Epsilon Hydrae

Epsilon Hydrae (ε Hydrae, abbreviated Epsilon Hya, ε Hya) is a multiple star system of a combined third magnitude in the constellation of Hydra. Based upon parallax measurements obtained during the Hipparcos mission, it is located roughly 129 light-years (40 parsecs) distant from the Sun.[1]

ε Hydrae
Hydra constellation map.svg
Red circle.svg
Location of ε Hydrae (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Hydra
Right ascension 8h 46m 46.51223s[1]
Declination +06° 25′ 07.6855″[1]
Apparent magnitude (V) AB: 3.38[2]
C: 7.5[3]
Spectral type AB: G5 III + F0 V[4]
C: F5[3]
U−B color index +0.37[5]
B−V color index +0.68[5]
Radial velocity (Rv)+36.4[6] km/s
Proper motion (μ) RA: −228.11[1] mas/yr
Dec.: −43.82[1] mas/yr
Parallax (π)25.23 ± 0.98 mas[1]
Distance129 ± 5 ly
(40 ± 2 pc)
Absolute magnitude (MV)+0.39[7]
Primaryε Hya A
Companionε Hya B
Period (P)5,478.98 ± 93.19 days
Semi-major axis (a)0.226[9]
Eccentricity (e)0.652 ± 0.020
Inclination (i)49.3°
Periastron epoch (T)2437060.85 ± 63.86
Argument of periastron (ω)
74.66 ± 0.21°
Semi-amplitude (K1)
8.05 ± 0.14 km/s
ε Hya A
Luminosity67[2] L
Temperature5,620[2] K
Metallicity [Fe/H]−0.08[7] dex
Rotational velocity (v sin i)19[9] km/s
Other designations
Ashlesha, ε Hya, 11 Hydrae, BD+06 2036, HD 74874, HIP 43109, HR 3482, NSV 4244, SAO 117112.
Database references

The system consists of a binary pair designated Epsilon Hydrae AB, whose two components are themselves designated Epsilon Hydrae A (formally named Ashlesha /ɑːʃˈlʃə/)[10] and B, orbited by a spectroscopic binary designated Epsilon Hydrae C. A possible fourth component, designated Epsilon Hydrae D, shares a common proper motion with the other components and thus is most likely a gravitationally-bound member of the system.


ε Hydrae (Latinised to Epsilon Hydrae) is the system's Bayer designation. The designations of the three constituents as Epsilon Hydrae AB, C and D, and those of AB's components - Epsilon Hydrae A and B - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[11]

In ancient Indian astronomy, the five-star cluster of Hydrae is collectively called आश्लेषा āślēṣā (Sanskrit pronunciation: [aːɕˈleːʂaː]) "the embrace", the 9th of the 27 nakshatras or lunar mansions in Hindu astrology.[12] In 2016, the IAU organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[14] It approved the name Ashlesha for the component Epsilon Hydrae A on 1 June 2018 and it is now so included in the List of IAU-approved Star Names.[10]

This system, along with Delta Hydrae (Lisan al Sudja), Zeta Hydrae, Eta Hydrae, Rho Hydrae and Sigma Hydrae (Minchir), were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".[15] According to a 1971 NASA technical memorandum, Min al Azʽal or Minazal were the title for five stars: Delta Hydrae as Minazal I, Eta Hydrae as Minazal II, Epsilon Hydrae as Minazal III, Rho Hydrae as Minazal IV and Zeta Hydrae as Minazal V.[16]

In Chinese, 柳宿 (Liǔ Sù), meaning Willow (asterism), refers to an asterism consisting of Epsilon Hydrae, Delta Hydrae, Sigma Hydrae, Eta Hydrae, Rho Hydrae, Zeta Hydrae, Omega Hydrae and Theta Hydrae[17] Consequently, Epsilon Hydrae itself is known as 柳宿五 (Liǔ Sù wǔ, English: the Fifth Star of Willow).[18]

Unwala, "The Crab", was the name given by the people of Groote Eylandt to the star cluster including this star, Delta Hydrae (Lisan al Sudja), Zeta Hydrae, Eta Hydrae, Rho Hydrae and Sigma Hydrae (Minchir).[19]


The Epsilon Hydrae AB binary has an orbital period of 15 years and an angular separation of 0.2 arcseconds. The spectroscopic binary Epsilon Hydrae C has a period of 9.9047 days[8] and an orbital eccentricity of 0.62,[9] while simultaneously orbiting AB with a period of 870 years. C is separated from AB by 3 arcseconds. Epsilon Hydrae D is separated from AB by 19 arcseconds and has an estimated orbital period of around 10,000 years.[8]

The primary, Epsilon Hydrae A, is a giant star that is shining with 67 times the luminosity of the Sun. This energy is being radiated from the star's outer atmosphere at an effective temperature of 5,620 K,[2] giving it the yellow hue of a G-type star. It is rotating rapidly with a projected rotational velocity of 19 km s−1.[9] It has an X-ray luminosity of 2.8 × 1029 erg s−1.[4]


  1. ^ a b c d e f van Leeuwen, F. (November 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 Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics, 352: 495–507, Bibcode:1999A&A...352..495M
  3. ^ a b Pourbaix, D.; et al. (September 2004), "SB9: The ninth catalogue of spectroscopic binary orbits", Astronomy and Astrophysics, 424: 727–732, arXiv:astro-ph/0406573, Bibcode:2004A&A...424..727P, doi:10.1051/0004-6361:20041213, S2CID 119387088
  4. ^ a b Makarov, Valeri V. (October 2003), "The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun", The Astronomical Journal, 126 (4): 1996–2008, Bibcode:2003AJ....126.1996M, doi:10.1086/378164
  5. ^ a b Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
  6. ^ Wilson, R. E. (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Carnegie Institute of Washington D.C. Bibcode:1953GCRV..C......0W.
  7. ^ a b 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.
  8. ^ a b c Bakos, G. A.; Tremko, J. (1987), "Spectroscopic orbital elements and photometry of the multiple system epsilon Hydrae", Contributions of the Astronomical Observatory Skalnaté Pleso, 16: 17–26, Bibcode:1987CoSka..16...17B
  9. ^ a b c d Strassmeier, Klaus G.; Hall, Douglas S. (August 1988), "Photometric variability in chromospherically active stars. I - The constant stars", Astrophysical Journal Supplement Series, 67: 439–451, Bibcode:1988ApJS...67..439S, doi:10.1086/191278
  10. ^ a b "Naming Stars". IAU.org. Retrieved 18 June 2018.
  11. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  12. ^ Arthur Llewellyn Basham, The Wonder that was India, 1954, appendix II: Astronomy, p.490
  13. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  14. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  15. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 249. ISBN 0-486-21079-0. Retrieved 2010-12-12.
  16. ^ Rhoads, Jack W. (November 15, 1971), Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars (PDF), Jet Propulsion Laboratory, California Institute of Technology
  17. ^ (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.
  18. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日
  19. ^ Helaine Selin, ed. (1997). Encyclopaedia of the History of Science, Technology, and Medicine in Non-Westen Cultures. Dordrecht: Springer Science & Business Media. p. 105. ISBN 978-0-7923-4066-9.

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