ANWAR AL FARKADAIN (Eta Ursae Minoris). The Big Dipper (the Plough in England), the major figure of Ursa Major (the Great Bear), is so well known that few except the dedicated pay much attention to its counterpart, the Little Dipper, which in parallel is the most (in fact the only) recognizable portion of Ursa Minor, the Small Bear. And no wonder, since the Little Dipper is so faint that it cannot be seen in any town with bright lights. The figure is recognizable mostly by the North Star (Polaris, Alpha Ursae Minoris) and by the two front bowl stars, Kochab and Pherkad (Beta and Gamma). Kochab was at one time called "Nair (or Anwar) al Farkadain," meaning the "bright one" (or "the lights") of the "two calves," and Pherkad was called "Alifa al Farkadain," meaning the "dim one of the two calves" (hence the name "Pherkad"). As so often happenes, the names have been transferred to other stars, in this case to Eta and Zeta Ursae Minoris. And here, at the bottom of the Little Dipper, we find dim fifth magnitude (4.95) "Anwar," the faintest star of the Dipper's seven. Physically, the star is nearly (but not quite) sunlike, a class F (in the middle of the range, F5) dwarf with an estimated temperature of 6400 Kelvin, right at the point at which we do not have to correct for infrared or ultraviolet radiation. The star's luminosity of only 7.4 times that of the Sun leads to a radius twice solar and a mass 1.4 solar. Rather well along in its hydrogen-fusing lifetime, Anwar is a bit brighter than normal for its temperature, and seems close to becoming a "subgiant," a star that has shut down hydrogen fusion, if it has not already done so. Anwar is listed as a double star with a rather distant companion, but the cool nature of the star and its dim 12th magnitude status place it at only about half Anwar's distance, showing the pairing to be merely a line-of-sight coincidence. Lower-mass solar type stars rotate slowly (the sun taking 25 days at an equatorial speed of 2 kilometers per second), while high-mass stars rotate quickly. The division in rotation is rather sharp, the "rotation break" falling in the middle of class F. Anwar, rotating at least 76 kilometers per second (with a period under 1.4 days), falls just above the limit. Like the Sun, however, the rotation (and convection in its outer layers) give Anwar an X-ray-emitting hot corona.
Written by Jim Kaler. Return to STARS.