Star trails. Time-exposure image of star trails around the north celestial pole. Star trails are formed by the apparent motion of the stars around the celestial pole as the Earth rotates. The trail of Polaris, the Pole Star (Alpha Ursae Minoris), in the constellation Ursa Minor is seen at the centre of the circular trails. The differing colours of the trails are due to the different intrinsic colours of the stars themselves. Photographed over British Columbia, Canada.