and (very rarely) by planets. It was occulted by Venus on October 25, 1947;[16] the next occultation by a planet will be by Mercury on 10 November 2052.[17] Both components are eclipsed (occulted) by the sun from about 7–9 November.[18] Thus the star can be viewed the whole night, crossing the sky, in early May.


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-janubiyyah الكفة الجنوبية "southern pan [of the scales]". Another name used in older astronomy texts, equivalent to "southern pan", was Lanx Australis.[19]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[20] 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.[15]

In Chinese, 氐宿 (Dī Xiù), meaning Root, refers to an asterism consisting of α2 Librae, ι Librae, γ Librae and β Librae.[21] Consequently, the Chinese name for α2 Librae itself is 氐宿一 (Dī Xiù yī), "the First Star of Root".[22]


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. They 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 magnitude 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.[8]

Alpha2 Librae[edit]

The brightest member, α2 Librae, is itself a spectroscopic binary system. The components have similar masses (mass ratio of 0.92) but the primary has a rotational velocity three times higher than its companion, which could be caused by them having significantly misaligned spin axes or if the primary is a merger product.[11]

Alpha1 Librae[edit]

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.

KU Librae[edit]

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,[8] but has a substantially different metallicity.[23]

See also[edit]


  1. ^ Calculated using an angular diameter of 0.6133287 mas and a distance of 23.354 parsecs (76.17 ly).
  2. ^ Calculated using an angular diameter of 1.12 mas and a distance of 23.24 parsecs (75.8 ly).


  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:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
  2. ^ a b Gutierrez-Moreno, Adelina; et al. (1966), "A System of photometric standards", Publ. Dept. Astron. Univ. Chile, 1, Publicaciones Universidad de Chile, Department de Astronomy: 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:1988mcts.book.....H.
  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 e 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/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637, S2CID 119476992
  6. ^ a b 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:0811.3982, Bibcode:2009A&A...501..941H, doi:10.1051/0004-6361/200811191, S2CID 118577511
  8. ^ 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:1001.5432, Bibcode:2010A&A...514A..98C, doi:10.1051/0004-6361/200913986, S2CID 118875432
  9. ^ a b Cruzalèbes, P.; Petrov, R. G.; Robbe-Dubois, S.; Varga, J.; Burtscher, L.; Allouche, F.; Berio, P.; Hofmann, K. -H.; Hron, J.; Jaffe, W.; Lagarde, S.; Lopez, B.; Matter, A.; Meilland, A.; Meisenheimer, K. (2019-12-01). "A catalogue of stellar diameters and fluxes for mid-infrared interferometry". Monthly Notices of the Royal Astronomical Society. 490 (3): 3158–3176. arXiv:1910.00542. Bibcode:2019MNRAS.490.3158C. doi:10.1093/mnras/stz2803. ISSN 0035-8711. Alpha Librae's database entry at VizieR.
  10. ^ 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" (PDF), Astronomy and Astrophysics, 520: A79, arXiv:1002.4391, Bibcode:2010A&A...520A..79M, doi:10.1051/0004-6361/200913725, S2CID 43455849, archived from the original (PDF) on 2017-09-22, retrieved 2018-11-04
  11. ^ a b Waisberg, Idel; Klein, Ygal; Katz, Boaz (2023-08-23). "Hidden Companions to Intermediate-mass Stars. X. Solving for the Mass Ratio in the Spectroscopic Binary Zubenelgenubi = α 2 Librae*". Research Notes of the AAS. 7 (8): 180. Bibcode:2023RNAAS...7..180W. doi:10.3847/2515-5172/acf28f. ISSN 2515-5172.
  12. ^ "* alf01 Lib". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2007-01-23.
  13. ^ "* alf02 Lib". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2007-01-23.
  14. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  15. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  16. ^ Können, G. P.; Van Maanen, J. (April 1981). "Planetary occultations of bright stars". Journal of the British Astronomical Association. 91: 148–157. Bibcode:1981JBAA...91..148K.
  17. ^ Peuschel, Marco (2003). "Astronomische Ereignisse der besonderen Art" Archived 2005-03-16 at the Wayback Machine. Retrieved July 10, 2005.
  18. ^ In the Sky Earth astronomy reference utility showing the ecliptic and relevant date as at J2000 - present
  19. ^ Philippe La Hire, "Tabulae Astronomicae" (1727), see star table, page 13.
  20. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  21. ^ (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.
  22. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  23. ^ Fuhrmann, K.; Chini, R. (2015). "Multiplicity among F-type stars. II". The Astrophysical Journal. 809 (1): 107. Bibcode:2015ApJ...809..107F. doi:10.1088/0004-637X/809/1/107. S2CID 126218052.

External links[edit]