Epoch J2000 Equinox J2000
|Right ascension||09h 27m 35.2433s|
|Declination||−08° 39′ 30.969″|
|Apparent magnitude (V)||+2.00|
|Spectral type||K3 II-III|
|U−B color index||+1.73|
|B−V color index||+1.44|
|Radial velocity (Rv)||−4.3 km/s|
|Proper motion (μ)||RA: −14.49 mas/yr
Dec.: 33.25 mas/yr
|Parallax (π)||18.40 ± 0.78 mas|
|Distance||177 ± 8 ly
(54 ± 2 pc)
|Absolute magnitude (MV)||−1.69 ± 0.09|
|Mass||3.03 ± 0.36 M☉|
|Radius||50.5 ± 4.0 R☉|
|Luminosity||780 ± 78 L☉|
|Surface gravity (log g)||1.54 cgs|
|Metallicity [Fe/H]||−0.06 dex|
|Rotational velocity (v sin i)||1.1 km/s|
|Age||(4.2 ± 1.6) × 108 years|
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.
In Chinese, 星宿 (Xīng Sù), meaning Star, refers to an asterism consisting of Alphard, τ1 Hydrae, τ2 Hydrae, ι Hydrae, 26 Hydrae, 27 Hydrae, HD 82477 and HD 82428. Consequently, Alphard itself is known as 星宿一 (Xīng Sù yī), "the First Star of Star".
Alphard has three times the mass of the Sun. Its estimated age 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. The angular diameter has been measured using long baseline interferometry, yielding a value of 9.09 ± 0.09 milliarcseconds, only beaten in it by Betelgeuse and R Doradus. It has expanded to 50 times the radius of the Sun.
Alphard's spectrum 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.
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.
- Perryman, M. A. C. et al. (1997), "The Hipparcos Catalogue", Astronomy & Astrophysics 323: L49–L52, Bibcode:1997A&A...323L..49P
- Piau, L. et al. (February 2011), "Surface convection and red-giant radius measurements", Astronomy and Astrophysics 526: A100, arXiv:1010.3649, Bibcode:2011A&A...526A.100P, doi:10.1051/0004-6361/201014442
- Pfleiderer, J.; Mayer, U. (October 1971). "Near-ultraviolet surface photometry of the southern Milky Way". Astronomical Journal 76: 691–700. Bibcode:1971AJ.....76..691P. doi:10.1086/111186.
- 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. Bibcode:1967IAUS...30...57E.
- da Silva, L.; et al. (November 2006). "Basic physical parameters of a selected sample of evolved stars". Astronomy and Astrophysics 458 (2): 609–623. arXiv:astro-ph/0608160. Bibcode:2006A&A...458..609D. doi:10.1051/0004-6361:20065105.
- 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. arXiv:astro-ph/0611618. Bibcode:2007MNRAS.374..664C. doi:10.1111/j.1365-2966.2006.11196.x.
- Setiawan, J. et al. (July 2004), "Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch", Astronomy and Astrophysics 421: 241–254, Bibcode:2004A&A...421..241S, doi:10.1051/0004-6361:20041042-1
- "Alphard". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-10-09.
- Olcott, William Tyler (2004). Star Lore: Myths, Legends, and Facts. Courier Dover Publications. p. 226. ISBN 0-486-43581-4.
- Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society 55: 429. Bibcode:1895MNRAS..55..429K.
- (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
- (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
- 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. arXiv:0812.2328. Bibcode:2009A&A...493..747L. doi:10.1051/0004-6361:200810613.
- Mennessier, M. O.; et al. (October 1997). "Barium Stars, Galactic Populations and Evolution". Astronomy and Astrophysics 326: 722–730. Bibcode:1997A&A...326..722M.
- 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. arXiv:astro-ph/0505184. Bibcode:2006MmSAI..77..510S.
- "Astronomy of the Brazilian Flag". FOTW Flags Of The World website.