Alpha Ursae Majoris
Dubhe / Dubə / is the name of the star Alpha Ursae Majoris ( α UMa ) in the constellation Ursa Major. It is visible in Central Europe throughout the year. The name comes from the Arabic word دب / dubb /, bear '.
Dubhe is despite the Bayer designation "alpha" only the second brightest star in the constellation Ursa Major and is located at the upper right corner of the box of the Big Dipper. Together with Merak it forms its completion, its extension beyond Dubhe addition to the Polar Star shows (Show 's star ).
He is not a member of the Ursa Major Group - the next physical star group with a common speed - and also includes another Spektralgruppe than most stars as Member Mizar or Phekda. Dubhe has an apparent magnitude of 1.7 mag. Its distance is about 124 light-years.
Construction of the system
The Dubhe system consists of two double stars, the star Dubhe B the much more massive Dubhe A orbits at a distance of 22.9 AU (slightly more than the maximum distance of Uranus to the sun ). Dubhe C is in an orbit of more than 8000 AU (0.12 light years ) and consists of a dark class F star and a close companion, which surrounded him in less than 7 days.
In A Dubhe is an orange- yellow giant star on the end of its development. He has 30 - times the radius of the sun as well as the 224 - times brighter. In contrast, he has, however, only a four-fold solar mass, resulting in a small envelope density results. The surface temperature is 4800 to 5000 degrees.
Dubhe B is located in contrast, still in the stage of hydrogen burning. As a star of the class K0 V he lights up for about 15 times as bright as our sun. It orbits the giant Dubhe A in a period of 44 years.
Dubhe C is about 8.000 AU (0.12 light years) away from Dubhe A and consists of a star of class F8 V and a close companion orbiting it with a period of slightly less than 7 days. He lit one and a half times as bright as our sun and is its mass and radius probably also similar.
Possibility for planetary systems
Both at Dubhe A and B Dubhe planets could move a small distance (so-called S- planet ). Here, however, a planetary system around Dubhe A would be more likely than at Dubhe B because the star can hold more satellites due to its greater gravity or Dubhe B the planet literally snatches.
Any planets around Dubhe C befänden outside a gravitational field that affect them the main star. The companion of Dubhe C does not interfere with the planetary orbits due to its small distance. To Dubhe C therefore could exist planets a greater number.