Beta Ursae Majoris - Merak

Beta Ursae Majoris is a star of magnitude 2.37 in Ursa Major, and ranks fifth in brightness in the Big Dipper. It is more familiar to northern hemisphere observers as the southern "pointer star" in the Big Dipper, since a line connecting it with nearby α UMa (Dubhe) extends to Polaris, the north star. β UMa has the traditional name Merak, derived from the Arabic "maraqq", meaning "loins" (of the bear).

Properties

While Dubhe is a cool orange helium-fusing giant, Merak is a hot (9000 K), white, hydrogen-fusing main sequence star of class A1 V. From its distance of 79 light years, Merak's luminosity is almost 60 suns, and its mass about triple the Sun's. Merak is distinguished by a surrounding disk of cool dust, much like those discovered around Fomalhaut and Vega. No planets have been discovered orbiting Merak, but the presence of the dust indicates they may exist or be forming. Merak's dust disk approaches the orbit of Saturn in size; the dust particles have temperatures of a few hundred degrees K.

Merak is also a prominent part of the Ursa Major moving group, along with the other middle five stars of the Big Dipper. All are class A stars about the same distance away, sharing the same motion through space. The view from one of Merak's planets, if it had any, would be quite lovely: the five easterly stars of the Dipper would all appear magnitude 0 or brighter, within a 25° circle, with the middle three stars in the handle clumped into a brilliant triangle even smaller.

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