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 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]


  1. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 250. ISBN 0-486-21079-0. Retrieved 2010-12-12.
  2. ^ Jack W. Rhoads - Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology; November 15, 1971
  3. ^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  4. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 29 日

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