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 Greek lettered 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.