Most comets have a nucleus (the center of a comet) that is less than about 6 miles (10 km) wide. The size of a comet changes depending on how close it is to the sun. As a comet gets closer to the sun, the ices on the surface of its nucleus vaporize and form a cloud called a coma around the nucleus that can expand out to 50,000 miles (80,000 km). A tail also forms on a comet as it approaches the sun. Comet tails can be over 600,000 miles (1 million km) long.