Beta Ursae Minoris - Kochab

At magnitude 2.08, Beta Ursae Minoris is the second brightest star in Ursa Minor, and represents the top front bowl star of the Little Dipper. It has the traditional name Kochab, which may originate from the Arabic "al-kawkab", and means simply "the star".

Kochab and Pherkad (γ UMi) make up a small asterism called the "Guardians of the Pole." They served as twin pole stars from 1500 B.C. until 500 A.D., although neither star was as close to the pole as Polaris is now. The succession of pole stars is a result of earth's precessional motion: the Sun and Moon act on the Earth's rotational bulge, and cause its axis to wobble over a 26,000 year period. The result is that the pole continually moves in a large, 47° circle against the background stars.

Properties

Kochab is an evolving orange class K4 III giant star that is fusing helium deep in its core. At a distance of 126 light years, 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 K, it radiates a fair amount of its light in the infrared. It is a marginal "barium star", in which the element is enhanced relative to what is found in the Sun.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]