Tau Hydrae

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The Bayer designation Tau Hydrae (τ Hya, τ Hydrae) is shared by two star systems in the equatorial constellation of Hydra. The two stars are separated by 1.74° in the sky.

The stars τ1 Hya and τ2 Hya, along with ι Hya and the 5th‑magnitude 33 Hya (A Hydrae), were Ptolemy's Καμπή (Kampē); but Kazwini knew them as ʽUḳdah, the Knot.[1] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Uḳdah were the title for four stars: τ1 Hya as Uḳdah I, τ2 Hya as Uḳdah II, 33 Hya (A Hydrae) as Uḳdah III and ι Hya as Uḳdah IV [2]

In Chinese, 柳宿 (Xīng Sù), meaning Star (asterism), refers to an asterism consisting of τ1 and τ2 Hydrae, Alphard, ι Hydrae, 26 Hydrae, 27 Hydrae, HD 82477 and HD 82428.[3] Consequently, τ1 and τ2 Hydrae are known as 星宿二 (Xīng Sù èr, English: the Second Star of Star) and 星宿三 (Xīng Sù sān, English: the Third Star of Star) respectively.[4]


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