Thuban

Observational Data
Designation |  HD 123299
HIP 68756
Right ascension |  14h 04m 23.34995s[1]
Declination |  +64° 22′ 33.0619″[1]
Constellation |  Draco
Type of object |  Binary star
Magnitude |  Apparent Mag: +3.65[1]
Absolute Mag: -1.19[2]
Astrometry
Distance from Earth |  303 ly[3]
Radial velocity |  -13.0±0.9 km/s[1]
Proper motion |  RA: -56.34 mas/yr[1]
Dec.: 17.21 mas/yr[1]
Parallax |  10.76±0.17 mas[1]

Thuban, also known as Alpha Draconis, is a fourth magnitude binary star, some 310 light years away in the constellation of Draco. The star is notable, because most stars this faint do not carry a proper name. Thuban, though, is a star that plays a significant historical role because it was once the pole, or north star of the ancient world. The name Thuban itself is derived from the Arabic ثعبان θu‘bān, meaning "the basilisk", which was also applied to the entire Draco constellation.[4]

Thuban has an apparent magnitude of 3.65, also making its alpha Bayer designation unusual. The designation is usually reserved for the brightest star in a constellation, in this case - Gamma Draconis. Alpha Draconis is only the fourth brightest. The star itself is easy to spot, despite its faintness, because of its location relative to the Big Dipper. The two inner stars of the Big Dipper can be used to locate Thuban, which appears very nearby in the night sky.

## The Star[edit]

Thuban is a rare white giant star of spectral class A0 III.[1] The star is much like Vega. As a giant star, however, Thuban has already evolved off of the main sequence. The star is 3.5 times the mass of our Sun, and is 265 times the radius. The star is 300 times as visually luminous as the Sun even though much of the energy emitted is in the ultraviolet. The surface temperature is 9,800 K.[5]

As a giant, Thuban has already used up its core hydrogen fuel. Depending on the stage of its life as a giant, the star may be either expanding and cooling to become an orange giant or that the helium fuel in the core has already been used up. If the latter is true, the star is actually contracting and will start to fuse carbon in the core, becoming a blue giant.[5]

It is known that Thuban has a small companion star. The companion, which has not been directly imaged, orbits the primary once every 51 days and may be either a red or white dwarf.[5]

## Thuban as a Pole Star[edit]

Thuban was the pole star, meaning the closest star to the celestial north pole that can be seen with the unaided eye. Due to the precession of the equinoxes, Thuban was the pole star from around 3900 BC until 1900 BC, when the much brighter Beta Ursae Minoris replaced it (although Thuban was still closer to the celestial pole until 1793 BC).[6] At present, Thuban is slowly moving away from the northern celestial pole.

## References[edit]

1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Alpha Draconis from the SIMBAD Astronomical Database
2. ↑ From definition of absolute magnitude, using apparent magnitude (+3.65) and distance (303 ly) given here.
3. ↑ Calculated from given parallax of 10.76 mas.
4. ↑ Allen, Richard Hinkley (1963). Star Names, Their Lore and Meaning. New York: Dover. ISBN 0-486-21079-0. Available online at: penelope.uchicago.edu
5. ↑ 5.0 5.1 5.2 Thuban from stars.astro.illinois.edu
6. ↑ Thuban from daviddarling.info

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