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η Hydrae
	Location of η Hydrae (circled)
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	08h 43m 13.47499s
Declination	+03° 23′ 55.1867″
Apparent magnitude (V)	4.294
Characteristics
Spectral type	B3 V
U−B color index	−0.726
B−V color index	−0.187
Variable type	Candidate β Cep
Astrometry
Proper motion (μ)	RA: −19.39 mas/yr ; Dec.: −1.08 mas/yr
Parallax (π)	5.56 ± 0.24 mas
Distance	590 ± 30 ly ; (180 ± 8 pc)
Absolute magnitude (MV)	−1.48
Details
Mass	7.0±0.1 M☉
Radius	3.9 R☉
Luminosity (bolometric)	2,680 L☉
Surface gravity (log g)	3.933 cgs
Temperature	18,630±411 K
Rotational velocity (v sin i)	101±5 km/s
Age	31.6±3.9 Myr
Other designations
	η Hya, 7 Hydrae, BD+03°2039, FK5 2687, HD 74280, HIP 42799, HR 3454, SAO 117050
Database references
SIMBAD	data

Eta Hydrae (η Hydrae) is a star in the equatorial constellation of Hydra. With an apparent visual magnitude of 4.3,^[2] it is visible to the naked eye. However, it is the faintest of the five stars that form the "head" of the hydra.^[11] Based upon an annual parallax shift of 5.56 mas,^[1] it is located roughly 590 light years from the Sun.

A light curve for Eta Hydrae, plotted from *TESS* data^[12]

This is a B-type main sequence star with a stellar classification of B3V.^[3] It has been classified as a candidate Beta Cephei variable with a period of 2.2 days, although this designation was rejected by Stankov and Handler (2005).^[4] The spectrum shows a slight underabundance of carbon, compared to the Sun.^[13] The star is around 32^[6] million years old and is spinning with a projected rotational velocity of 101^[9] km/s. It has an estimated seven^[6] times the mass of the Sun and nearly four^[7] times the Sun's radius. Eta Hydrae radiates 2,680^[8] times the solar luminosity from its outer atmosphere at an effective temperature of 18,630^[9] K.

Name and etymology

This star, along with δ Hya (Lisan al Sudja), ε Hya, ζ Hya, ρ Hya and σ Hya (Minchir), were Ulug Beg's Min al Azʽal, "Belonging to the Uninhabited Spot".^[14] 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).^[15]

In Chinese, 柳宿 (Liǔ Sù), meaning Willow, refers to an asterism consisting of η Hydrae, δ Hydra, σ Hydrae, ρ Hydrae, ε Hydrae, ζ Hydrae, ω Hydrae and θ Hydrae^[16] Consequently, η Hydrae itself is known as 柳宿三 (Liǔ Sù sān, English: the Third Star of Willow).^[17]

The people of Groote Eylandt called Unwala, "The Crab", for the star cluster including this star, δ Hya (Lisan al Sudja), ε Hya, ζ Hya, ρ Hya and σ Hya (Minchir).^[18]

References

^ ^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.
^ ^a ^b ^c ^d Oja, T. (August 1986), "UBV photometry of stars whose positions are accurately known. III", Astronomy and Astrophysics Supplement Series, 65 (2): 405–409, Bibcode:1986A&AS...65..405O.
^ ^a ^b Levenhagen, R. S.; Leister, N. V. (2006), "Spectroscopic Analysis of Southern B and Be Stars", Monthly Notices of the Royal Astronomical Society, 371 (1): 252–62, arXiv:astro-ph/0606149, Bibcode:2006MNRAS.371..252L, doi:10.1111/j.1365-2966.2006.10655.x, S2CID 16492030.
^ ^a ^b Stankov, Anamarija; Handler, Gerald (June 2005), "Catalog of Galactic β Cephei Stars", The Astrophysical Journal Supplement Series, 158 (2): 193–216, arXiv:astro-ph/0506495, Bibcode:2005ApJS..158..193S, doi:10.1086/429408, S2CID 119526948.
^ Ammler-von Eiff, Matthias; Reiners, Ansgar (June 2012), "New measurements of rotation and differential rotation in A-F stars: are there two populations of differentially rotating stars?", Astronomy & Astrophysics, 542: A116, arXiv:1204.2459, Bibcode:2012A&A...542A.116A, doi:10.1051/0004-6361/201118724, S2CID 53666672.
^ ^a ^b ^c ^d Tetzlaff, N.; et al. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x, S2CID 118629873.
^ ^a ^b Pasinetti Fracassini, L. E.; et al. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)", Astronomy and Astrophysics, 367 (3rd ed.): 521–524, arXiv:astro-ph/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451, S2CID 425754.
^ ^a ^b Hohle, M. M.; et al. (April 2010), "Masses and luminosities of O- and B-type stars and red supergiants", Astronomische Nachrichten, 331 (4): 349, arXiv:1003.2335, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355, S2CID 111387483.
^ ^a ^b ^c ^d ^e Huang, W.; Gies, D. R. (August 2008), "Stellar Rotation in Field and Cluster B Stars", The Astrophysical Journal, 683 (2): 1045–1051, arXiv:0805.2133, Bibcode:2008ApJ...683.1045H, doi:10.1086/590106, S2CID 18926523, 1045−1051.
^ "eta Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-03.
^ Moore, Patrick (2013), The Observer's Year: 366 Nights of the Universe, The Patrick Moore Practical Astronomy Series, Springer Science & Business Media, p. 79, ISBN 978-1447136132.
^ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
^ Schnerr, R. S.; et al. (June 2008), "Magnetic field measurements and wind-line variability of OB-type stars", Astronomy and Astrophysics, 483 (3): 857–867, arXiv:1008.4260, Bibcode:2008A&A...483..857S, doi:10.1051/0004-6361:20077740, S2CID 53454915.
^ 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