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.