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Pi Hydrae (π Hya, π Hydrae) is a star in the constellation Hydra with an apparent visual magnitude of 3.3,[2] making it visible to the naked eye. Parallax measurements put this star at a distance of about 101 light-years (31 parsecs) from the Earth.

Pi Hydrae
Diagram showing star positions and boundaries of the Hydra constellation and its surroundings
Cercle rouge 100%.svg
Location of π Hydra (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Hydra
Right ascension  14h 06m 22.29749s[1]
Declination –26° 40′ 56.5024″[1]
Apparent magnitude (V) 3.25[2]
Spectral type K1 III–IV[3]
U−B color index +1.040[4]
B−V color index +1.120[4]
Radial velocity (Rv)+26.7[2] km/s
Proper motion (μ) RA: +43.70[1] mas/yr
Dec.: −141.18[1] mas/yr
Parallax (π)32.30 ± 0.16[1] mas
Distance101.0 ± 0.5 ly
(31.0 ± 0.2 pc)
Absolute magnitude (MV)0.79[5]
Mass1.76[5] to 2.45[6] M
Radius12–13[7] R
Surface gravity (log g)2.65[8] cgs
Temperature4,670[8] K
Metallicity [Fe/H]0.04[9] dex
Rotational velocity (v sin i)2.25[8] km/s
Other designations
Markeb, 49 Hydrae, HD 123123, HR 5287, SAO 182244, FK5 519, CPD−26 5170, HIP 68895.[10]
Database references

The spectrum of this star shows it to have a stellar classification of K1 III-IV,[3] with the luminosity class of 'III-IV' suggesting it is in an evolutionary transition stage somewhere between a subgiant and a giant star. It has a low projected rotational velocity of 2.25 km s−1.[8] Pi Hydrae is radiating energy from its outer envelope with an effective temperature of 4,670 K,[8] giving it the orange hue of a K-type star.[11]

Pi Hydrae is a type of giant known as a cyanogen-weak star, which means that its spectrum displays weak absorption lines of CN relative to the metallicity. (The last is a term astronomers use when describing the abundance of elements other than hydrogen and helium.) Otherwise, it appears to be a normal star of its evolutionary class, having undergone first dredge-up of nuclear fusion by-products onto its surface layers.[9] The measured angular diameter of this star, after correction for limb darkening, is 3.76 ± 0.04 mas.[12] At its estimated distance, this yields a physical size of about 12–13 times the radius of the Sun.[7] It has an estimated mass of 2.45 times the mass of the Sun.[6]


  1. ^ a b c d e van Leeuwen, F. (November 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 Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35), Bibcode:1999VeARI..35....1W
  3. ^ a b Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637
  4. ^ a b Jennens, P. A.; Helfer, H. L. (September 1975), "A new photometric metal abundance and luminosity calibration for field G and K giants", Monthly Notices of the Royal Astronomical Society, 172 (3): 667–679, Bibcode:1975MNRAS.172..667J, doi:10.1093/mnras/172.3.667
  5. ^ a b Hekker, S.; et al. (August 2006), "Precise radial velocities of giant stars. I. Stable stars", Astronomy and Astrophysics, 454 (3): 943–949, arXiv:astro-ph/0604502, Bibcode:2006A&A...454..943H, doi:10.1051/0004-6361:20064946
  6. ^ a b Edvardsson, B. (January 1988), "Spectroscopic surface gravities and chemical compositions for 8 nearby single sub-giants", Astronomy and Astrophysics, 190 (1–2): 148–166, Bibcode:1988A&A...190..148E
  7. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
  8. ^ a b c d e Hekker, S.; Meléndez, J. (December 2007), "Precise radial velocities of giant stars. III. Spectroscopic stellar parameters", Astronomy and Astrophysics, 475 (3): 1003–1009, arXiv:0709.1145, Bibcode:2007A&A...475.1003H, doi:10.1051/0004-6361:20078233
  9. ^ a b Luck, R. Earle (February 1991), "Chemical abundances for cyanogen-weak giants", Astrophysical Journal Supplement Series, 75: 579–610, Bibcode:1991ApJS...75..579L, doi:10.1086/191542
  10. ^ "49 Hya -- Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-16
  11. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16
  12. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431 (2): 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039

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