PHERKAD (Gamma Ursae Minoris).
Unless you live south of the Tropic of Cancer, you will
have no trouble finding Pherkad any time of the year, as it glows
at mid-third magnitude in the bowl of the Little Dipper and is
circumpolar from anywhere north of 20 degrees north latitude along
with its mate Kochab. Third brightest
within the Little Dipper, the major figure of Ursa Minor, the Smaller Bear, Pherkad received Bayer's
Gamma designation, its superiors being second magnitude Kochab
(Beta) and famed Polaris (Alpha), which
lies just short of the north celestial pole. (The other stars in
the Dipper are fourth and fifth magnitude and hard to see with any
kind of lighting). The name derives from the Arabic for "the two
calves," which originally referred to both Kochab and Pherkad.
Together, the two stars are also called "the Guardians of the
Pole," as they nightly draw a close circle around one of the sky's
most significant stars, Polaris. Like its bowl-mate Kochab,
Pherkad is a giant star, but one considerably hotter, at the warm
side of class A with a temperature of 8600 Kelvin. From its
distance of 480 light years, we calculate a high luminosity 1100
times that of the Sun,
double that of Kochab, yielding a radius 15
times solar. As a warm giant, and a bright one at that, the star
is evolving, probably with a for-now quiet helium core surrounded
by a ring of fusing hydrogen, its current temperature and
luminosity suggesting a mass of around five times solar. If that
is the case, it left the "main sequence," where it once (like the
Sun) fused core hydrogen, only about 100 million years ago, and
will, by odd coincidence, before long turn into a star much like
Kochab is today. Many class A stars have odd chemical compositions
resulting from selective settling and lofting of atoms in quiet
atmospheres, Though evolved, Pherkad is still spinning rapidly,
over 170 kilometers per second at the equator, 85 times solar,
which keeps things stirred up and the composition "normal." The
star nevertheless exudes mystery. It is of interest for its subtle
and confusing variability, changing over less than a tenth of a
magnitude with a period of only a couple hours. No one seems to
know where to classify it. It is too hot and bright to be a well-
understood pulsator of the main sequence. It was once thought to
be a member of the "Maia" class
(after a star in the Pleiades), but the whole class has since disappeared,
showing the difficulty of understanding stellar stability. It was
also classified as "shell star," one with a surrounding shroud of
dust, but that seems to have disappeared as well. If nothing else,
the star is easy to study, and maybe the mysteries will be solved
before long.