Eta Hydrae

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Eta Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 08h 43m 13.47499s[1]
Declination +03° 23′ 55.1867″[1]
Apparent magnitude (V) 4.294[2]
Spectral type B3 V[3]
U−B color index −0.726[2]
B−V color index −0.187[2]
Variable type Candidate β Cep[4]
Proper motion (μ) RA: −19.39[1] mas/yr
Dec.: −1.08[1] mas/yr
Parallax (π) 5.56 ± 0.24[1] mas
Distance 590 ± 30 ly
(180 ± 8 pc)
Mass 7.0±0.1[5] M
Radius 3.9[6] R
Luminosity (bolometric) 2,680[7] L
Surface gravity (log g) 3.933[8] cgs
Temperature 18,630±411[8] K
Rotational velocity (v sin i) 101±5[8] km/s
Age 31.6±3.9[5] Myr
Other designations
η Col, 7 Hydrae, BD+03° 2039, FK5 2687, HD 74280, HIP 42799, HR 3454, SAO 117050.[9]

Eta Hydrae (η Hya) is the Bayer designation for 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.[10] Based upon an annual parallax shift of 5.56 mas,[1] it is located roughly 590 light years from the Sun.

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.[11] The star is around 32[5] million years old and is spinning with a projected rotational velocity of 101[8] km/s. It has an estimated seven[5] times the mass of the Sun and nearly four[6] times the Sun's radius. Eta Hydrae radiates 2,680[7] times the solar luminosity from its outer atmosphere at an effective temperature of 18,630[8] K.


  1. ^ a b c d e f van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357. 
  2. ^ 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. 
  3. ^ 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: 252–62, arXiv:astro-ph/0606149Freely accessible, Bibcode:2006MNRAS.371..252L, doi:10.1111/j.1365-2966.2006.10655.x. 
  4. ^ a b Stankov, Anamarija; Handler, Gerald (June 2005), "Catalog of Galactic β Cephei Stars", The Astrophysical Journal Supplement Series, 158 (2): 193−216, Bibcode:2005ApJS..158..193S, doi:10.1086/429408. 
  5. ^ 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:1007.4883Freely accessible, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x. 
  6. ^ a b Pasinetti Fracassini, L. E.; et al. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)", Astronomy and Astrophysics (3rd ed.), 367: 521–524, arXiv:astro-ph/0012289Freely accessible, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451. 
  7. ^ 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:1003.2335Freely accessible, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355. 
  8. ^ 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), arXiv:0805.2133Freely accessible, Bibcode:2008ApJ...683.1045H, doi:10.1086/590106, 1045−1051. 
  9. ^ "eta Hya -- Variable Star of beta Cep type", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-01-03. 
  10. ^ 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 1447136136. 
  11. ^ 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:1008.4260Freely accessible, Bibcode:2008A&A...483..857S, doi:10.1051/0004-6361:20077740.