KOCHAB (Beta Ursae Minoris). Kochab, an obscure Arabic name
that might simply mean "star," is just barely the second brightest,
and appropriately the Beta, star in
Ursa Minor, and represents the
top front bowl star of the Little Dipper. Second magnitude (2.08) and
only 15 degrees from the
north celestial pole, middle northerners can see it every night as
it plies its small circular path. Together with the other bowl
star (Pherkad, the Gamma star),
it makes a small asterism called
the "Guardians of the Pole," the two seeming in myth to "protect"
the pole star. Though we are quite familiar with the major two
motions of the Earth, daily rotation and annual revolution, the
third motion, precession, is more obscure. The Moon and Sun act on
the Earth's rotational bulge, and cause the axis to wobble over a
26,000 year period. The result is that the axis continually moves
in a small circle against the background stars. Polaris is thus
only a temporary pole star that will get better into the next
century and then will begin to shift away. About the year 1100 BC,
the pole made a reasonably close pass to Kochab, and there are old
references to THIS star being called "Polaris." Precession also
causes the vernal equinox (where we find the Sun on the first day
of spring) to move backwards through the constellations of the
zodiac; the equinox is now in Pisces rather than in Aries where it
was when the constellations were being named. Unlike the
Sun,
Kochab (which is not particularly unusual among its class), has run
out of internal hydrogen fuel, and is an evolving orange
class K (K4) giant star
that is now running for awhile on the fusion of helium deep in its
core. At a distance of 126 light years, we calculate that it is
almost 500 times more luminous (and about 50 times bigger) than our
Sun. It appears about the same brightness as much more luminous
Polaris because it is much closer and because, at a temperature of
4000 degrees Kelvin, it radiates a fair amount of its light in the
infrared where we cannot see it. It has a reputation as a marginal
"barium star," the element only a small bit enhanced relative to
what is found in the Sun.