Iota Hydrae (ι Hydrae, abbreviated Iota Hya, ι Hya), formally named Ukdah /ˈʌkdə/,[7] is a star in the constellation of Hydra, about 8° to the north-northwest of Alphard (Alpha Hydrae)[8] and just to the south of the celestial equator.[9] Visible to the naked eye, it is a suspected variable star with an apparent visual magnitude that ranges between 3.87 and 3.91.[3] Based upon an annual parallax shift of 12.39 mas measured during the Hipparcos mission,[1] it is located around 263 light-years from the Sun.

Iota Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 09h 39m 51.36145s[1]
Declination −01° 08′ 34.1135″[1]
Apparent magnitude (V) 3.91[2]
Spectral type K2.5 III[2]
B−V color index 1.32
Variable type Suspected[3]
Radial velocity (Rv)+24.19±0.36[4] km/s
Proper motion (μ) RA: +46.96[1] mas/yr
Dec.: −62.39[1] mas/yr
Parallax (π)12.39 ± 0.14 mas[1]
Distance263 ± 3 ly
(80.7 ± 0.9 pc)
Absolute magnitude (MV)−0.63[5]
Mass1.92[2] M
Radius33[4] R
Luminosity83[2] L
Surface gravity (log g)2.2[4] cgs
Temperature4,244±32[2] K
Metallicity [Fe/H]−0.05[2] dex
Rotational velocity (v sin i)4.5[4] km/s
Age2.47[2] Gyr
Other designations
Ukdah, ι Hya, 35 Hydrae, BD−00° 2231, FK5 1250, HD 83618, HIP 47431, HR 3845, SAO 137035.[6]
Database references


ι Hydrae (Latinised to Iota Hydrae) is the star's Bayer designation.

This star along with Tau¹ Hydrae, Tau² Hydrae and 33 Hydrae (A Hydrae), were Ptolemy's Καμπή (Kampē); but Kazwini knew them as عقدة ʽuqdah (or ʽuḳdah) "knot".[10] According to a 1971 NASA memorandum,[11] Ukdah was the name of an asterism of four stars: Tau¹ Hydrae as Uḳdah I, Tau² Hydrae as Uḳdah II, 33 Hydrae as Uḳdah III and Iota Hydrae as Uḳdah IV. In 2016, the IAU organized a Working Group on Star Names (WGSN)[12] to catalog and standardize proper names for stars. The WGSN approved the name Ukdah for Iota Hydrae on 1 June 2018 and it is now so included in the List of IAU-approved Star Names.[7]

In Chinese, 柳宿 (Xīng Sù), meaning Star (asterism), refers to an asterism consisting of ι Hydrae, Alphard, τ1 Hydrae, τ2 Hydrae, 26 Hydrae, 27 Hydrae, HD 82477 and HD 82428.[13] Consequently, ι Hydrae are known as 星宿四 (Xīng Sù sì, English: the Fourth Star of Star).[14]


This is an evolved K-type giant star with a stellar classification of K2.5 III.[2] It is a Barium star, which means that, for a giant star, it displays unusually strong absorption lines of singly-ionized barium and strontium.[15] Iota Hydrae has nearly twice[2] the mass of the Sun and has expanded to 33 times the Sun's radius.[4] It is around 2.5 billion years old and is spinning with a leisurely projected rotational velocity of 4.5[4] km/s. It may be a member of the Wolf 630 moving group of stars that share a common trajectory through space.[16]


  1. ^ a b c d e f van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
  2. ^ a b c d e f g h i Luck, R. Earle (September 2015), "Abundances in the Local Region. I. G and K Giants", The Astronomical Journal, 150 (3): 23, arXiv:1507.01466, Bibcode:2015AJ....150...88L, doi:10.1088/0004-6256/150/3/88, S2CID 118505114, 88.
  3. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (2009), "General Catalogue of Variable Stars", VizieR On-line Data Catalog, 1, Bibcode:2009yCat....102025S.
  4. ^ a b c d e f Massarotti, Alessandro; et al. (January 2008), "Rotational and radial velocities for a sample of 761 HIPPARCOS giants and the role of binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209.
  5. ^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  6. ^ "iot Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-04.{{cite web}}: CS1 maint: postscript (link)
  7. ^ a b "Naming Stars". Retrieved 18 June 2018.
  8. ^ O'Meara, Steve (2007), Herschel 400 Observing Guide, Cambridge University Press, p. 84, ISBN 978-0521858939.
  9. ^ Moore, Patrick (2013), The Observer's Year: 366 Nights of the Universe, The Patrick Moore Practical Astronomy Series, Springer Science & Business Media, p. 71, ISBN 978-1447136132.
  10. ^ 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.
  11. ^ 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
  12. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  13. ^ (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.
  14. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 29 日
  15. ^ Lu, Phillip K. (June 1991), "Taxonomy of barium stars", Astronomical Journal, 101: 2229–2254, Bibcode:1991AJ....101.2229L, doi:10.1086/115845.
  16. ^ McDonald, A. R. E.; Hearnshaw, J. B. (August 1983), "The Wolf 630 moving group of stars", Monthly Notices of the Royal Astronomical Society, 204 (3): 841–852, Bibcode:1983MNRAS.204..841M, doi:10.1093/mnras/204.3.841.