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.