Comet Ikeya-Zhang. Comets are bodies of ice and dust, which move around the Sun in long, elliptical orbits. As a comet nears the Sun its surface is heated and some of the ice evaporates. The released gas is ionized by ultraviolet light from the Sun and glows. The gas and dust can form two separate tails; a gas tail and a dust tail. A gas tail consists of ionised gas, swept outwards by the solar wind, whereas a dust tail is composed of grains of dust, blown outwards by solar radiation pressure. Ikeya-Zhang is thought to orbit the Sun with a period of around 367 years. Here, the spiral galaxy Messier 31 (M31) can also be seen. Photographed in 2002.