Epoch J2000.0 Equinox J2000.0
|Right ascension||8h 46m 46.51223s|
|Declination||+06° 25′ 07.6855″|
|Apparent magnitude (V)||AB: 3.38
|Spectral type||AB: G5 III + F0 V
|U−B color index||+0.37|
|B−V color index||+0.68|
|Radial velocity (Rv)||+36.4 km/s|
|Proper motion (μ)||RA: −228.11 mas/yr
Dec.: −43.82 mas/yr
|Parallax (π)||25.23 ± 0.98 mas|
|Distance||129 ± 5 ly
(40 ± 2 pc)
|Primary||ε Hya A|
|Companion||ε Hya B|
|Period (P)||5,478.98 ± 93.19 days|
|Semi-major axis (a)||0.226"|
|Eccentricity (e)||0.652 ± 0.020|
|Periastron epoch (T)||2437060.85 ± 63.86|
|Argument of periastron (ω)
|74.66 ± 0.21°|
|8.05 ± 0.14 km/s|
|ε Hya A|
|Rotational velocity (v sin i)||19 km/s|
Epsilon Hydrae (ε Hya, 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) 135 light-years from Earth.
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 and an orbital eccentricity of 0.62, 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.
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, giving it the yellow hue of a G-type star. It is rotating rapidly with a projected rotational velocity of 19 km s–1. It has an X-ray luminosity of 2.8 × 1029 erg s–1.
Name and etymology
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)
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 Consequently, ε Hydrae itself is known as 柳宿五 (Liǔ Sù wǔ, English: the Fifth Star of Willow.)
In ancient Indian astronomy, the five-star cluster of Hydrae is collectively called Ashlesha (Ayilyam in southern India) Nakshatras
- 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
- Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics 352: 495–507, Bibcode:1999A&A...352..495M
- 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
- 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
- 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
- Wilson, R. E. (1953). General Catalogue of Stellar Radial Velocities. Carnegie Institute of Washington D.C. Bibcode:1953GCRV..C......0W.
- Bakos, J.; Tremko (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
- 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
- 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.
- Rhoads, Jack W. (November 15, 1971), Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology
- (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.
- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日
- Encyclopaedia of the history of science, technology, and medicine in non-western cultures, ed. Helaine Selin, Dordrecht, Kluwer Academic Publishers, 1997, p.105.
- "HD 74874 -- Variable Star". SIMBAD Astronomical Database. Retrieved 2006-11-05.