The constellation Libra marked on a naked eye view.
Overall, there are 83 stars within the constellation's borders brighter than or equal toapparent magnitude 6.5.[a][2]
The brightest stars in Libra form aquadrangle 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, 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 (Beta Librae) is the corresponding "northern claw" to Zubenelgenubi. The brightest star in Libra, Zubeneschamali is a green-tinged star of magnitude 2.6, 160 (or 185 ± 2) light-years from Earth.Gamma Librae is called Zubenelakrab, which means "the scorpion's claw", completing the suite of names referring to Libra's archaic status. It is anorange giant of magnitude 3.9, 152 light-years from Earth.[3]
Iota Librae is a complex multiple star, 377 light-years from Earth, with both optical and true binary components in it. 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 is a star of magnitude 6.1, 219 light-years from Earth and visible in binoculars.Mu Librae 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.[3]
Delta Librae is anAlgol-typeeclipsing 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, designated48 Librae, is ashell star of magnitude 4.9. Shell stars, likePleione andGamma Cassiopeiae, are stars with irregular variations caused by their abnormally high speed of rotation. This ejects gas from the star's equator.[3]
Sigma Librae (the proper name is Brachium[4]) was formerly known as Gamma Scorpii despite being well inside the boundaries of Libra. It was not redesignated as Sigma Librae until 1851 byBenjamin A. Gould.
Libra as depicted inUrania's Mirror, a set of constellation cards published in London c.1825
Libra was known inBabylonian astronomy asMUL Zibanu (the "scales" or "balance"), or alternatively as the Claws of the Scorpion. The scales were held sacred to the sun godShamash, who was also the patron of truth and justice.[5] Since these times, Libra has been associated with law, fairness and civility.
It was also seen as the Scorpion's Claws inancient Greece.[3] This is the name used repeatedly by Aratus, which made it widespread in Hellenistic times: "few conspicuous stars the Claws can boast,and their dim light mid brighter gems is lost"[6] In Arabiczubānā 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.[citation needed] 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 theecliptic at theautumnal equinox, the days and nights are equal.[7] 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 theprecession of the equinoxes.
In ancient Egypt the three brightest stars of Libra (α, β, and σ Librae) formed a constellation that was viewed as a boat.[8] Libra is not mentioned byEudoxus orAratus as a constellation.[7] Libra is mentioned byManetho (3rd century B.C.) andGeminus (1st century B.C.), and included byPtolemy in his 48asterisms. Ptolemy catalogued 17 stars,Tycho Brahe 10, andJohannes Hevelius 20.[7] It only became a constellation inancient Rome, when it began to represent the scales held byAstraea, the goddess of justice, associated with Virgo in the Greek mythology.[3]
Libra is bordered by the head ofSerpens to the north, Virgo to the northwest, Hydra to the southwest, the corner ofCentaurus to the southwest, Lupus to the south,Scorpius to the east andOphiuchus to the northeast. Covering 538.1 square degrees and 1.304% of the night sky, it ranks 29th of the 88 constellations in size.[2] The three-letter abbreviation for the constellation, as adopted by theInternational Astronomical Union in 1922, is "Lib".[9] The official constellation boundaries, as set byEugène Delporte in 1930, are defined by a polygon of 12 segments (illustrated in infobox). In theequatorial coordinate system, theright ascension coordinates of these borders lie between14h 22m 08.08s and16h 02m 17.23s, while thedeclination coordinates are between −0.47° and −30.00°.[10] The whole constellation is visible to observers south of latitude 60°N.[b]
Libra is home to theGliese 581 planetary system, which consists of the star Gliese 581 and three confirmed planets. This system gained attention in the late 2000s and early 2010s as the subject of some of the earliest claims of potentially habitable exoplanets,[11][12] but it is now known thatGliese 581c is too hot to be potentially habitable, and the planet candidatesGliese 581d andg likely do not exist.[13] At the time of its discovery in 2009,Gliese 581e was the smallest massexoplanet known orbiting a normal star.[14]
Libra is home to one brightglobular cluster,NGC 5897. It is a loose cluster, 50,000 light-years from Earth; it is fairly large and has an integrated magnitude of 9.[3]IC 1059 is agalaxy in the constellation Libra.[15]
As of 2002[update], the Sun appears in the constellation Libra from October 31 to November 22. Intropical astrology, the Sun is considered to be in the signLibra from the northern autumnal equinox (c. September 23) to on or about October 23, and insidereal astrology, from October 16 to November 15.
^Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[1]
^While parts of the constellation technically rise above the horizon to observers between 60°N and 89°N, stars within a few degrees of the horizon are to all intents and purposes unobservable.[2]
.svg)
