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Thuban

The star (or star system) Thuban, also known as Alpha Draconis would be totally unknown if it weren't that -- due to the precession of Earth's rotational axis -- it was the star closest to the north pole from some time prior to 3000 BC until 1900 BC (when it was superseded by Kochab[?]). It was at its closest approach to the pole in 2700 BC, when it was 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.

Thuban has a spectral class of A0III, indicating that it's very similar 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 it's 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 Hadar.

Thuban is binary, 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.



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