ALIOTH (Epsilon Ursae Majoris). The graceful curve of handle of the Big Dipper (the Plough in Great Britain), among the most famed of celestial sights, represents the tail of Ursa Major, the Greater Bear. Third star in from the end, "Alioth" relates not to a bear, but to a "black horse," the name corrupted from the original and mis- assigned to the naked-eye companion of Mizar, which took on the vaguely similar name "Alcor." Bayer's rough rule of assigning Greek-letter names more or less in order of brightness is quite violated here, as the Bear's bright stars are named from west to east, hence "Epsilon" for Ursa Major's brightest (bright second magnitude, 1.77) star, indeed for the 32nd brightest star in the whole sky. A white class A (A0) star with a measured temperature of 9400 Kelvin, Alioth shines at us from a distance of 81 light years with a luminosity 108 times that of the Sun, from which we derive a diameter of four times solar and a mass close to triple that of the Sun. Large and luminous for its class, Alioth is probably ageing, and is nearing the end of its main sequence hydrogen-fusing lifetime. Of greater significance, Alioth is the brightest of the "peculiar A (Ap) stars," magnetic stars in which a variety of chemical elements are either depleted or enhanced, and in addition appear to change with great regularity as the star rotates. "Chemically peculiar" behavior in class A and B stars generally comes not from creation of elements, but from their separation in the relatively thin stellar atmospheres, some falling downward within the star's gravitational field, others lofted upward as a result of an outward push by radiation. Here, they are also apparently related to the Alioth's magnetic field. Alioth is classed as an "Alpha Canum Venaticorum" star (after the prototype, Cor Caroli). Its magnetic field -- and the chemical composition -- change from our perspective during the star's 5.1-day stellar rotation period. Some elements are highly concentrated into distinct regions that swing in and out of sight as the star spins. For example, the abundance of oxygen is 100,000 times greater near the magnetic equator than near the magnetic poles (which are displaced from the rotational equator and poles); chromium behaves similarly. Heavier elements, such as the rare earth europium, also display strong variations. Though visually the brightest of the peculiar A stars, Alioth is also noted for having one of the weakest magnetic fields among its class, only about 100 times that of the Earth, 15 times weaker than that observed for Cor Caroli.
Written by Jim Kaler. Return to STARS.