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Polar star trails. Optical long exposure image of star trails around the celestial north pole. As the Earth rotates on its axis, the stars appear to rotate around the celestial poles. The long exposure allows the colours of the stars to be clearly seen. The colour of a star depends largely on its age and temperature, with blue/white stars generally being young and very hot and red stars being old and cool. The brightest trail seen here (white, centre) is that of Polaris, the Pole Star. It lies within one degree of the celestial north pole, in the constellation of Ursa Minor. (KEYSTONE/SCIENCE PHOTO LIBRARY/JOHN SANFORD)