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ζ Hydrae
	Location of ζ Hydrae (circled)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Hydra
Right ascension	08h 55m 23.62614s
Declination	+05° 56′ 44.0354″
Apparent magnitude (V)	3.10
Characteristics
Spectral type	G9 II-III
U−B color index	+0.82
B−V color index	+1.00
Astrometry
Radial velocity (Rv)	+22.8 km/s
Proper motion (μ)	RA: –100.06 mas/yr ; Dec.: +15.46 mas/yr
Parallax (π)	19.51 ± 0.18 mas
Distance	167 ± 2 ly ; (51.3 ± 0.5 pc)
Absolute magnitude (MV)	–0.24
Details
Mass	3.09±0.1 M☉
Radius	17.61±0.19 R☉
Luminosity	174.2±9.3 L☉
Surface gravity (log g)	2.48 cgs
Temperature	4,996±30 K
Metallicity [Fe/H]	–0.21 dex
Rotational velocity (v sin i)	2.5 km/s
Age	370±40 Myr
Other designations
	16 Hydrae, BD+06 2060, FK5 334, HD 76294, HIP 43813, HR 3547, SAO 117264.
Database references
SIMBAD	data

Zeta Hydrae (ζ Hya, ζ Hydrae) is a solitary star in the equatorial constellation of Hydra. This is a generally faint constellation, so, at an apparent visual magnitude of +3.10,^[2] this is the third-brightest member after Alphard and Gamma Hydrae.

Distance

The distance to this star has been measured using the parallax technique, yielding a value of roughly 167 light-years (51 parsecs).^[1] At this distance, the visual magnitude of the star is diminished by 0.03 as a result of extinction from intervening gas and dust.^[5] Delta Hydrae is about 12.9 light-years (4.0 parsecs) from Zeta Hydrae and may be a largely co-moving object.^[10] The star has one of the lower-error margin readings among those of the Gaia spacecraft which computes a parallax of 20.7182 ± 0.3925 mas and, if correct, a distance of 157 ± 3 light years.^[11]

Characteristics

With a stellar classification of G9 II-III,^[3] this is an evolved giant star that is radiating 174 times the luminosity of the Sun from its outer envelope at an effective temperature of around 5,000 K.^[6] At this heat, the star glows with the yellow hue of a G-type star.^[12] The radius of this star, measured using interferometry, is 17.6 times the Sun's radius. It has an estimated 3.1 times the mass of the Sun and is around 370 million years old.^[6]

Name and etymology

This star, along with δ Hya (Lisan al Sudja), ε Hya, η Hya, ρ Hya and σ Hya (Minchir), were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".^[13] 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).^[14]

In Chinese, 柳宿 (Liǔ Sù), meaning Willow, refers to an asterism consisting of ζ Hydrae, δ Hydra, σ Hydrae, η Hydrae, ρ Hydrae, ε Hydrae, ω Hydrae and θ Hydrae^[15] Consequently, ζ Hydrae itself is known as 柳宿六 (Liǔ Sù liù, English: the Sixth Star of Willow).^[16]

The people of Groote Eylandt called Unwala, "The Crab", for the star cluster including this star, δ Hya (Lisan al Sudja), ε Hya, η Hya, ρ Hya and σ Hya (Minchir).^[17]

References

^ ^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
^ ^a ^b ^c ^d Jennens, P. A.; Helfer, H. L. (September 1975), "A new photometric metal abundance and luminosity calibration for field G and K giants.", Monthly Notices of the Royal Astronomical Society, 172 (3): 667–679, Bibcode:1975MNRAS.172..667J, doi:10.1093/mnras/172.3.667
^ ^a ^b Morgan, W. W.; Keenan, P. C. (1973), "Spectral Classification", Annual Review of Astronomy and Astrophysics, 11 (1): 29–50, Bibcode:1973ARA&A..11...29M, doi:10.1146/annurev.aa.11.090173.000333
^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veroeffentlichungen des Astronomischen Rechen-Instituts Heidelberg, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1, Bibcode:1999VeARI..35....1W
^ ^a ^b Takeda, Yoichi; Sato, Bun'ei; Murata, Daisuke (August 2008), "Stellar parameters and elemental abundances of late-G giants", Publications of the Astronomical Society of Japan, 60 (4): 781–802, arXiv:0805.2434, Bibcode:2008PASJ...60..781T, doi:10.1093/pasj/60.4.781, S2CID 16258166
^ ^a ^b ^c Baines, Ellyn K.; et al. (2018), "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer", The Astronomical Journal, 155 (1), 30, arXiv:1712.08109, Bibcode:2018AJ....155...30B, doi:10.3847/1538-3881/aa9d8b, S2CID 119427037.
^ Ando, Hiroyasu; Tsuboi, Yusuke; Kambe, Eiji; Sato, Bun'ei (August 2010), "Oscillations in G-Type Giants", Publications of the Astronomical Society of Japan, 62 (4): 1117–1126, arXiv:1007.1286, Bibcode:2010PASJ...62.1117A, doi:10.1093/pasj/62.4.1117, S2CID 118677219
^ ^a ^b 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, S2CID 121883397
^ "zet Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-05.
^ Shaya, Ed J.; Olling, Rob P. (January 2011), "Very Wide Binaries and Other Comoving Stellar Companions: A Bayesian Analysis of the Hipparcos Catalogue", The Astrophysical Journal Supplement, 192 (1): 2, arXiv:1007.0425, Bibcode:2011ApJS..192....2S, doi:10.1088/0067-0049/192/1/2, S2CID 119226823
^ Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2013-12-03, retrieved 2012-01-16
^ 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 (PDF), Jet Propulsion Laboratory, California Institute of Technology
^ (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.
^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 28 日 Archived 2011-05-22 at the Wayback Machine
^ Selin, Helaine, ed. (1997), Encyclopaedia of the history of science, technology, and medicine in non-western cultures, Dordrecht: Kluwer Academic Publishers, p. 105, ISBN 9780792340669