As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. The evaporating gases carry small grains with them, forming the comet's coma of gas and dust. When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce.
Comets Are Important Because They . . .
May be the oldest, most primitive bodies in the solar system preserving the earliest record of material from the nebula which formed the sun and the planets. Impact the Earth and other planets at hypervelocities, causing major changes in climate and dramatically affecting the ecological balance, possibly including the extinction of the Dinosaurs. Are the building blocks of planetary systems around other stars. Also, see this letter written by Fred Whipple .