The ice consists primarily of frozen water, with smaller amounts of methane, ammonia, and carbon dioxide radicals. So far approximately 850 comets have been recorded. The tail of a comet always faces away from the sun as a result of the solar wind. The sublimed gas also contains dust particles. The gas and dust spread out around the nucleus to form a large, thin atmosphere that is called the coma.
Sunlight causes the gas atoms in the coma to glow.
The amount of gasses present in the coma determine how bright the comet will be. Short-period comets have orbital periods of less than 200 years and are usually members of the inner solar system. Long-period comets, on the other hand, can have periods on the order of millions of years and may have orbits that extend halfway to the nearest stars. They are powered by thermonuclear fusion which is the process of combining two light elements to form a heavier element plus large amounts of energy. The brightness of a star depends upon its size and temperature.