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. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking ...

of a combined third magnitude in the constellation of Hydra. Based upon

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

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 of celestial obj ...

mission, it is located roughly distant from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared rad ...

. 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 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 celestial objects and phenomena that originate outsi ...

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 or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. T ...

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

(IAU). In ancient

Indian astronomy Astronomy has long history in Indian subcontinent stretching from pre-historic to modern times. Some of the earliest roots of Indian astronomy can be dated to the period of Indus Valley civilisation or earlier. Astronomy later developed as a di ...

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

nakshatra Nakshatra ( sa, नक्षत्रम्, translit=Nakṣatram) is the term for lunar mansion in Hindu astrology and Indian Astronomy. A nakshatra is one of 27 (sometimes also 28) sectors along the ecliptic. Their names are related to ...

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 its orbit around the Earth. The concept was used by several ancient cultures as part of their calendrical system. Stations in ...

s in

Hindu astrology Jyotisha or Jyotishya (from Sanskrit ', from ' “light, heavenly body" and ''ish'' - from Isvara or God) is the traditional Hindu system of astrology, also known as Hindu astrology, Indian astrology and more recently Vedic astrology. It is o ...

. 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 List of proper names of stars, proper names for stars for the international astronomical community. It operates under ...

(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 (Lisan al Sudja),

Zeta Hydrae Zeta Hydrae (ζ Hya, ζ Hydrae) is a solitary star in the equatorial constellation of Hydra. This is a generally faint constellation, so, at an apparent visual magnitude of +3.10, this is the third-brightest member after Alphard and G ...

Eta Hydrae Eta (uppercase , lowercase ; grc, ἦτα ''ē̂ta'' or ell, ήτα ''ita'' ) is the seventh letter of the Greek alphabet, representing the close front unrounded vowel . Originally denoting the voiceless glottal fricative in most dialect ...

Rho Hydrae Rho Hydrae, equally written ρ Hydrae, is a binary star in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.34. The distance to this system, based upon an annual parallax shif ...

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 agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...

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 Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of v ...

, (), meaning ''

Willow Willows, also called sallows and osiers, from the genus ''Salix'', comprise around 400 speciesMabberley, D.J. 1997. The Plant Book, Cambridge University Press #2: Cambridge. of typically deciduous trees and shrubs, found primarily on moist ...

(asterism)'', refers to an asterism consisting of Epsilon Hydrae, Delta Hydrae, Sigma Hydrae, Eta Hydrae, Rho Hydrae, Zeta Hydrae,

Omega Hydrae Omega Hydrae, Latinised from ω Hydrae, is a golden-hued star in the equatorial constellation of Hydra, located to the west-southwest of the brighter star Zeta Hydrae. Based upon an annual parallax shift of just 3.64 mas as seen from ...

and

Theta Hydrae Theta Hydrae, Latinized from θ Hydrae, is a binary star system in the constellation Hydra. It is visible to the naked eye with an apparent visual magnitude of 3.9. The star system has a high proper motion with an annual parallax shift ...

Consequently, Epsilon Hydrae itself is known as (, en, the Fifth Star of Willow). ''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" i ...

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 years and an

angular separation Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two sightlines, or between two point objects as viewed from an observer. Angular distance appears in mathematics (in pa ...

of 0.2

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

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 b ...

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 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 ...

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 nomin ...

. 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 A G-type main-sequence star (Spectral type: G-V), also often, and imprecisely called a yellow dwarf, or G star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective tempe ...

. 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 bulge ...

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 Multiple star systems