PSI UMA (Psi Ursae Majoris). In the middle of the Ursa Major's middle leg (the Great Bear has but three,
each ending in pairs of unrelated stars that were the Arabs'
"leaps" of the gazelle), Psi UMa has little actually going for it
but its position, level of brightness, and its ordinariness --
which in studying stars is sometimes a plus rather than a minus.
At a distance of 147 light years, it is not quite twice as far away
as the bulk of the Dipper stars (that is, the Ursa Major cluster). At magnitude 3.01, the star
almost perfectly defines third magnitude. If you are in the
northern hemisphere, compare it with Polaris (magnitude 2.02) to see the
relative brightnesses change to the eye caused by of a difference
of a full magnitude (if in the southern hemisphere, use Nunki, Sigma Sagittarii, also magnitude
2.02). Spica (magnitude 1.04) is as close
as you can get to defining mid-first magnitude, while Vega (0.03) and Arcturus (-0.04) do it fairly well for
magnitude zero. (Upsilon Ceti, the southernmost
star in Cetus, at 4.00 defines fourth.) Each full magnitude division
corresponds to a factor in brightness of 2.5. As a typical class
K (K1) giant, Psi UMa is physically right in the middle of the pack
as well. With a temperature of 4545 Kelvin, the star radiates at
a rate of 170 times that of the Sun, much of
the light coming out in the infrared part of the spectrum.
Temperature and luminosity then lead to a radius of 21 times solar.
Direct measure of angular diameter with an interferometer yields an
agreeable 20 times solar. Ordinariness continues with rotation and
composition. As one would expect from a swollen giant star, the
spin rate is slow, the projected equatorial velocity just 1.1
kilometers per second (half that of the Sun). If the rotation axis
is perpendicular to the line of sight, the star takes 2.6 years to
complete a full rotation (as opposed to just under a month for the
Sun). The metal content of the star is near solar. Psi UMa is a
classic example of helium-fusing giant, one of a large number
scattered across the sky, all readily identifiable by their yellow-
orange colors. It began life some 300 million years ago as a hot,
blue (class B7), hydrogen-fusing dwarf. Its fate is to end life as
a relatively massive white
dwarf of about 0.7 solar masses. showing how much mass is lost
during the giant phases through winds. Psi UMa will lose three-
quarters of itself back into the dusty gases of interstellar space
from which it came. By comparison, the Sun, having lost nearly
half its bulk, will wind up as a 0.55 solar mass white dwarf.
Written by Jim Kaler 7/20/07. Return to STARS.