KOCHAB (Beta Ursae Minoris). Kochab, an obscure Arabic name that might simply mean "star," is just barely the second brightest and thus appropriately the Beta star in Ursa Minor, the Smaller Bear, and represents the top front bowl star of the Little Dipper. Second magnitude (2.08, ranking 59th brightest in the sky) and only 16 degrees from the North Celestial Pole, middle northerners can see it every night as it plies its small circular daily path. Together with the other bowl star (Pherkad, Gamma Ursae Minoris), it makes a small asterism called the "Guardians of the Pole," the two seeming in myth to "protect" the so-important pole star. Though we are 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 circle of 23 1/2 degrees radius against the background stars. Polaris is therefore only a temporary pole star that will get better into the next century and will then begin to shift away. About the year 1100 BC, the pole made a reasonably close pass to Kochab, and there are old references to Kochab 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 has run out of internal hydrogen fuel, and is an evolving orange class K (K4) giant star that is either brightening with a dead helium core or is dimming after already having fired its internal helium as it prepare to be an ordinary helium burner like so many class K giants. At a distance of 131 (second Hipparcos reduction) light years (give or take just 1), it shines with a luminosity of 450 times that of the Sun. It appears about the same brightness as much more luminous Polaris because it is much closer and because, at a temperature of 4130 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, maybe accreted from an undetected close companion that has already gone through its evolution and is now a white dwarf. Then again, maybe not. It does seem to be a bit low in iron, about 60 percent that found in the Sun (relative to hydrogen). Luminosity and temperature yield a radius of 42 times that of the Sun, very close to the value of 44 solar found from interferometer measures of angular diameter. Theory suggests a mass around 3 Suns, though analysis of subtle oscillations give a lower, and probably more accurate, mass of 1.3 Suns. At a separation of 3.5 minutes of arc lies an 11th magnitude "companion" whose rapid motion shows it merely to lie in the line of sight.

Written by Jim Kaler 5/29/98; revised 12/20/13. Return to STARS.