Tau Hydrae

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τ¹ Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Hydra
Right ascension 9h 29m 8.9s
Declination -2° 46' 8"
Apparent magnitude (V) +4.59
Distance 56 ly
(17.18 pc)
Spectral type F6V
Other designations
Ukdah, 31 Hydrae HR 3759,
HD 81997, SAO 136895,
HIP 46509, CCDM J09291-0246A

The Bayer designation Tau Hydrae (τ Hya, τ Hydrae) is shared by two star systems, τ¹ Hydrae and τ² Hydrae, in the constellation Hydra. The two stars are separated by 1.74° in the sky.

This star (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]

τ¹ Hydrae[edit]

Tau-1 Hydrae is a binary star lying 56 light years from Earth. It is a spectral type F6 main sequence star which has an apparent magnitude of +4.59.


τ² Hydrae[edit]

τ² Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Hydra
Right ascension 9h 31m 58.9s
Declination -1° 11' 6"
Apparent magnitude (V) +4.54
Distance 460 ly
(141.10 pc)
Spectral type A3V
Other designations
Ukdah, 32 Hydrae, HR 3787,
HD 82446, SAO 136932,
HIP 46776

Tau-2 Hydrae is a spectral type A3 main sequence star which has an apparent magnitude of +4.54. It is about 460 light years from Earth.

Name[edit]

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 Hydra and τ² Hydra, Alphard, ι Hydrae, 26 Hydrae, 27 Hydrae, HD 82477 and HD 82428.[3] Consequently, τ¹ Hydra and τ² themself is 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.)[4]

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