Alpha Ursae Majoris - Dubhe

Alpha Ursae Majoris shines at magnitude 1.79 in the front of the bowl of the Big Dipper in Ursa Major, the Great Bear. It is the second-brightest star in Ursa Major, and received the α designation when Bayer simply lettered the Dipper's stars from west to east. It is the northern of the two "Pointer" stars (α and β UMa) which point towards Polaris, the North Star (and point in the other direction to Regulus in Leo).

The star's traditional name, Dubhe, comes from the Arabic phrase "zahr ad-dubb al-akbar", meaning "the back of the Greater Bear". It is known as the First Star of the Northern Dipper or the Celestial Pivot in Chinese. Dubhe is the official star of the State of Utah.

Properties and Companions

Dubhe is a cool class K0 III giant with a temperature of 4500 K; its orange is color easily noted. Dubhe is an evolved, helium fusing giant with a luminosity of 300 suns, and a radius 30 times solar.

The star is orbited every 44 years at a distance of about 23 AU by a warmer, much dimmer, and less massive class F0 V star. Over 400 times farther away, at a distance of about 8000 AU, is another class F8 star that also has a companion with a 6-day period - making Dubhe a quadruple system.

Unlike the middle five stars of the Big Dipper, Dubhe is not part of the Ursa Major Moving Cluster. It is half again as distant (124 light years) - in fact the farthest of the Dipper stars. It is also moving in a different direction - the Dipper will fall apart over the next few tens of thousands of years.

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