Omega Hydrae

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Omega Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 09h 05m 58.36642s[1]
Declination +05° 05′ 32.3360″[1]
Apparent magnitude (V) 5.00[2]
Spectral type K2 II-III[3]
U−B color index +1.22[2]
B−V color index +1.22[2]
Radial velocity (Rv) +24.3±0.8[4] km/s
Proper motion (μ) RA: −19.58[1] mas/yr
Dec.: −11.07[1] mas/yr
Parallax (π) 3.64 ± 0.31[1] mas
Distance 900 ± 80 ly
(270 ± 20 pc)
Absolute magnitude (MV) −2.19[5]
Mass 4.32±0.37 M
Radius 48.49±5.55 R
Luminosity 944.3±178.3 L
Surface gravity (log g) 1.74±0.12 cgs
Temperature 4,789[7] K
Metallicity [Fe/H] −0.12±0.10 dex
Rotational velocity (v sin i) 2.3[8] km/s
Age 180±70 Myr
Other designations
ω Lyr, 18 Hya, BD+05° 2116, HD 77996, HIP 44659, HR 3613, SAO 117420.[9]
Database references

Omega Hydrae, Latinized from ω Hydrae, is a golden-hued star in the equatorial constellation of Hydra, located to the west-southwest of the brighter star Zeta Hydrae.[10] Based upon an annual parallax shift of just 3.64 mas as seen from Earth, it is located roughly 900 light years from the Sun. It is faintly visible to the naked eye with an apparent visual magnitude of 5.00.[2]

This is an evolved K-type star with a stellar classification of K2 II-III,[3] which indicates a spectrum showing traits intermediate between the giant and bright giant stages. It is most likely (98% chance) on the horizontal branch, indicating that the star is generating energy through the thermonuclear fusion of helium at its core. With 4.32 times the Sun's mass, it has expanded to around 48 times the radius of the Sun. Omega Hydrae is about 180[6] million years old and spinning with a leisurely projected rotational velocity of 2.3 km/s.[8] The star is radiating roughly 944[6] times the solar luminosity from its photosphere at an effective temperature of 4,789 K.[7]


  1. ^ a b c d e 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 ^ a b c d Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV data, SIMBAD, Bibcode:1986EgUBV........0M. 
  3. ^ a b Tetzlaff, N.; et al. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410: 190. arXiv:1007.4883Freely accessible. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. 
  4. ^ de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048Freely accessible, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, A61. 
  5. ^ 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. 
  6. ^ a b c Reffert, Sabine; et al. (2015), "Precise radial velocities of giant stars. VII. Occurrence rate of giant extrasolar planets as a function of mass and metallicity", Astronomy & Astrophysics, 574: A116, arXiv:1412.4634Freely accessible, Bibcode:2015A&A...574A.116R, doi:10.1051/0004-6361/201322360.  Values are for the higher probability horizontal branch model fit.
  7. ^ a b McDonald, I.; et al. (2012), "Fundamental Parameters and Infrared Excesses of Hipparcos Stars", Monthly Notices of the Royal Astronomical Society, 427 (1): 343–57, arXiv:1208.2037Freely accessible, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x. 
  8. ^ a b Lèbre, A.; et al. (May 2006), "Lithium abundances and rotational behavior for bright giant stars", Astronomy and Astrophysics, 450 (3): 1173–1179, Bibcode:2006A&A...450.1173L, doi:10.1051/0004-6361:20053485 
  9. ^ "ome Hya -- Star", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-03-17. 
  10. ^ O'Meara, Stephen James (2016), Deep-Sky Companions: The Caldwell Objects, Cambridge University Press, p. 224, ISBN 1107083974 

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