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.