Thuban

Thuban (α Dra / α Draconis / Alpha Draconis) is a star (or star system) in the constellation of Draco.

Related Topics:
Alpha - Star - Constellation - Draco

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Due to the precession of Earth's rotational axis, it was the naked-eye star closest to the north pole from some time prior to 3000 B.C. until 1900 B.C., when it was superseded by Beta Ursae Minoris. It was closest to the pole in 2700 B.C.: a mere 10 seconds of arc away (1/180th the width of the full Moon). Thuban is just barely fourth magnitude, so it cannot be seen from light-polluted areas. Even though Thuban is the alpha star in Draco, at apparent magnitude 3.65 it is over a magnitude fainter than the brightest star in the constellation, γ Dra (Etamin), whose apparent magnitude is 2.23.

Related Topics:
Precession - Earth - North pole - 3000 B.C. - 1900 B.C. - Beta Ursae Minoris - 2700 B.C. - Full Moon - Magnitude - Light-polluted - γ Dra

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Thuban has a spectral class of A0III, indicating its similarity to Vega in temperature and spectrum, but more powerful and more massive. Thuban is not a main sequence star; it has now ceased hydrogen fusion in its core and is fusing helium. That makes it a bright giant star, being 250 times more powerful than our Sun but over 300 light-years distant. Thuban has no real anomalies except that it's quite rare to have a giant star in the A class, A being usually reserved for main sequence and the occasional supergiant. This indicates that Thuban has not been a giant star for very long and is likely still in the process of expanding, probably to eventually become a K class red/orange giant of the Aldebaran sort. It may also mean that it has recently run out of helium to fuse and is contracting before starting to burn carbon, in which case it may end up a blue giant such as Beta Centauri.

Related Topics:
Spectral class - Vega - Spectrum - Main sequence - Hydrogen - Fusion - Helium - Sun - Light-year - Aldebaran - Carbon - Blue giant - Beta Centauri

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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.

Related Topics:
Binary star - Red dwarf - White dwarf

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The name comes from the Arabic ثعبان (θu?bān, "the basilisk"), the Arabic name for the constellation Draco.

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