About 4000 years ago, the star Thuban was the North Star. Since then, precession of the Earth's axis has changed where the North Pole points, so the North Star is now Polaris. In another 10,000 years or so, the North Star will be Vega.
Earth takes 25000years for a complete cycle of Precession, for Polaris to be the Pole Star again.
The North Celestial Pole changes along a Circle through Polaris, Vega, Thuban.
See the image,
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