Gamma Librae (γ Librae, abbreviated Gamma Lib, γ Lib) is a suspected

binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

system 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. It is visible to the naked eye, having an apparent visual magnitude of +3.91. Based upon an annual parallax shift of 19.99 mas as seen from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, it lies at a distance of 163

light year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...

s. The primary component (designated Gamma Librae A) has been formally named Zubenelhakrabi , the traditional name of the system.

Nomenclature

''γ Librae'' ( Latinised to ''Gamma 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 ...

. The designations of the two components as ''Gamma Librae A'' and ''B'' derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by 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 ...

(IAU). Gamma Librae bore the traditional name ''Zuben (el) Hakrabi'' (also rendered as ''Zuben-el-Akrab'' and corrupted as ''Zuben Hakraki''). The name is a modification of 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 ...

زبانى العقرب ''Zubān al-ʿAqrab'' "the claws of the scorpion", a name that dates to before Libra was a distinct constellation from Scorpius. In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. It approved the name ''Zubenelhakrabi'' for the component Gamma Librae A on 5 September 2017 and it is now so included in the List of IAU-approved 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 Gamma Librae, Alpha² Librae, Iota Librae and

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 ...

. Consequently, the Chinese name for Gamma Librae itself is (), "the Third Star of Root".

Properties

Because the star lies near 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 ...

it is subject to

occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks f ...

s 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 ...

, allowing the angular size to be measured by this method. As of 1940, the pair had an angular separation of 0.10

arc seconds A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

along a

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 191°. The yellow-hued primary, component Aa, is an

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

G-type giant star with a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of G8.5 III and an estimated age of 4.3 billion years. It has 1.15 times the mass of the Sun and has expanded to 11.14 times the Sun's radius. The star is radiating around 72 times the Sun's luminosity from its enlarged

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

at an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 4,786 K. There is a magnitude 11.2 visual companion, component B, at an angular separation of 42.5 arc seconds along a position angle of 157°, as of 2013. At its distance, the visual magnitude is diminished by an

extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

of 0.11 due to interstellar dust. The system is moving closer to the Sun with a radial velocity of −26.71 km/s.

Planetary system

Gamma Librae host two known planets, discovered in 2018 by

doppler spectroscopy Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in ...

. The planets, named Gamma Librae b and Gamma Librae c, have

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

s of and , and orbital separations of 1.24 and 2.2

astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

s. Due to limitations of the doppler spectroscopy method, only a lower limit in the planetary masses can be measured, assuming the orbital inclination relative to Earth is a

right angle In geometry and trigonometry, a right angle is an angle of exactly 90 Degree (angle), degrees or radians corresponding to a quarter turn (geometry), turn. If a Line (mathematics)#Ray, ray is placed so that its endpoint is on a line and the ad ...

. A lower inclination would imply a larger mass. Nonetheless, the inclination must be larger than 70º for the planetary system to be stable. Planet b and c have minimum masses of respectively, and hence are

gas giants A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

. They are in a 7:3

orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...

, so planet c make three orbits in the same amount of time planet b make seven orbits.

References

{{DEFAULTSORT:Gamma Librae Zubenelhakrabi Librae, 38 Librae, Gamma 076333 G-type giants 138905 5787 CD-27 10464 Libra (constellation) Planetary systems with two confirmed planets