ALPHARD (Alpha Hydrae). Not well known, but surprisingly prominent, Alphard dominates the dim constellation Hydra, the Water Serpent, and is hence also known as Alpha Hydrae. The star, right in the middle of the range of second magnitude (1.98), is made more noticeable by lying within a fairly blank region of sky to the southwest of brighter Regulus. Appropriately, its Arabic name means "the solitary one." A dark sky, or better, binoculars, will show the star glowing a pale orange color, indicative of a giant star, one of the most common kinds that inhabit the naked-eye sky. Its distance of 175 light years allows us to calculate a true luminosity that to the eye would be 400 times that of the Sun. With a temperature of 4000 degrees Kelvin, the term "giant" is apt, as the star is some 40 times larger than the Sun. Placed at the solar position, Alphard would extend halfway to the orbit of Mercury. As such the star is in league with several others that include both Arcturus and Aldebaran, which are more prominent because they are closer to us even though Alphard is actually a bit more luminous (making it a so-called "bright giant."). Alphard also distinguishes itself by being a mild form of "barium star," in which barium and other elements that are formed by the slow capture of neutrons are enhanced. Barium stars are thought to be doubles. When Alphard was young, it had a more massive companion that died first, and in the process contaminated it with the by-products of nuclear fusion that had been shovelled to the top. The companion has now died as a dim white dwarf, and the star that had been contaminated is now itself dying, presenting us with evidence for what once happened.
Written by Jim Kaler. Return to STARS.