Alpha Hydrae

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Alpha Hydrae
Observational Data
Designation Alpha Hydrae
α Hydra
30 Hydrae
Right ascension 09h 27m 35.2427s[1][2]
Declination -08° 39′ 30.9583″[1][2]
Constellation Hydra
Type of object Bright giant star
Magnitude Apparent Mag: +1.98[3]
Absolute Mag: -1.83[3]
Distance from Earth 177 ly[4]
Radial velocity -4.27±0.05 km/s[1][5]
Proper motion R.A.: -15.23 mas/yr[1][2]
Dec.: 34.37 mas/yr[1][2]
Parallax 18.09±0.18 mas[1][2]

Alpha Hydrae (Alphard, α Hydra, Alpha Hya, α Hya) is a bright giant star in the constellation of Hydra, the water snake.[6] Its name of "Alphard," like many star names, derives from the Arabic al-fard, meaning "the solitary one."[4] This comes from the fact it is the only bright star in a constellation of dim ones.[6] Arabs also knew the star as the "backbone of the serpent". Tycho Brahe called the star "Cor Hydrae, or “the heart of the snake.” In China, it is part of the asterism known only as "Star", and the star is called the "First Star of Star." Its Bayer designation is Alpha Hydra. Alphard appears on the national flag of Brazil, where it represents or symbolizes the state of Mato Grosso do Sul.[7]

Alpha Hydra and the rest of the constellation of Hydra can be seen ascending in the east in spring. The 47th brightest star in the night sky, it can be seen without binoculars and appears slightly orange.[8]

Properties and structure

Located 177 light years from Earth, Alphard has an apparent magnitude of +2.0, making it the brightest star in the constellation of Hydra.[4] It has a stellar classification of K3 II-III, placing it halfway between an orange giant star and a bright giant star. It is thought to have a mass of 3 solar masses. The star's angular diameter has been measured to be 9.09±0.09 milliarcseconds, corresponding to a radius 50 times larger than that of the Sun.[9][10] The star's surface temperature is estimated at around 4,100 kelvin.[8] Alphard is believed to have a luminosity roughly 946 times that of the Sun.[11] The star has a rotational speed of 1.1 km/s at its equator.[7]

The star is unusual as its spectrum suggests it contains a large abundance of barium.[7][12] Barium is often produced through "s process nucleosynthesis." This is common in massive stars with white dwarf stars as companions, but no such companion has been observed for Alphard. Such stars are sometimes called "barium stars."[13] The star is also of interest for studying astroseismological events. Multi-periodic oscillations with periods that range from a couple hours to nearly two days.[9][14] The short-term oscillations are thought to be similar to those that are caused by stellar pulsations such as those that occur on the Sun, but the exact reason or reasons for the correlation between the variations in the star’s spectral line and radial velocity remains unknown.[7]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Alphard from the SIMBAD Astronomical Database Alphard. Simbad Astronomical Database. Retrieved on 2019-05-03.
  2. 2.0 2.1 2.2 2.3 2.4 van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy & Astrophysics 474 (2): 653-664. doi:10.1051/0004-6361:20078357. Bibcode2007A&A...474..653V.  arXiv:0708.1752
  3. 3.0 3.1 Alphard (Star). Retrieved on 2019-04-11.
  4. 4.0 4.1 4.2 Hydra Constellation. Retrieved on 2019-04-11.
  5. Jofré, E.; Petrucci, R.; Saffe, C. et al. (2015). "Stellar parameters and chemical abundances of 223 evolved stars with and without planets". Astronomy and Astrophysics 574 (A50): A50. doi:10.1051/0004-6361/201424474. Bibcode2015A&A...574A..50J.  arXiv:1410.6422
  6. 6.0 6.1 Bruce McClure (2019-03-06). Alphard is the Snake’s Heart.
  7. 7.0 7.1 7.2 7.3 Peter Christoforou (2018-05-12). Star Facts: Alphard. Retrieved on 2019-04-11.
  8. 8.0 8.1 Alphard (Alpha Hydrae, 30 Hydrae). Retrieved on 2019-04-11.
  9. 9.0 9.1 Alphard. Retrieved on 2019-04-11.
  10. da Silva, L.; Girardi, L.; Setiawan, J. (2006). "Basic physical parameters of a selected sample of evolved stars". Astronomy and Astrophysics 458 (2): 609-623. doi:10.1051/0004-6361:20065105. Bibcode2006A&A...458..609D.  arXiv:astro-ph/0608160
  11. Jim Kaler (2015-06-16). Alphard. Stars. Retrieved on 2019-04-11.
  12. Mennessier, M. O.; Luri, X.; Figueras, F. (1997). "Barium stars, galactic populations and evolution". Astronomy and Astrophysics 326: 722-730. Bibcode1997A&A...326..722M. 
  13. Alphard (Alpha Hydrae). Retrieved on 2019-04-11.
  14. Setiawan, J.; Weise, P.; Roth, M. (2006). "Multi-periodic oscillations of alpha Hya". Memorie della Societa Astronomica Italiana 77: 510–514.  arXiv:astro-ph/0505184