10 DRA (10 Draconis = CU Draconis). Among the most famed of stars is fourth magnitude Thuban, Alpha Draconis (in Draco, the Dragon), which, compliments of the 26,000-year precession, or wobble, of the Earth's axis, was the pole star in and around 2700 BC, during the time of ancient Egypt. Indeed, there were not one, but TWO pole stars, though the other one, fifth magnitude (4.64) 10 Draconis (Flamsteed 10, no Greek letter), which lies just 1.4 degrees to the west of Thuban, is never mentioned alongside its far better known neighbor. And too bad, as of the two it may be the more interesting. If not, it certainly is the more colorful, a red class M (M3.5) giant that through binoculars or a wide field telescope contrasts very nicely with white Thuban. Cool (3660 Kelvin), from a distance of 390 light years 10 Dra shines with the light of 840 Suns, though the great majority of the radiation lies in the invisible infrared part of the spectrum. Typical of class M giants, it's big, with a radius 72 times that of the Sun, 0.34 Astronomical Units, 87 percent the size of Mercury's orbit. Its mass and exact state of evolution are hard to gauge, as stars like this one but with a range of masses are so close in characteristics. It is most likely a 1.5 to 2 solar mass star that is either just starting to fire up its helium core to fuse to carbon and oxygen, or it has already used its internal helium core fuel and is brightening for the second time in a more advanced state. Again typical of such stars, it's a slight irregular variable with about a half-magnitude range. As such, it also goes by the variable star name CU Draconis (fitting, since the Star of the Week is written from Champaign-Urbana). At the same time, the star oscillates much more subtly over a period of two hours. There is no known rotation period (such stars very slow rotators), nor are there significant abundance anomalies, though the iron-to-hydrogen ratio is a bit low, about 75 percent that of the Sun. A minute to a minute and a half of arc away lie two "companions," 13th magnitude 10-B and 12th magnitude 10-C, which relative motions show merely lie along the line of sight, the star, so far as we know, single.
Written by Jim Kaler 5/15/09. Return to STARS.