Alpha Hydri, Latinized from α Hydri, is the second brighteststar in the southern circumpolar constellation of Hydrus. It is readily visible to the naked eye in locations south of 28°N with an apparent visual magnitude of +2.9. It is sometimes informally known as the Head of Hydrus.[8] 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 east of the prominent star Achernar in the constellation Eridanus.[9]
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 three[3] times larger and twice as massive as the Sun, with a stellar classification of F0 IV.[3] It is about 810 million years old[6] and is radiating 21 times the Sun's luminosity from its outer atmosphere at an effective temperature of 7,087 K.[3] Alpha Hydri emits X-rays similar to Altair.[10] The space velocity components of this star are [U, V, W] = [−14, −14, -2] km/s.[11]
Naming
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.)[12]
^ abGutierrez-Moreno, Adelina; et al. (1966). "A System of photometric standards". Publications of the Department of Astronomy University of Chile. 1. Publicaciones Universidad de Chile, Department de Astronomy: 1–17. Bibcode:1966PDAUC...1....1G.
^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
