Alkaid

Physical properties

Alkaid is a B-type main sequence star with a stellar classification of B3 V. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.

Alkaid has 5.07 times the mass of the Sun. It is a young star, roughly six million years old, and is in the beginning of its life in the main sequence (ZAMS). The star rotates rapidly, with a projected rotational velocity of 154.3±9.1 km/s. Correcting for its inclination angle of 42° implies an equatorial rotational velocity of 240 km/s. As a consequence of its rapid rotation, Alkaid is oblate, with a radius of 2.89 solar radii (R_☉) at the equator, and 2.66 solar radii at the poles. The effective temperature also varies across latitudes, being of 16,300 K at the equator and 17,700 K at the poles. Such a temperature gives it the blue-white hue of a B-type star. This star is an X-ray emitter with a luminosity of 9.3 × 10²⁸ erg s⁻¹.

Eta Ursae Majoris was listed as a standard star for the spectral type B3 V. It has broadened absorption lines due to its rapid rotation, which is common in stars of this type. However, the lines are very slightly distorted and variable, which may be caused by some emission from a weak disk of material produced by the rapid rotation.

Alkaid is a relatively nearby and bright star and has been examined closely, but no exoplanets or companion stars have been discovered.