Tau Hydrae

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.


 * τ1 Hydrae, triple star system
 * τ2 Hydrae, astrometric binary

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. 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 as Uḳdah III and ι Hya as Uḳdah IV

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. Consequently, τ1 and τ2 Hydrae are known as 星宿二 (Xīng Sù èr, the Second Star of Star) and 星宿三 (Xīng Sù sān, the Third Star of Star) respectively.