The dust tail is characterized by a spectrum of sunlight. The sunlight is reflected from the dust emitted by the coma. In 1950 an American astronomer named Fred L. Whipple developed the Dirty Snowball Comet Model to explain the nature of the nuclei of a comet. He stated that the nuclei are compact, solid bodies of frozen water, ammonia, and methane bounded by a rocky material. When most comets near the perihelion, the nucleus splits up and leaves pieces of comet matter.
Periodic comets are comets that have an established pattern of orbiting the sun.
Short period comets have small orbit swings and orbit in the direction planets do. Long orbits have unstable orbit swings. Both comets either dissipate, collide with another planet, or are ejected from their orbit and the solar system after a million years or less. Jan Oort, a Dutch astronomer, developed the Oort model, which says that most comets are near Pluto and never reach the Earth. Infrequently, the gravity of a passing star or cloud may lead a comet to orbit the sun.