PHECDA (Gamma Ursae Majoris). Few figures in the sky move us more than the Big Dipper, its seven bright stars laid out in a long bent row that the British call the Plough. All but one of its stars are second magnitude, though Phecda, the third one in from the end of the bowl, and southernmost of the bowl stars, is just on the edge of third (2.44). Though it ranks sixth in brightness in the Dipper, indeed in the whole parent constellation, Ursa Major (the Greater Bear), Phecda still received the Gamma designation from Bayer, who simply lettered the Dipper stars from front to back, the brightest (Alkaid) getting Eta. The name refers directly to the Bear; Phecda is a rough re-pronunciation of the Arabic word that means "thigh," reflecting the star's placement within the Bear's hindquarters. The five middle stars of the Dipper are all part of a group, a loose open cluster of stars (the Ursa Major Cluster) whose members are moving through space together. (Dubhe and Alkaid, at the Dipper's ends, are not a part; the result will be the Dipper's eventual dissolution.) All about the same distance away from us, Phecda, at 84 light years, is just barely the most distant of the five. They are the collective centerpiece of a much larger structure called the Ursa Major Moving Group that, among many other stars, includes Sirius, and is about 300 million years old. Like its Dipper counterparts, Phecda is a class A (at the hot end of A, A0) main sequence dwarf, a common hydrogen-burner like the Sun (though Alioth -- Epsilon -- is probably close to burning out). Phecda is a white, "colorless," star, meaning that it appears the same relative brightness in a photograph as it does to the eye. With a temperature of 9500 Kelvin, it radiates 64 times as much energy as the Sun from a surface three times the solar diameter, from which we infer a mass about 2.7 times solar. Phecda's spectrum once suggested evidence for a close companion, but there is no further evidence that such exists, and the effect is probably spurious. Phecda's greatest claim to stellar fame involves emissions from hydrogen that tell of a circulating cloud or disk of spinning gas, making it formally an "Ae" star. Most stars that behave like Phecda are hotter, in class B (which stretches from just over 9500 Kelvin to 30,000 Kelvin). Such "Be stars" are fairly common, and include Achernar, Alcyone, and (at the extreme) Gamma Cas. All are rapid rotators, the fast rotation probably the cause of the spinning cloud that surrounds them. These "emission stars" extend coolward into class A, but they are rare; only about 100 Ae stars are known. Phecda, rotating at least at 168 kilometers per second at its equator, 84 times faster than the Sun, is the brightest. There is no evidence of any dust cloud that might hold planets, a pity as the other Dipper stars would present quite a sight. From Phecda, Merak (the Beta star) would look like Sirius does in our sky. In the opposite direction, the stars of the handle would be strung out like bright jewels along wide string.
Written by Jim Kaler 3/19/00. Return to STARS.