THUBAN (Alpha Draconis). Fourth magnitude (though at 3.65 just barely), Thuban is one of the fainter stars that carries a proper name, almost certainly because of the immense historical role played by its position in the northern sky. The star's significance is further highlighted by being Draco's (the Dragon's) Alpha star even though it is not close to being the brightest within this long and rambling constellation, the star easily outshined by Gamma, Beta, and even Eta Draconis. Among the most famed stars of the sky is Polaris, the North Star. Lying close to the North Celestial Pole, Polaris shows the way north to within about half a degree. But it was not always so. The Earth's rotational axis undergoes a slow, 26,000 year wobble around the perpendicular to its orbit around the Sun. Called precession, the motion was discovered by Hipparchus in the second century BC. As a result, the position of the sky's rotational pole, around which all the stars seem to go, constantly changes. About the time of the Greek poet Homer, Kochab in Ursa Minor was a (rather poor) pole star. Among the best ever, however, was our Thuban, which was almost exactly at the pole in 2700 BC. It remained better than Kochab up to around 1900 BC, and was therefore the pole star during the time of the ancient Egyptian civilizations. Even though the star is in the Dragon's tail, its name confusingly derives from an Arabic phrase meaning "the Serpent's head," having been borrowed from that of another star. Listed as a class A (A0) giant (see below) 303 light years away (give or take just 5), Thuban glows with a temperature of 9910 Kelvin, from which we calculate a small amount of ultraviolet radiation. The result is a total luminosity of 294 times that of the Sun, which along with temperature gives a radius of 5.8 times solar. A projected equatorial rotation velocity of 23 kilometers per second yields a rotation period under 12.7 days. The lack of abundance anomalies suggests that the star is really rotating much faster so as to stir up the stellar atmosphere, which disallows gravitational settling and radiational lofting of various chemical elements. While the star is spectroscopically classed as a giant, theory applied to luminosity and temperature shows that, with a mass of 3.4 to 3.5 Suns, Thuban is either in the end-stages of "dwarfhood" with a hydrogen-fusing core or has just turned into a subgiant with the core burned out to helium. Thuban has a faint spectroscopically-detected companion in an orbit with a 51.42 day period. Assuming low mass, the companion orbits at an averaged distance of 0.41 Astronomical Units, just over Mercury's distance from the Sun. A small amount of excess infrared radiation implies the possibility of a thin surrounding disk, though there is no evidence for any planets.

Written byJim Kaler 8/28/98; revised 8/28/15. Return to STARS.