Scientists hypothesize that comets and rocky asteroids are leftover materials from the formation of the solar system. A typical comet is made up of a small chunk of ice, about 5 to 10 miles across. This chunk is called the nucleus. Mixed in with the ice of the nucleus is dust, making the comet like a dirty snowball. As a comet nears the sun, heat causes part of the comet's surface to become a gas.
Dust is mixed with the gas.
The mixture is blown from the comet's surface to form a large cloud called a coma. A coma can be tens of thousands of miles across, which makes a comet look fuzzy. Solar winds, streams of particles from the sun's surface, blow the coma behind the nucleus as the comet approaches the sun. This trailing cloud becomes the comet's tail. As the comet orbits the sun, the tail changes direction because it is always blown away from the sun by solar winds.