DUBHE (Alpha Ursae Majoris). Almost first magnitude,
shining for us at the front of the
bowl of the Big Dipper in Ursa Major, the
Great Bear, Dubhe (the "h" silent, the final "e" pronounced almost
any way you wish) leads the Dipper in its northeasterly climb above
the horizon. The Arabic name means "the bear" itself, and comes
from a longer phrase that indicates the star's location on the back
of the Great Bear. Though not quite the brightest of its
constellation, just two percent short of
Alioth (the Eta star,
third in from the handle), Dubhe received the Alpha designation
when Bayer simply lettered the Dipper's stars from west to east,
from Dubhe to Alkaid, the latter bringing
up the end of the bear's tail. Together with
Merak, the Beta star,
Dubhe makes the famed "Pointers," which lead north to the North
Star, Polaris. In the other direction
they point toward Regulus in
Leo. As
appropriate for the Dipper's lead star, Dubhe quite stands out
among the others that make the famed figure. The middle five
stars, which include Mizar along with its
little companion
Alcor, are all warm class A stars that are part of
a physical cluster all about 80 light years away. Dubhe, however,
is not a part of the system (nor is Alkaid), and is half again as
distant, 124 light years, and the most distant of the Dipper stars.
As a class K giant with a temperature of 4500 Kelvin, it is also
the coolest of them (its orange color easily noted), and the only
one that is evolved and in the long process of dying, though for
now it is temporarily stabilized by the fusion of helium in its
core. With a luminosity 300 times that of the
Sun, Dubhe is the
second most luminous of the seven stars, topped only by hot Alkaid,
the luminosity and temperature implying a radius 30 times solar.
Dubhe is orbited at a distance of about 23 Earth-Sun distances
(somewhat greater than the distance between Uranus and the Sun) by
a warmer and much dimmer and less massive class F star that takes
44 years to go around. Someone riding a planet orbiting the F star
would see vastly brighter Dubhe as second orange sun with about
half the brightness of the Sun in our sky. Over 400 times farther
away is another class F star that also has a companion (with a six-
day period), from which Dubhe would appear as a brilliant orange
star over 10 times brighter then Venus, making a total of four
stars in the system. The Dipper's middle five stars are all moving
together, while Dubhe and Alkaid are going in the other direction,
the Dipper destined to fall apart over the next tens of thousands
of years.