Alpha Librae (α Librae, abbreviated Alpha Lib, α Lib) is a double star and, despite its '

alpha Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...

' designation, it is the second-brightest star system (or star) in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

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 The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

now regards that name as only applying to α² Librae. Alpha² Librae is 0.33 degrees north of the

ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...

so it can be occulted by the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

and (very rarely) by

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

s. It was occulted by

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

on October 25, 1947; the next occultation by a planet will be by Mercury on 10 November 2052.Peuschel, Marco (2003)
"Astronomische Ereignisse der besonderen Art"
. Retrieved July 10, 2005. Both components are eclipsed ( occulted) by the sun from about 7–9 November. Thus the star can be viewed the whole night, crossing the sky, in early May.

Nomenclature

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

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

. ''Zubenelgenubi'' , also rendered ''Zuben Elgenubi'', derives from the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

ّ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 Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

translation of the Arabic ''al-kiffah al-janubiyyah'' ''الكفة الجنوبية'' "southern pan f the scales. Another name used in older astronomy texts, equivalent to "southern pan", was ''Lanx Australis''. In 2016, the

organized a Working Group on Star Names (WGSN) 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. In Chinese, (), meaning ''

Root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...

'', refers to an asterism consisting of α² Librae, ι Librae,

γ Librae Gamma Librae (γ Librae, abbreviated Gamma Lib, γ Lib) is a suspected binary star system in the constellation of Libra (constellation), Libra. It is visible to the naked eye, having an Apparent magnitude, apparent visual magnitude of +3. ...

and β Librae. Consequently, the Chinese name for α² Librae itself is (), "the First Star of Root".香港太空館 - 研究資源 - 亮星中英對照表
, Hong Kong Space Museum. Accessed on line November 23, 2010.

Properties

Alpha Librae is about 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 Angular distance or angular separation is the measure of the angle between the orientation (geometry), orientation of two straight lines, ray (geometry), rays, or vector (geometry), vectors in three-dimensional space, or the central angle subtende ...

of 231" (3'51"). The

position angle In astronomy, position angle (usually abbreviated PA) is the convention for measuring angles on the sky. The International Astronomical Union defines it as the angle measured relative to the Celestial pole, north celestial pole (NCP), turning pos ...

of the companion is 314 degrees. The brighter of the two is a white star of spectral type A3, with an

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

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.

Alpha² Librae

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.

Alpha¹ Librae

The second member, α¹ Librae, is separated from the primary system by around . 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

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 The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

. It is sufficiently close to be gravitationally bound to the other members, but has a substantially different

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

, making it unlikely to be physically bound to the system.

Notes

References

External links

*Kaler, Jim (2004)
"STARS: Zubenelgenubi"
Retrieved July 10, 2005. * {{DEFAULTSORT:Alpha Librae Librae, Alpha Binary stars Libra (constellation) A-type subgiants F-type main-sequence stars Zubenelgenubi Castor Moving Group Librae, 08 9 072603 22 5530 1 130819 41 BD-15 3965 Am stars A-type main-sequence stars