At magnitude 2.44, Gamma Ursae Majoris is the sixth brightest star in Ursa Major, the Great Bear; it is also the southernmost star in the bowl of the Big Dipper. The star has the traditional names Phecda, Phekda or Phad, which come from the Arabic word "fakhdh", meaning "thigh" (of the Great Bear), reflecting the star's placement within the Bear's hindquarters.
Properties
The five middle stars of the Dipper are all part of the Ursa Major moving group, a loose open cluster of stars about 300 million years old, whose members are moving together through space. All are about the same distance away from us; Phecda, at 84 light years, is the most distant of the five in the Big Dipper. Like its counterparts, Phecda is a class A0 V main sequence star, fusing hydrogen into helium in its core, like the Sun. Phecda is a white star with a temperature of 9500 K; it radiates 64 times the Sun's luminosity, has three times its diameter, and has a mass about 2.7 times solar.
Phecda's spectrum shows hydrogen emissions from a surrounding disk of spinning gas, making it an "Ae" star. Most stars that behave like Phecda are hotter, class B stars; only about 100 Ae stars are known. Phecda, rotating at least at 168 km/sec at its equator, is the brightest. All are rapid rotators, whose fast rotation is probably the cause of the gas disk that surrounds them. Phecda's spectrum also shows evidence of a faint, close companion, but nothing else is known about it. There is no evidence of any dust disk that might indicate planets.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]