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.
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