MERAK (Beta Ursae Majoris). High in the sky in northern spring evenings, just climbing above the northern horizon in southern hemisphere autumn, the Big Dipper -- the "plough" in England -- is among the most recognized and recognizable of figures, one of the first learned in a quest to know the constellations. Leading the westward moving parade are Dubhe at the lip of the Dipper's bowl and Merak, also at the bowl's front and just to the south of Dubhe, the two making the Big Dipper's "Pointers" that lead the way to the North Star. While often considered a constellation, the Dipper is a small part -- an asterism -- of the ancient figure of Ursa Major, the Greater Bear, much of which is circumpolar, never setting for far northerners. The names of all but two of the Dipper's stars (Alioth and Alkaid) refer to the Bear, "Merak" coming from an Arabic description that means "the flank of the Greater Bear." The two front bowl stars make a nice contrast, Dubhe a cool orange giant, Merak a seemingly standard hot (9000 Kelvin) white class A (A1) "main sequence" dwarf star, one that is quietly fusing hydrogen to helium its core, as does the Sun. With an apparent magnitude of 2.37 (faint second), Merak ranks fifth in brightness in the Dipper, right after Mizar in the figure's handle. In spite of its ranking, however, it received the Beta designation from Bayer, who lettered the Dipper's stars from front to back. From its distance of 79 light years, Merak's luminosity is seen to be almost 60 times solar, its mass about triple that of the Sun. While these class A stars are not all that common, they are bright enough to be seen at large distances and thus seem disproportionately numerous in nighttime sky. Merak has two special features that set it off from the others. Like Fomalhaut and some others, it is a Vega kind of star, one that radiates extra infrared light that seems to be coming from a disk-like shroud of heated dust, one reminiscent of the dusty disk that produced our planets. Merak's detected disk approaches the orbit of Saturn in size, the dust particles having temperatures of a few hundred degrees Kelvin, similar to that found in our own planetary system. Does the star have planets too? We do not know. Merak is also a prominent part of the Ursa Major Cluster, as are all the Dipper's stars but the two at the ends, the middle five all class A stars about the same distance away. The sight from one of Merak's planets, were it to have any, would be quite lovely, the five easterly stars of the Dipper all "zeroth" magnitude or brighter within a 25 degree-wide segment, the middle three stars of the handle (Megrez, Alioth, and Mizar) clumped into a small brilliant triangle.
Written by Jim Kaler. Return to STARS.