Omicron Hydrae

Omicron Hydrae (ο Hya) is the Bayer designation for a solitary[8] star in the equatorial constellation Hydra. At one time it bore the Flamsteed designation 25 Crateris, but this is no longer used by astronomers so as to avoid confusion.[9] With an apparent visual magnitude of 4.70,[2] this star is visible to the naked eye. Based upon an annual parallax shift of 7.27 mas,[1] it is located around 449 light years from the Sun.

Omicron Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 11h 40m 12.78970s[1]
Declination −34° 44′ 40.7733″[1]
Apparent magnitude (V) 4.70[2]
Spectral type B9 V[3]
U−B color index −0.20[2]
B−V color index −0.08[2]
Radial velocity (Rv)+5.9[4] km/s
Proper motion (μ) RA: −43.24[1] mas/yr
Dec.: −1.61[1] mas/yr
Parallax (π)7.27 ± 0.16 mas[1]
Distance449 ± 10 ly
(138 ± 3 pc)
Absolute magnitude (MV)−0.99[5]
Mass3.56±0.04 M
Luminosity309 L
Temperature10,495 K
Rotational velocity (v sin i)160 km/s
Other designations
ο Hya, CD−34° 7610, FK5 439, HD 101431, HIP 56922, HR 4494, SAO 202695.[7]
Database references

This is a B-type main sequence star with a stellar classification of B9 V.[3] It has completed an estimated 98.4%±1.1% of its lifetime on the main sequence. With 3.56[6] times the mass of the Sun, it radiates 309 times the Sun's luminosity from its outer atmosphere at an effective temperature of 10,495 K.[6] The rate of spin is relatively high, with a projected rotational velocity of 160 km/s.[6]


  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:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
  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 ^ a b c d Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.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 Houk, N. (1982), Michigan Catalogue of Two-dimensional Spectral Types for the HD stars, vol. 3, Ann Arbor, MI: Department of Astronomy, University of Michigan,
  4. ^ Wielen, R.; et al. (2000), "Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions", Veröffentlichungen Astronomisches Rechen-Institut Heidelberg, Karlsruhe: Verlag G. Braun, vol. 37, no. 37, pp. 1–308, Bibcode:2000VeARI..37....1W, ISBN 3-7650-0536-3.
  5. ^ 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.
  6. ^ a b c d Zorec, J.; Royer, F. (January 2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", Astronomy & Astrophysics, 537: A120, arXiv:1201.2052, Bibcode:2012A&A...537A.120Z, doi:10.1051/0004-6361/201117691, S2CID 55586789.
  7. ^ "* omi Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-06.
  8. ^ Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
  9. ^ Wagman, M. (August 1987), "Flamsteed's Missing Stars", Journal for the History of Astronomy, 18 (3): 216, Bibcode:1987JHA....18..209W, doi:10.1177/002182868701800305, S2CID 118445625.