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For the programming language, see Alphard (programming language).
For the vehicle, see Toyota Alphard.
Alpha Hydrae
Hydra constellation map.png
Alphard is the α star in the constellation of Hydra.
Observation data
Epoch J2000      Equinox J2000
Constellation Hydra
Right ascension 09h 27m 35.2433s[1]
Declination –08° 39′ 30.969″[1]
Apparent magnitude (V) +1.98
Spectral type K3 II-III
U-B color index +1.73[2]
B-V color index +1.44[2]
Radial velocity (Rv) -4.3[3] km/s
Proper motion (μ) RA: –14.49[1] mas/yr
Dec.: 33.25[1] mas/yr
Parallax (π) 18.40 ± 0.78[1] mas
Distance 177 ± 8 ly
(54 ± 2 pc)
Absolute magnitude (MV) −1.69 ± 0.09[4]
Mass 3.03 ± 0.36[4] M
Radius 50.5 ± 4.0[4] R
Surface gravity (log g) 1.54[5]
Temperature 4,120[5] K
Metallicity \begin{smallmatrix}\frac{Fe}{H}\ =\ -0.06\end{smallmatrix}[5]
Age (4.2 ± 1.6) × 108[4] years
Other designations
Alphard, Alfard, Alphart, Kalbelaphard, Cor Hydrae, 30 Hydrae, HR 3748, BD -08°2680, HD 81797, SAO 136871, FK5 354, HIP 46390.[6]

Alphard (α Hya / α Hydrae / Alpha Hydrae) is the brightest star in the constellation Hydra. The name Alphard is from the Arabic الفرد (al-fard), "the solitary one", there being no other bright stars near it. It was also known as the "backbone of the Serpent" to the Arabs. In ancient China it formed part of an asterism called the "red bird". The European astronomer Tycho Brahe dubbed it Cor Hydræ, the heart of the snake.[7]

Alphard has three times the mass of the Sun. The estimated age of this star is 420 million years and it has evolved away from the main sequence to become a giant star with a spectral classification of K3 and luminosity class between II and III.[4] The angular diameter of this star has been measured using long baseline interferometry, yielding a value of 9.09 ± 0.09 milliarcseconds.[8] It has expanded to 50 times the radius of the Sun.[4]

The spectrum of this star shows a mild excess of barium, an element that is normally produced by the s-process of nucleosynthesis. Typically a barium star belongs to a binary system and the anomalies in abundances are explained by mass transfer from a companion white dwarf star.[9]

Precise radial velocity measurements have shown variations in the stellar radial velocities and spectral line profiles. The oscillations are multi-periodic with periods from several hours up to several days. The short-term oscillations were assumed to be a result of stellar pulsations, similar to the solar ones. A correlation between the variations in the asymmetry of the spectral line profile and the radial velocity has also been found. The multi-periodic oscillations make HD 81797 (Alphard) an object of interest for asteroseismologic investigations.[10]


  1. ^ a b c d e Perryman, M. A. C. et al (April 1997). "The HIPPARCOS Catalogue". Astronomy & Astrophysics 323: L49–L52. Bibcode1997A&A...323L..49P.  
  2. ^ a b Pfleiderer, J.; Mayer, U. (October 1971). "Near-ultraviolet surface photometry of the southern Milky Way". Astronomical Journal 76: 691–700. doi:10.1086/111186.  
  3. ^ Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". in Batten, Alan Henry; Heard, John Frederick. Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. University of Toronto: International Astronomical Union. Retrieved 2009-09-10.  
  4. ^ a b c d e f da Silva, L.; et al. (November 2006). "Basic physical parameters of a selected sample of evolved stars". Astronomy and Astrophysics 458 (2): 609-623. doi:10.1051/0004-6361:20065105.  
  5. ^ a b c Cenarro, A. J.; et al. (January 2007). "Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters". Monthly Notices of the Royal Astronomical Society 374 (2): 664–690. doi:10.1111/j.1365-2966.2006.11196.x.  
  6. ^ "Alphard". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-10-09.  
  7. ^ Olcott, William Tyler (2004). Star Lore: Myths, Legends, and Facts. Courier Dover Publications. p. 226. ISBN 0486435814.  
  8. ^ Le Bouquin, J.-B.; et al. (January 2009). "Post-processing the VLTI fringe-tracking data: first measurements of stars". Astronomy and Astrophysics 493 (2): 747–752. doi:10.1051/0004-6361:200810613.  
  9. ^ Mennessier, M. O.; et al. (October 1997). "Barium Stars, Galactic Populations and Evolution". Astronomy and Astrophysics 326: 722–730. Bibcode1997A&A...326..722M.  
  10. ^ Setiawan, J., Roth, M., Weise, P., Dölinger, M. P. (2006). "Multi-periodic oscillations of HD 32887 and HD 81797". Memorie della Societa Astronomica Italiana 77: 510–514. Bibcode2006MmSAI..77..510S.  


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary


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From Arabic الفرد (al-fard) ‘the solitary one’.

Proper noun




  1. (astronomy) An orange giant star in the constellation of Hydra; Alpha (α) Hydrae,


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