ANWAR AL FARKADAIN (Eta Ursae Minoris). The Big Dipper (the Plough in England), the major figure of
Ursa Major (the Great Bear), is so well
known that few except the dedicated pay much attention to its
counterpart, the Little Dipper, which in
parallel is the most (in fact the only) recognizable portion of
Ursa Minor,
the Small Bear. And no wonder, since the Little Dipper
is so faint that it cannot be seen in any town with bright lights.
The figure is recognizable mostly by the North Star (Polaris, Alpha Ursae Minoris) and by the
two front bowl stars, Kochab and Pherkad (Beta and Gamma). Kochab was at
one time called "Nair (or Anwar) al Farkadain," meaning the "bright
one" (or "the lights") of the "two calves," and Pherkad was called
"Alifa al Farkadain," meaning the "dim one of the two calves"
(hence the name "Pherkad"). As so often happenes, the names have
been transferred to other stars, in this case to
Eta and Zeta Ursae
Minoris. And here, at the bottom of the Little Dipper, we find dim
fifth magnitude (4.95) "Anwar," the faintest star of the Dipper's
seven. Physically, the star is nearly (but not quite) sunlike, a
class F (in the middle of the range, F5) dwarf with an estimated
temperature of 6400 Kelvin, right at the point at which we do not
have to correct for infrared or ultraviolet radiation. The star's
luminosity of only 7.4 times that of the Sun
leads to a radius twice solar and a mass 1.4 solar. Rather well
along in its hydrogen-fusing lifetime, Anwar is a bit brighter than
normal for its temperature, and seems close to becoming a
"subgiant," a star that has shut down hydrogen fusion, if it has
not already done so. Anwar is listed as a double star with a
rather distant companion, but the cool nature of the star and its
dim 12th magnitude status place it at only about half Anwar's
distance, showing the pairing to be merely a line-of-sight
coincidence. Lower-mass solar type stars rotate slowly (the sun
taking 25 days at an equatorial speed of 2 kilometers per second),
while high-mass stars rotate quickly. The division in rotation is
rather sharp, the "rotation break" falling in the middle of class
F. Anwar, rotating at least 76 kilometers per second (with a
period under 1.4 days), falls just above the limit. Like the Sun,
however, the rotation (and convection in its outer layers) give
Anwar an X-ray-emitting hot corona.