Star trails. Long exposure image of star trails over a forest. Star trails are formed by the apparent motion of the stars through the sky as the Earth rotates. The stars appear to move around a point known as the celestial pole (out of frame to upper left). The long exposure allows the different colours of the stars to be seen clearly, from the bright orange trail at lower right, through yellow and white to blue. The colour of a star depends largely on its age and temperature, with red stars generally being old and cool, and blue/white stars being young and very hot. Photographed in Finland with a 4 hour 55 minute exposure.