Alpha Draconis - Thuban

Thuban is a relatively inconspicuous star of apparent magnitude 3.65, fourth-brightest in the constellation Draco. Even though Thuban carries the Bayer designation Alpha Draconis, it is over a magnitude fainter than the brightest star in the constellation, γ Dra (Etamin). The name Thuban comes from Thu'ban, "the basilisk", the Arabic name for the constellation Draco.

Thuban is not bright enough to see from light-polluted areas. But given good viewing conditions, Thuban is relatively easy to spot, due to its location in relation to the Big Dipper. While it is well known that the two outer stars of the Dipper point to the modern-day pole star Polaris, it is less well known that the two inner stars, Phecda and Megrez, point to Thuban, just 7-1/2° degrees north of Megrez.

Pole Star

Thuban is historically significant as having been the north pole star in ancient times. Due to the precession of Earth's rotational axis (a slow, 26,000-year wobble around the line perpendicular to the Earth's solar orbit), the position of the celestial pole - around which all stars appear to revolve - constantly changes. During the time of the ancient Egyptian civilizations, Thuban was the naked-eye star closest to the north pole. In 3942 BC, it moved farther north than θ Boo; it was closest to the pole - less than 2.5' away - in 2787 BC.

Thuban was considered the pole star until about 1900 BC, when the much brighter Kochab in Ursa Minor began to approach the pole as well. Drifting away from the pole over the last 4,800 years, Thuban will be nearly 47 degrees off by 10000 AD, then gradually move back toward the pole. In 20346 AD, it will be the pole star once again.

Properties and Companion

At a rather large distance of 310 light years, Thuban's spectral class is A0 III, making it a fairly rare hot giant star. Its temperature of 9800 K is near that of Vega; but it is over five times as luminous as Vega (and 300 times more luminous than the Sun). Its high luminosity indicates that Thuban, unlike Vega, has ceased fusing hydrogen in its core and has begun to evolve into a class K red-orange giant, like Aldebaran. Unlike many stars of its class, Thuban no particular chemical anomalies. It is in fact somewhat poor in metals as compared with the Sun.

Thuban is a binary star, with a companion star in a 51-day orbit. The companion has not been directly imaged and from its mass is likely a red dwarf or a low mass white dwarf.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]