ALIFA AL FARKADAIN (Zeta Ursae Minoris). Many stars carry multiple
names. The front bowl stars of the Little Dipper of Ursa Minor are best known as Kochab (Beta UMi) and Pherkad (Gamma), but in Arabia Beta was
also called both Nair al Farkadain and Anwar al Farkadain,
together meaning "the bright one" and "the lights" of the "two
calves," while Pherkad was Alifa al Farkadain, the "dim one of the
two calves." Anwar and Alifa (al Farkadain) were subsequently and
erroneously transferred to Eta and Zeta
Umi, in spite of the fact that Eta is the fainter of the two (all
according to Allen). Since Beta and Gamma already have perfectly
good names, here we let Zeta and Eta have their alternatives.
Alifa (etc.), rather Zeta, an easier term, is one of the more
neglected of stars, rather odd since at fourth magnitude (4.32) it
is decently bright (ranking fourth in the Little Dipper) and since
it is but 12 degrees from the pole always visible from most of the
northern hemisphere. It is the subject of a mere 17 scientific
references over the past 20 years. This white class A (A3, rarely
A1) dwarf shines with a
luminosity 200 times that of the Sun from a
distance of 375 light years, the one measure of temperature a
consistent 8700 Kelvin. Such a high luminosity is anomalous for an
A3 dwarf. Visually, the star is over two magnitudes brighter than
it "ought" to be, which carries it well out of hydrogen-fusing
dwarfhood and into the realm of the giant stars. A closer
comparison with theory reveals that this 3.4 solar mass star is
right at the edge of giving up its hydrogen fusion and is about to
become a true giant with a dead helium core. Some 280 million
years ago, it began life as a bluer B7 dwarf. Zeta is a
particularly fast rotator. The equatorial velocity is measured at
206 kilometers per second, which with a calculated radius 6.2 times
larger than the Sun gives a rotation period less than 1.5 days (as
opposed to the Sun's 25 days). There is some evidence that Zeta is
a slightly variable, pulsating "Delta
Scuti" star that subtly changes its magnitude, though no one
has confirmed it. There is no evidence for duplicity. From here,
Polaris (Alpha UMi) shines at mid-second magnitude. Since Zeta UMi
lies close to the direction of Polaris and is 88 percent Polaris's
distance, from Zeta Polaris would be almost as bright as Sirius is in our sky.