| 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 | |
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]
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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.
| 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 | |
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.
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]