Thuban‒The Once and Future Pole Star

Original article appears in GuideStar April, 2015.

It’s easy to observe the current pole star, Polaris, the past pole star, Thuban, and the future pole star Gamma Cep. Binoculars will get the job done easly. Thuban is easy to find on a line from the bowl of the Little Dipper asterism.

This is a spectral class A star, meaning it has a very significant hydrogen absorption line in its spectrum. Spectra of Thuban indicate that it has completed its hydrogen fusion phase and that it is now burning helium with a shell of non-burning hydrogen. (Burning here means that it is fusing helium to heavier elements including carbon and oxygen.) So, Thuban has begun its move off the main sequence of the HR diagram on its way to becoming, perhaps, a K (red) giant, similar to Aldebaran. Another possibility is that it has nearly exhausted its helium and will begin a carbon burning phase on its way to becoming a blue giant.

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