Comet Hyakutake. Close-up of the head or coma (bottom right) and the tail of Comet Hyakutake, one of the brightest comets to appear in the sky. Comets are bodies, mostly formed by water ice, which orbit the Sun with usually highly elongated orbits. When a comet approaches the Sun the water ice vaporises forming the coma, a cloud of gas and dust which can be up to 15 times the diameter of the Earth, and the tail. Comets have two types of tails: a gas tail (blue) which shows long filaments of excited gases, and a dust tail (not clearly seen here) in which dust particles reflect sunlight. North is at centre right.