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Eta Hydrae
Star in the constellation Hydra From Wikipedia, the free encyclopedia
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Eta Hydrae (η Hydrae) is a star in the equatorial constellation of Hydra. With an apparent visual magnitude of 4.3,[2] it is visible to the naked eye. However, it is the faintest of the five stars that form the "head" of the hydra.[11] Based upon an annual parallax shift of 5.56 mas,[1] it is located roughly 590 light years from the Sun.

This is a B-type main sequence star with a stellar classification of B3V.[3] It has been classified as a candidate Beta Cephei variable with a period of 2.2 days, although this designation was rejected by Stankov and Handler (2005).[4] The spectrum shows a slight underabundance of carbon, compared to the Sun.[13] The star is around 32[6] million years old and is spinning with a projected rotational velocity of 101[9] km/s. It has an estimated seven[6] times the mass of the Sun and nearly four[7] times the Sun's radius. Eta Hydrae radiates 2,680[8] times the solar luminosity from its outer atmosphere at an effective temperature of 18,630[9] K.
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Name and etymology
This star, along with δ Hya (Lisan al Sudja), ε Hya, ζ Hya, ρ Hya and σ Hya (Minchir), were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".[14] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Min al Azʽal or Minazal were the title for five stars:δ Hya as Minazal I, η Hya as Minazal II, ε Hya as Minazal III, ρ Hya as Minazal IV and ζ Hya as Minazal V (exclude σ Hya).[15]
In Chinese, 柳宿 (Liǔ Sù), meaning Willow, refers to an asterism consisting of η Hydrae, δ Hydra, σ Hydrae, ρ Hydrae, ε Hydrae, ζ Hydrae, ω Hydrae and θ Hydrae[16] Consequently, η Hydrae itself is known as 柳宿三 (Liǔ Sù sān, English: the Third Star of Willow).[17]
The people of Groote Eylandt called Unwala, "The Crab", for the star cluster including this star, δ Hya (Lisan al Sudja), ε Hya, ζ Hya, ρ Hya and σ Hya (Minchir).[18]
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References
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