Libra is a constellation of the

zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The pat ...

and is located in the

Southern celestial hemisphere The southern celestial hemisphere, also called the Southern Sky, is the southern half of the celestial sphere; that is, it lies south of the celestial equator. This arbitrary sphere, on which seemingly fixed stars form constellations, appea ...

. Its name is

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

for

weighing scales A scale or balance is a device used to measure weight or mass. These are also known as mass scales, weight scales, mass balances, and weight balances. The traditional scale consists of two plates or bowls suspended at equal distances from a ...

. Its old astronomical symbol is (♎︎). It is fairly faint, with no first magnitude stars, and lies between

Virgo Virgo may refer to: *Virgo (astrology), the sixth astrological sign of the zodiac * Virgo (constellation), a constellation *Virgo Cluster, a cluster of galaxies in the constellation Virgo *Virgo Stellar Stream, remains of a dwarf galaxy * Virgo Su ...

to the west and

Scorpius Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation that pre-dates the Gr ...

to the east.

Beta Librae Beta Librae (β Librae, abbreviated Beta Lib, β Lib), formally named Zubeneschamali , is (despite its 'beta' designation) the brightest star in the zodiac constellation of Libra. From parallax measurements, its distance can be estimated ...

, also known as Zubeneschamali, is the brightest star in the constellation. Three star systems are known to have planets.

Features

Stars

Overall, there are 83 stars within the constellation's borders brighter than or equal to

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

6.5. The brightest stars in Libra form a quadrangle that distinguishes it for the unaided observer. Traditionally, Alpha and Beta Librae are considered to represent the scales' balance beam, while Gamma and Sigma are the weighing pans.

Alpha Librae 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 Libra ...

, called Zubenelgenubi, is a multiple star system divisible into two stars when seen through binoculars, The primary (Alpha² Librae) is a blue-white star of magnitude 2.7 and the secondary (Alpha¹ Librae) is a white star of magnitude 5.2 and spectral type F3V that is 74.9 ± 0.7 light-years from Earth. Its traditional name means "the southern claw". Zubeneschamali (

) is the corresponding "northern claw" to Zubenelgenubi. The brightest star in Libra, it is a green-tinged star of magnitude 2.6, 160 light-years from Earth.

Gamma Librae Gamma Librae (γ Librae, abbreviated Gamma Lib, γ Lib) is a suspected binary star system in the constellation of Libra. It is visible to the naked eye, having an apparent visual magnitude of +3.91. Based upon an annual parallax shift o ...

is called Zubenelakrab, which means "the scorpion's claw", completing the suite of names referring to Libra's archaic status. It is an

orange giant A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...

of magnitude 3.9, 152 light-years from Earth.

Iota Librae The Bayer designation Iota Librae (ι Lib / ι Librae) is shared by two star systems, in the constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically re ...

is a complex multiple star, 377 light-years from Earth, with both optical and true binary components. The primary appears as a blue-white star of magnitude 4.5; it is a binary star indivisible in even the largest amateur instruments with a period of 23 years. The secondary, visible in small telescopes as a star of magnitude 9.4, is a binary with two components, magnitudes 10 and 11. There is an optical companion to Iota Librae;

25 Librae 5 (five) is a number, numeral and digit. It is the natural number, and cardinal number, following 4 and preceding 6, and is a prime number. It has attained significance throughout history in part because typical humans have five digits on eac ...

is a star of magnitude 6.1, 219 light-years from Earth and visible in binoculars.

Mu Librae μ Librae (Latinised as Mu Librae) is the Bayer designation for a probable triple star system in the zodiac constellation of Libra. They have a combined apparent visual magnitude of 5.32, which is bright enough to be faintly visible to th ...

is a binary star divisible in medium-aperture amateur telescopes, 235 light-years from Earth. The primary is of magnitude 5.7 and the secondary is of magnitude 6.8. Delta Librae is an Algol-type eclipsing variable star, 304 lightyears from Earth. It has a period of 2 days, 8 hours; its minimum magnitude of 5.9 and its maximum magnitude is 4.9. FX Librae, designated 48 Librae, is a

shell star A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The broa ...

of magnitude 4.9. Shell stars, like Pleione and

Gamma Cassiopeiae Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a bright star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual mag ...

, are

blue supergiant A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They have luminosity class I and spectral class B9 or earlier. Blue supergiants are found towards the top left of the Hertzsprung–Russell diagram, above ...

s with irregular variations caused by their abnormally high speed of rotation. This ejects gas from the star's equator.

Sigma Librae Sigma Librae (σ Librae, abbreviated Sigma Lib, σ Lib) is a binary star in the constellation of Libra. The apparent visual magnitude is +3.29, making it visible to the naked eye. Based upon parallax measurements, this system is at a dist ...

(the proper name is Brachium) was formerly known as Gamma Scorpii despite being well inside the boundaries of Libra. It was not redesignated as Sigma Librae until 1851 by Benjamin A. Gould.

History and mythology

Libra was known in

Babylonian astronomy Babylonian astronomy was the study or recording of celestial objects during the early history of Mesopotamia. Babylonian astronomy seemed to have focused on a select group of stars and constellations known as Ziqpu stars. These constellations ...

as ''MUL Zibanu'' (the "scales" or "balance"), or alternatively as the Claws of the Scorpion. The scales were held sacred to the sun god Shamash, who was also the patron of truth and justice. It was also seen as the Scorpion's Claws in

ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean Sea, Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, classical antiquity ( AD 600), th ...

. Since these times, Libra has been associated with law, fairness and civility. In Arabic ''zubānā'' means "scorpion's claws", and likely similarly in other Semitic languages: this resemblance of words may be why the Scorpion's claws became the Scales. Indeed, Zubenelgenubi and Zubeneschamali, the names of the constellation's two main stars, in Arabic mean "southern claw" and "northern claw" respectively. It has also been suggested that the scales are an allusion to the fact that when the sun entered this part of the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

at the autumnal equinox, the days and nights are equal. Libra's status as the location of the equinox earned the equinox the name "First Point of Libra", though this location ceased to coincide with the constellation in 730 BC because of the

precession of the equinoxes In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In partic ...

. In ancient Egypt the three brightest stars of Libra (α, β, and σ Librae) formed a constellation that was viewed as a boat. Libra is a constellation not mentioned by Eudoxus or

Aratus Aratus (; grc-gre, Ἄρατος ὁ Σολεύς; c. 315 BC/310 BC240) was a Greek didactic poet. His major extant work is his hexameter poem ''Phenomena'' ( grc-gre, Φαινόμενα, ''Phainómena'', "Appearances"; la, Phaenomena), the ...

. Libra is mentioned by Manetho (3rd century B.C.) and

Geminus Geminus of Rhodes ( el, Γεμῖνος ὁ Ῥόδιος), was a Greek astronomer and mathematician, who flourished in the 1st century BC. An astronomy work of his, the ''Introduction to the Phenomena'', still survives; it was intended as an int ...

(1st century B.C.), and included by

Ptolemy Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importance ...

in his 48 asterisms. Ptolemy catalogued 17 stars,

Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was ...

10, and

Johannes Hevelius Johannes Hevelius Some sources refer to Hevelius as Polish: * * * * * * * Some sources refer to Hevelius as German: * * * * *of the Royal Society * (in German also known as ''Hevel''; pl, Jan Heweliusz; – 28 January 1687) was a councillor ...

20. It only became a constellation in

ancient Rome In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 BC ...

, when it began to represent the scales held by

Astraea Astraea, Astrea or Astria ( grc, Ἀστραία, Astraía; "star-maiden" or "starry night"), in ancient Greek religion, is a daughter of Astraeus and Eos. She is the virgin goddess of justice, innocence, purity and precision. She is closely as ...

, the goddess of justice, associated with

in the Greek mythology.

The constellation

Libra is bordered by the head of

Serpens Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...

to the north,

to the northwest, Hydra to the southwest, the corner of

Centaurus Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. ...

to the southwest, Lupus to the south,

to the east and

Ophiuchus Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constella ...

to the northeast. Covering 538.1 square degrees and 1.304% of the night sky, it ranks 29th of the 88 constellations in size. The three-letter abbreviation for the constellation, as adopted by the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

in 1922, is "Lib". The official constellation boundaries, as set by

Eugène Delporte Eugene is a common male given name that comes from the Greek εὐγενής (''eugenēs''), "noble", literally "well-born", from εὖ (''eu''), "well" and γένος (''genos''), "race, stock, kin".equatorial coordinate system The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fu ...

, the right ascension coordinates of these borders lie between and , while the declination coordinates are between −0.47° and −30.00°. The whole constellation is visible to observers south of latitude 60°N.

Planetary systems

Libra is home to the

Gliese 581 planetary system Gliese 581 () is a red dwarf star of spectral type M3V at the center of the Gliese 581 planetary system, about 20 light years away from Earth in the Libra constellation. Its estimated mass is about a third of that of the Sun, and it is t ...

, which consists of the star Gliese 581, three confirmed planets, and two unconfirmed planets. Both Gliese 581d, and

Gliese 581g Gliese 581g , unofficially known as Zarmina (or Zarmina's World), was a candidate exoplanet postulated to orbit within the Gliese 581 system, twenty light-years from Earth. It was discovered by the Lick–Carnegie Exoplanet Survey, and was the si ...

are debatably the most promising candidates for life, although Gliese 581g's existences has been disputed and has not been entirely confirmed or agreed on in the scientific community. Gliese 581c is considered to be the first Earth-like

extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

to be found within its parent star's

habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kast ...

. Gliese 581e is possibly the smallest mass exoplanet orbiting a normal star found to date All of these exoplanets are of significance for establishing the likelihood of

life Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...

outside of the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

. The family of candidate habitable planets was extended in late September 2010 to include exoplanets around red dwarf stars because of Gliese 581g, which is a tidally locked planet in the middle of the

. Weather studies show that tidally locked planets may still have the ability to support life.

Deep-sky objects

Libra is home to one bright

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

, NGC 5897. It is a loose cluster, 50,000 light-years from Earth; it is fairly large and has an integrated magnitude of 9. IC 1059 is a galaxy in the constellation Libra.csligman.com
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Astrology

, the Sun appears in the constellation Libra from October 31 to November 22. In tropical astrology, the Sun is considered to be in the sign Libra from the northern autumnal equinox (c. September 23) to on or about October 23, and in

sidereal astrology '' Sidereal'' and ''tropical'' are terms used to describe two different definitions of a year, applied in sidereal solar calendars or tropical solar calendars. In astrology, they refer to two different systems of ecliptic coordinates used to d ...

, from October 16 to November 15.

Namesakes

* ''Libra'' (AKA-12) was a

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

navy ship named after the constellation. * Tropical Storm Tembin - Four tropical cyclones in the western Pacific have been given its Japanese name.

Notes

References

Sources

* * Ian Ridpath and Wil Tirion (2007). ''Stars and Planets Guide'', Collins, London. . Princeton University Press, Princeton. .

External links

The Deep Photographic Guide to the Constellations: Libra

Libra constellation

Warburg Institute Iconographic Database (medieval and early modern images of Libra)
{{DEFAULTSORT:Libra (Constellation) Constellations Southern constellations Constellations listed by Ptolemy