Epsilon Hydrae (ε Hydrae, abbreviated Epsilon Hya, ε Hya) is a

multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a ''st ...

of a combined third magnitude 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 Hydra. Based upon

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements obtained during the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

mission, it is located roughly distant from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

. The system consists of a binary pair designated Epsilon Hydrae AB, whose two components are themselves designated Epsilon Hydrae A (formally named Ashlesha ) and B, orbited by a

spectroscopic binary 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 ...

designated Epsilon Hydrae C. A possible fourth component, designated Epsilon Hydrae D, shares a

common proper motion This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of astronomical object, celestial objects and phenomena ...

with the other components and thus is most likely a gravitationally-bound member of the system.

Nomenclature

''ε Hydrae'' ( Latinised to ''Epsilon Hydrae'') 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 three constituents as ''Epsilon Hydrae AB'', ''C'' and ''D'', and those of ''AB's'' components - ''Epsilon Hydrae A'' and ''B'' - derive from the convention used by the Washington Multiplicity Catalog (WMC) for

s, 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). In ancient

Indian astronomy Astronomy has a long history in the Indian subcontinent, stretching from History of India, pre-historic to History of India (1947–present), modern times. Some of the earliest roots of Indian astronomy can be dated to the period of Indus Valle ...

, the five-star cluster of Hydrae is collectively called आश्लेषा '' āślēṣā'' () "the embrace", the 9th of the 27

nakshatra Nakshatra () is the term for Lunar mansion in Hindu astrology and Buddhist astrology. A nakshatra is one of 27 (sometimes also 28) sectors along the ecliptic. Their names are related to a prominent star or asterisms in or near the respective s ...

s or

lunar mansion Often called lunar mansion, a lunar station or lunar house is a segment of the ecliptic through which the Moon passes in orbit of the Moon, its orbit around the Earth. The concept was used by several ancient cultures as part of their calendrical ...

s in Hindu astrology. In 2016, the IAU organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(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 ''Ashlesha'' for the component Epsilon Hydrae A on 1 June 2018 and it is now so included in the List of IAU-approved Star Names. This system, along with

Delta Hydrae Delta Hydrae, Latinized from δ Hydrae, is a triple star in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.146. Based upon an annual parallax shift of 20.34 mas, it ...

(Lisan al Sudja), Zeta Hydrae, Eta Hydrae, Rho Hydrae and Sigma Hydrae (Minchir), were Ulug Beg's ''Min al Azʽal'', "Belonging to the Uninhabited Spot". According to a 1971

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

technical memorandum, ''Min al Azʽal'' or ''Minazal'' were the title for five stars: Delta Hydrae as ''Minazal I'', Eta Hydrae as ''Minazal II'', Epsilon Hydrae as ''Minazal III'', Rho Hydrae as ''Minazal IV'' and Zeta Hydrae as ''Minazal V''. In Chinese, (), meaning ''

Willow Willows, also called sallows and osiers, of the genus ''Salix'', comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions. Most species are known ...

(asterism)'', refers to an asterism consisting of Epsilon Hydrae, Delta Hydrae, Sigma Hydrae, Eta Hydrae, Rho Hydrae, Zeta Hydrae, Omega Hydrae and Theta Hydrae Consequently, Epsilon Hydrae itself is known as (, ). ''Unwala'', "The Crab", was the name given by the people of

Groote Eylandt Groote Eylandt ( Anindilyakwa: ''Ayangkidarrba''; meaning "island" ) is the largest island in the Gulf of Carpentaria and the fourth largest island in Australia. It was named by the explorer Abel Tasman in 1644 and is Dutch for "large island" ...

to the star cluster including this star, Delta Hydrae (Lisan al Sudja), Zeta Hydrae, Eta Hydrae, Rho Hydrae and Sigma Hydrae (Minchir).

Properties

The Epsilon Hydrae AB binary has an

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 ...

of 15.07 years and an

angular separation Angular distance or angular separation is the measure of the angle between the orientation of two straight lines, rays, or vectors in three-dimensional space, or the central angle subtended by the radii through two points on a sphere. When t ...

of 0.252

arcsecond 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 ...

s. The spectroscopic binary Epsilon Hydrae C has a period of 9.9047 days and an

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...

of 0.62, while simultaneously orbiting AB with a period of 870 years. C is separated from AB by 3 arcseconds. Epsilon Hydrae D is separated from AB by 19 arcseconds and has an estimated orbital period of around 10,000 years. The primary, Epsilon Hydrae A, is a

giant star A giant star has a substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsp ...

that is shining with 67 times the

luminosity of the Sun The solar luminosity () is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal s ...

. This energy is being radiated from the star's outer atmosphere 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 5,620 K, giving it the yellow hue of a

G-type star In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

. It is rotating rapidly with a

projected rotational velocity Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bu ...

of 19 km s⁻¹. It has an X-ray luminosity of .

References

External links

* {{DEFAULTSORT:Epsilon Hydrae Hydrae, Epsilon Hydra (constellation) Ashlesha Hydrae, 11 043109 G-type giants F-type main-sequence stars Suspected variables 07874 3482 Durchmusterung objects 5