This hydrogen cloud may be as big as 2 million miles across. When comets come near the Sun, their tails become visible because heat from the Sun vaporizes some of the icy nucleus or head and sunlight reflects from the vapor. Solar wind pushes the vapor in a direction away from the Sun to form the comet's tail. For this reason, comet tails always point away from the Sun. Comet tails are often very long.
Some comets also develop separate, curved tails or dust clouds.
From the study of spectrograms, it is known that comets are made largely of compounds of carbon, oxygen, nitrogen, and hydrogen. These compounds include water, ammonia, methane, carbon monoxide, and smaller amounts of other, more complex compounds. The dust in comets is composed of silicates. Recently, ethane gas was detected in Comet Hyakutake.