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"Hale-Bopp comet. Optical image of the Hale-Bopp comet and the Aurora Borealis seen over trees and snow. The comet's head and tails are clearly visible (upper centre). The head comprises ice and rock (nucleus) surrounded by a region of glowing gas (coma). The gas or \ion\"" tail (blue) is formed by the solar wind blowing ionised gas away from the comet's head. Hale-Bopp was discovered on the 23rd June 1995, it is one of the brightest comets of the 20th century and was last seen around 2000 BC. The Aurora Borealis (northern lights) is also visible (lower centre). Photographed in Fairbanks, Alaska, USA.""" (KEYSTONE/SCIENCE PHOTO LIBRARY/JACK FINCH)