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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.