Epsilon Hydrae

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Epsilon Hydrae
Diagram showing star positions and boundaries of the Hydra constellation and its surroundings
Cercle rouge 100%.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[1] mas
Distance 129 ± 5 ly
(40 ± 2 pc)
Absolute magnitude (MV) +0.39[7]
Primary ε Hya A
Companion ε 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
Luminosity 67[2] L
Temperature 5,620[2] K
Metallicity [Fe/H] −0.08[7] dex
Rotational velocity (v sin i) 19[9] km/s
Other designations
ε Hya, 11 Hydrae, BD+06 2036, HD 74874, HIP 43109, HR 3482, NSV 4244, SAO 117112.
Database references

Epsilon Hydrae, Latinized from ε Hydrae, is a Class G5, third-magnitude star system in the constellation Hydra. This stellar system is located about 129 light-years (40 parsecs) from Earth.[1]

It is a multiple star system having at least four stellar components, the pair ε Hya AB form a binary star with an orbital period of 15 years and an angular separation of 0.2 arcseconds. Component ε Hya C is separated from the AB pair by 3 arcseconds. It is a spectroscopic binary system with a period of 9.9047 days[8] and an orbital eccentricity of 0.62,[9] while simultaneously orbiting ε Hya AB with a period of 870 years. The fourth component, ε Hya D, shares a common proper motion with the other components and thus is most likely a gravitationally-bound member of the system, it is separated from the AB pair by 19 arcseconds and has an estimated orbital period of around 10,000 years.[8]

The primary, ε Hya 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]

Name and etymology[edit]

This star, along with δ Hya (Lisan al Sudja), ζ Hya, η Hya, ρ Hya and σ Hya (Minhar al Shija), were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".[10]

According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Min al Azʽal or Minazal were the title for five stars:δ Hya as Minazal I, η Hya as Minazal II, ε Hya as Minazal III, ρ Hya as Minazal IV and ζ Hya as Minazal V (exclude σ Hya)[11]

In Chinese, 柳宿 (Liǔ Sù), meaning Willow (asterism), refers to an asterism consisting of ε Hydrae, δ Hydra, σ Hydrae, η Hydrae, ρ Hydrae, ζ Hydrae, ω Hydrae and θ Hydrae[12] Consequently, ε Hydrae itself is known as 柳宿五 (Liǔ Sù wǔ, English: the Fifth Star of Willow.)[13]

In ancient Indian astronomy, the five-star cluster of Hydrae is collectively called Ashlesha (Ayilyam in southern India) Nakshatras

The people of Groote Eylandt called Unwala, "The Crab", for the star cluster including this star, δ Hya (Lisan al Sudja), ζ Hya, η Hya, ρ Hya and σ Hya (Minhar al Shija).[14]


  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:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  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/0406573Freely accessible, Bibcode:2004A&A...424..727P, doi:10.1051/0004-6361:20041213 
  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), Bibcode:1966CoLPL...4...99J 
  6. ^ Wilson, R. E. (1953). General Catalogue of Stellar Radial Velocities. 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:1108.4971Freely accessible, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015. 
  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. ^ 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. 
  11. ^ 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 
  12. ^ (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.
  13. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日
  14. ^ Encyclopaedia of the history of science, technology, and medicine in non-western cultures, ed. Helaine Selin, Dordrecht, Kluwer Academic Publishers, 1997, p.105.

External links[edit]