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 (3.05) in the bowl of the Little Dipper and is circumpolar (never setting) from anywhere
north of 18 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, beaten out by
second magnitude Kochab (Beta Ursae Minoris) and famed Polaris (Alpha), which lies just short of
the North Celestial Pole. (The other
stars in the Little 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 (actually a "bright
giant"), but one considerably warmer, at the warm side of class A
(A3) with a temperature of 8280 Kelvin. From its distance of 487
light years (second Hipparcos reduction, give or take 8) we
calculate a high luminosity 1050 times that of the Sun, more than 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 4.8 times
solar, which gives an age of somewhat over 100 million years. It
will first turn into an ordinary giant much like Kochab is today
(though brighter) and after sloughing off its outer layers, will
end its life as a fairly massive (about 0.85 Suns) white dwarf. During its
travels on the HR diagram, Pherkad may well become a Cepheid variable. 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 175 kilometers per
second at the equator, more than 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 disk, but that seems to have disappeared as well. If
nothing else, Pherkad is easy to study, and maybe the mysteries
will be solved before long.
Written by Jim Kaler 5/17/99; revised
12/20/13. Return to STARS.