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Omega Hydrae
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	09h 05m 58.36642s
Declination	+05° 05′ 32.3360″
Apparent magnitude (V)	5.00
Characteristics
Spectral type	K2 II-III
U−B color index	+1.22
B−V color index	+1.22
Astrometry
Radial velocity (Rv)	+24.3±0.8 km/s
Proper motion (μ)	RA: −19.58 mas/yr ; Dec.: −11.07 mas/yr
Parallax (π)	3.64 ± 0.31 mas
Distance	900 ± 80 ly ; (270 ± 20 pc)
Absolute magnitude (MV)	−2.19
Details
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 K
Metallicity [Fe/H]	−0.12±0.10 dex
Rotational velocity (v sin i)	2.3 km/s
Age	180±70 Myr
Other designations
	ω Hya, 18 Hydrae, BD+05°2116, HD 77996, HIP 44659, HR 3613, SAO 117420
Database references
SIMBAD	data

Omega Hydrae, Latinised 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]

References

^ ^a ^b ^c ^d ^e 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^ ^a ^b ^c ^d Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.
^ ^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 (1): 190–200. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. S2CID 118629873.
^ 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:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, S2CID 59451347, A61.
^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
^ ^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:1412.4634, Bibcode:2015A&A...574A.116R, doi:10.1051/0004-6361/201322360, hdl:10722/215277, S2CID 59334290. Values are for the higher probability horizontal branch model fit.
^ ^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:1208.2037, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x, S2CID 118665352.
^ ^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
^ "* ome Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-03-17.{{cite web}}: CS1 maint: postscript (link)
^ O'Meara, Stephen James (2016), Deep-Sky Companions: The Caldwell Objects, Cambridge University Press, p. 224, ISBN 978-1107083974