THUBAN (Alpha Draconis). Fourth magnitude (though just barely) Thuban is one of the fainter stars that carries a proper name, almost certainly because of its immense historical role as a result of its position in the northern sky. Its importance is further highlighted in that it is the Alpha star of Draco (the Dragon) even though it not close to being the brightest of this long and rambling constellation, easily exceeded in visibility by Gamma, Beta, and even Eta Draconis. Among the most famed stars of the sky is Polaris, the north star, its prominence the result of its position close to the north celestial pole, the star showing the way north to within about half a degree. It was not always so, however. The Earth's rotational axis undergoes a slow, 26,000 year wobble around the perpendicular to its orbit around the Sun. As a result, the position of the sky's rotational pole, around which all the stars seem to go, constantly changes. Around 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 the name for another star. Thuban is among a fairly rare class of hot giant stars, its temperature of 9800 Kelvin near that of Vega. It is, however, over five times as luminous as Vega (and 300 times more luminous than the Sun), its fourth magnitude status the result of its rather large distance of 310 light years. Its relative brightness tells us that the star, unlike Vega, has ceased hydrogen fusion in its core and has begun to die. Thuban has an faint unseen companion in an orbit with a 51 day period and, unlike many stars of its class, has no particular abundance anomalies. It is in fact somewhat poor in metals as compared with the Sun.
Written by Jim Kaler. Return to STARS.