Alphard dominates the dim constellation Hydra, the Water Snake, and is also known as Alpha Hydrae; it marks the heart of the snake. The star, of magnitude 1.98, is made surprisingly prominent by lying in a fairly blank region of sky to the southwest of Leo's Regulus. Appropriately, its Arabic name "Al Fard" means "the solitary one."
Properties
Alphard is a bright orange giant star of spectral class K3 II-III and a surface temperature of 4000 K. Its distance of 175 light years implies a true luminosity of 400 suns. The star is some 40 times larger than the Sun; Alphard would extend halfway to the orbit of Mercury if placed at the center of our solar system. It is physically similar to Arcturus and Aldebaran, which are more prominent because they are closer to us, even though Alphard is actually a bit more luminous.
Alphard is also a mild barium star, containing enhanced levels of barium and other elements that are formed by the slow capture of neutrons. Barium stars are thought to be binaries. When Alphard was young, it had a more massive companion , before it became a white dwarf, contaminated its partner with the byproducts of nuclear fusion that had been dredged to its surface.
Precise measurements have shown oscillations in the star's radial velocity and spectral lines. 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 Sun's. A correlation between the variations the spectral line profile and the radial velocity has also been found. These oscillations make Alphard an object of interest for astro-seismology.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]