Alpha Hydri

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Alpha Hydri
Diagram showing star positions and boundaries of the Hydrus constellation and its surroundings
Cercle rouge 100%.svg
Location of α Hydri (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Hydrus
Right ascension  01h 58m 46.19467s[1]
Declination −61° 34′ 11.4948″[1]
Apparent magnitude (V) +2.90[2]
Spectral type F0 IV[2]
U−B color index +0.189[3]
B−V color index +0.290[3]
Radial velocity (Rv)+7[4] km/s
Proper motion (μ) RA: +263.66[1] mas/yr
Dec.: +26.77[1] mas/yr
Parallax (π)45.43 ± 0.44[1] mas
Distance71.8 ± 0.7 ly
(22.0 ± 0.2 pc)
Absolute magnitude (MV)+1.153[5]
Mass2.0[6] M
Radius1.8[7] R
Luminosity32[6] L
Surface gravity (log g)3.73[6] cgs
Temperature7,077[5] K
Metallicity [Fe/H]0.11[5] dex
Rotational velocity (v sin i)118[8] km/s
Age0.81[5] Gyr
Other designations
Gl 83, CD−62° 71, HD 12311, HIP 9236, HR 591, SAO 248474.[2]
Database references

Alpha Hydri (α Hyi, α Hydri) is the second brightest star in the southern circumpolar constellation of Hydrus. It is readily visible to the naked eye with an apparent visual magnitude of +2.9. It is sometimes informally known as the Head of Hydrus.[9] This should not be confused with Alpha Hydrae (Alphard) in the constellation Hydra. Alpha Hydri is one of only three stars in the constellation Hydrus that are above the fourth visual magnitude. This star can be readily located as it lies to the south and west of the prominent star Achernar in the constellation Eridanus.[10]

Based upon parallax measurements from the Hipparcos mission, Alpha Hydri is located at a distance of about 71.8 light-years (22.0 parsecs) from Earth. This subgiant star is 80%[7] larger and twice as massive as the Sun, with a stellar classification of F0 IV.[2] It is about 810 million years old[5] and is radiating 32 times the Sun's luminosity from its outer atmosphere at an effective temperature of 7,077 K.[5] Alpha Hydri emits X-rays similar to Altair.[11] The space velocity components of this star are [U, V, W] = [−14, −14, -2] km/s.[12]


In Chinese caused by adaptation of the European southern hemisphere constellations into the Chinese system, 蛇首 (Shé Shǒu), meaning Snake's Head, refers to an asterism consisting of α Hydri and β Reticuli. Consequently, α Hydri itself is known as 蛇首一 (Shé Shǒu yī, English: the First Star of Snake's Head.)[13]

See also[edit]


  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 d "LTT 1059". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-10-13.
  3. ^ a b Gutierrez-Moreno, Adelina; et al. (1966). "A System of photometric standards". 1. Publicaciones Universidad de Chile, Department de Astronomy: 1–17. Bibcode:1966PDAUC...1....1G. Cite journal requires |journal= (help)
  4. ^ Wilson, R. E. (1953). General Catalogue of Stellar Radial Velocities. Carnegie Institute of Washington D.C. Bibcode:1953GCRV..C......0W.
  5. ^ a b c d e f Lachaume, R.; et al. (August 1999). "Age determinations of main-sequence stars: combining different methods". Astronomy and Astrophysics. 348: 897–909. Bibcode:1999A&A...348..897L. Retrieved 2009-10-13.
  6. ^ a b c Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series, 85 (3): 1015–1019, Bibcode:1990A&AS...85.1015M
  7. ^ a b Wesselink, A. J.; Paranya, K.; DeVorkin, K. (November 1972). "Catalogue of stellar dimensions". Astronomy and Astrophysics Supplement. 7: 257. Bibcode:1972A&AS....7..257W. Retrieved 2009-10-13. Via Vizier table II/224/cadars.
  8. ^ Royer, F.; et al. (October 2002). "Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i". Astronomy and Astrophysics. 393 (3): 897–911. arXiv:astro-ph/0205255. Bibcode:2002A&A...393..897R. doi:10.1051/0004-6361:20020943.
  9. ^ Allen, Richard Hinkley (1963). Star Names, Their Lore and Meaning. New York: Dover. p. 377. ISBN 0-486-21079-0.
  10. ^ Moore, Patrick (2005). The observer's year: 366 nights of the universe (2nd ed.). Springer. p. 4. ISBN 1-85233-884-9.
  11. ^ Schmitt, J. H. M. M.; Golub, L.; Harnden, F. R. Jr.; Maxson, C. W.; Rosner, R.; Vaiana, G. S. (March 1985). "An Einstein Observatory X-ray survey of main-sequence stars with shallow convection zones". Astrophysical Journal. 290 (Part 1): 307–20. Bibcode:1985ApJ...290..307S. doi:10.1086/162986.
  12. ^ Gliese, W. (1969). Catalogue of Nearby Stars. Karlsruhe.
  13. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 27 日