Thuban

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Thuban also known by its Bayer designation Alpha Draconis (α Draconis, α Dra) is a star (or star system) in the constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star in ancient times. Thuban is an Arabic word for snake.

Even though Johann Bayer gave Thuban the designation Alpha, its apparent magnitude of 3.65 is 3.7 times fainter than the brightest star in the constellation, Gamma Draconis (Eltanin), whose apparent magnitude is 2.24.

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Visibility

Given good viewing conditions, Thuban is relatively easy to spot in the night sky, due to its location in relation to the Big Dipper asterism of Ursa Major. 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 15 degrees of arc from Megrez. Thuban is not bright enough to be viewed from light-polluted areas.

Pole star

Due to the precession of Earth's rotational axis, Thuban was the naked-eye star closest to the north pole from 3942 BC, when it moved farther north than Theta Boötis, until 1793 BC, when it was superseded by Kappa Draconis. It was closest to the pole in 2787 BC, when it was less than two and a half arc-minutes away from the pole. It remained within one degree of true north for nearly 200 years afterwards, and even 900 years after its closest approach, was just five degrees off the pole. Thuban was considered the pole star until about 1900 BC, when the much brighter Kochab began to approach the pole as well.

Having gradually drifted away from the pole over the last 4,800 years, Thuban is now seen in the night sky at a declination of 64° 20' 45.6", RA 14h 04m 33.58s. After moving nearly 47 degrees off the pole by 10000 AD, Thuban will gradually move back toward the north celestial pole. In 20346 AD, it will again be the pole star, that year reaching a maximum declination of 88° 43' 17.3", RA 19h 08m 54.17s.

Properties

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 white giant star, being 250 times more powerful than our Sun but over 300 light-years distant.

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