# Tau Hydrae

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The [Bayer designation](/source/Bayer_designation) **Tau Hydrae** (τ Hya, τ Hydrae) is shared by two [star systems](/source/Star_system) in the equatorial [constellation](/source/Constellation) of [Hydra](/source/Hydra_(constellation)). The two stars are separated by 1.74° in the sky.

- [τ1 Hydrae](/source/Tau1_Hydrae), [triple star system](/source/Triple_star_system)

- [τ2 Hydrae](/source/Tau2_Hydrae), [astrometric binary](/source/Astrometric_binary)

The stars τ1 Hya and τ2 Hya, along with [ι Hya](/source/Iota_Hydrae) and the 5th‑magnitude [33 Hya](/source/33_Hydrae) (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* was the title for four stars: τ1 Hya as *Uḳdah I*, τ2 Hya as *Uḳdah II*, 33 Hya as *Uḳdah III* and [ι Hya](/source/Iota_Hydrae) as *Uḳdah IV*.[2] The name Ukdah is now officially applied to ι Hydrae.

In [Chinese](/source/Chinese_language), 柳宿 (*Xīng Sù*), meaning *[Star](/source/Star_(Chinese_constellation)) (asterism)*, refers to an asterism consisting of τ1 and τ2 Hydrae, [Alphard](/source/Alphard), [ι Hydrae](/source/Iota_Hydrae), [26 Hydrae](/source/26_Hydrae), [27 Hydrae](/source/27_Hydrae), [HD 82477](https://en.wikipedia.org/w/index.php?title=HD_82477&action=edit&redlink=1) and [HD 82428](https://en.wikipedia.org/w/index.php?title=HD_82428&action=edit&redlink=1).[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]

## References

1. **[^](#cite_ref-1)** [Allen, R. H.](/source/Richard_Hinckley_Allen) (1963). [*Star Names: Their Lore and Meaning*](https://archive.org/details/starnamestheirlo00alle/page/250) (Reprint ed.). New York, NY: Dover Publications Inc. p. [250](https://archive.org/details/starnamestheirlo00alle/page/250). [ISBN](/source/ISBN_(identifier)) [0-486-21079-0](https://en.wikipedia.org/wiki/Special:BookSources/0-486-21079-0). Retrieved 2010-12-12. {{[cite book](https://en.wikipedia.org/wiki/Template:Cite_book)}}: ISBN / Date incompatibility ([help](https://en.wikipedia.org/wiki/Help:CS1_errors#invalid_isbn_date))

1. **[^](#cite_ref-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](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720005197_1972005197.pdf)

1. **[^](#cite_ref-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](/source/ISBN_(identifier)) [978-986-7332-25-7](https://en.wikipedia.org/wiki/Special:BookSources/978-986-7332-25-7).

1. **[^](#cite_ref-4)** (in Chinese) [AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 29 日](http://aeea.nmns.edu.tw/2006/0605/ap060529.html) [Archived](https://web.archive.org/web/20110522120541/http://aeea.nmns.edu.tw/2006/0605/ap060529.html) 2011-05-22 at the [Wayback Machine](/source/Wayback_Machine)

v t e Constellation of Hydra List of stars in Hydra Hydra in Chinese astronomy Stars Bayer α (Alphard) β γ (Naga) δ ε (Ashlesha) ζ η θ ι (Ukdah) κ λ μ ν ξ ο π ρ σ (Minchir) τ1 τ2 υ1 (Zhang) υ2 φ1 φ2 φ3 χ1 χ2 ψ ω b1 b3 C F Flamsteed 1 2 3 6 (a) 9 10 12 (D) 14 15 17 19 20 21 23 24 25 26 27 (P) 28 29 33 (A) 34 37 44 47 48 50 51 (k) 52 (l) 54 (m) 55 56 57 58 (Solitaire) 59 60 10 Crt 17 Crt 20 Crt 2 Sex Variable R U V W TW EX HS KU LQ V361 V419 V421 V478 V484 HR 3378 3538 3749 (G) 3750 3858 (I) 3862 3919 3923 (Felis) 4162 4328 4339 (β Ant) 4445 5265 HD 72659 74156 82943 86226 86264 90156 (γ Ant) Other 2MASS 1114−2618 ESO 439-26 Gliese 328 Gliese 357 Gliese 433 Gliese 453 GJ 3634 HAT-P-30 HE 1256−2738 HE 1327−2326 LHS 2065 LHS 3003 LTT 3780 SDSS J090745.0+024507 WASP-25 WASP-36 WASP-84 WASP-166 (Filetdor) WISE 0855−0714 WISEA J1141−3326 Exoplanets Gliese 357 d HD 72659 b HD 74156 b c HD 82943 b c HD 86226 b HD 86264 b TW Hydrae b (unconfirmed) WASP-193b Star clusters Messier 48 Messier 68 NGC 5694 Price-Whelan 1 Nebulae Abell 33 Abell 35 NGC 3242 Southern Owl Nebula Galaxies Messier 83 NGC 2617 2642 2697 2708 2758 2835 2848 2865 2890 2935 2936 2937 2960 2962 2986 2992 3030 3052 3054 3081 3109 3124 3200 3285 3285B 3290 3305 3307 3308 3309 3311 3312 3313 3314 3315 3316 3336 3369 3383 3393 3402 3463 3464 3585 3621 3673 3717 3904 3923 3936 4980 4993 5042 5061 5078 5085 5101 5135 5260 5264 5495 5626 Other 3C 196.1 AM 1316−241 Arp 7 CTQ 327 ESO 510-G13 GAMA202627 GLEAM J0917−0012 Hydra A IC 535 MRC 1138−262 Peekaboo Galaxy PKS 1151−348 RX J0911.4+0551 SDSS J0841+0101 TN J0924−2201 Tololo 1247−232 ZwCl 1693 BCG Galaxy clusters Abell 754 Abell 3411 Abell 3412 Bullet Group Hickson 40 Astronomical events GW170817 Category

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Adapted from the Wikipedia article [Tau Hydrae](https://en.wikipedia.org/wiki/Tau_Hydrae) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Tau_Hydrae?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.