Rho Hydrae

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Rho Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension  08h 48m 25.97057s[1]
Declination +05° 50′ 16.1283″[1]
Apparent magnitude (V) 4.34[2]
Spectral type A0 Vn[3]
U−B color index −0.04[2]
B−V color index −0.04[2]
Radial velocity (Rv)+32.8[4] km/s
Proper motion (μ) RA: −17.33[1] mas/yr
Dec.: −29.41[1] mas/yr
Parallax (π)9.21 ± 0.21[1] mas
Distance354 ± 8 ly
(109 ± 2 pc)
Absolute magnitude (MV)−0.83[5]
Mass3.24±0.05 M
Radius2.0[6] R
Luminosity242 L
Temperature9,795 K
Rotational velocity (v sin i)128 km/s
Age350[7] Myr
Other designations
ρ Hya, 13 Hya, BD+06° 2040, HD 75137, HIP 43234, HR 3492, SAO 117146.[8]
Database references

Rho Hydrae, Latinized from ρ Hydrae, is a binary star[9] in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.34.[2] This system forms part of the ring-shaped asterism that represents the head of the hydra constellation.[10] The distance to this system, based upon an annual parallax shift of 9.21 mas,[1] is about 354 light years. At that distance, the visual magnitude is diminished by an interstellar extinction factor of 0.06, due to intervening dust.[7]

The primary component is an A-type main sequence star with a stellar classification of A0 Vn.[3] It has around double[6] the radius of the Sun and 3.2 times the Sun's mass. Rho Hydrae is around 350[7] million years old and has a high rate of spin, with a projected rotational velocity of 128 km/s. It radiates 242 times the solar luminosity from its outer atmosphere at an effective temperature of 9,795 K.[3] The companion is a magnitude 11.9 star at an angular separation of 12.1 arc seconds along a position angle of 146°, as of 2000.[11]


  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.
  2. ^ a b c d Mermilliod, J.-C. (1986), Compilation of Eggen's UBV data, transformed to UBV (unpublished), SIMBAD, Bibcode:1986EgUBV........0M.
  3. ^ 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.
  4. ^ Wilson, Ralph Elmer (1953), General Catalogue of Stellar Radial Velocities, Washington: Carnegie Institution of Washington, Bibcode:1953GCRV..C......0W.
  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.
  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/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451.
  7. ^ a b c Gontcharov, G. A. (November 2012), "Spatial distribution and kinematics of OB stars", Astronomy Letters, 38 (11): 694–706, arXiv:1606.09028, Bibcode:2012AstL...38..694G, doi:10.1134/S1063773712110035.
  8. ^ "rho Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-08.
  9. ^ 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.
  10. ^ Burnham, Robert (2013), Burnham's Celestial Handbook, An Observer's Guide to the Universe Beyond the Solar System, 2, Courier Corporation, p. 1014, ISBN 0486317935.
  11. ^ Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122: 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920.