Definition
A comet is a celestial body mainly composed of ices, dust, and volatile compounds. It orbits a star along typically elliptical or parabolic trajectories. Unlike asteroids, comets show visible activity when they approach the star, due to the sublimation of their volatile materials. They do not shine by themselves, but reflect stellar light.
Structure
The structure of a comet includes a solid nucleus made of ice mixed with mineral particles, known as the cometary nucleus. Around this core forms a gaseous and dusty envelope called the coma when the comet nears the star. Two types of tails form: a curved dust tail due to radiation pressure, and a straight plasma tail directed by the stellar wind.
Functioning
Cometary activity depends on the distance to the star. As the comet approaches, the temperature rises and causes sublimation of the ices, releasing gases and particles that form the coma. This material is pushed away from the nucleus by radiation and magnetic fields, creating the tails. As the comet recedes, the activity decreases and it becomes an inactive nucleus again.
Evolution
Comets evolve according to their approaches to the star. At each orbit, part of their mass is lost as gas and dust, causing gradual changes to their surface. Some comets may fragment, become extinct, or become indistinguishable from inactive asteroids.
Limits
A comet is defined by its mass, volatile content, and orbit. Its activity only occurs when it is close enough to the star for sublimation to happen. Beyond a certain distance, it remains inert. Its stability may also be influenced by gravitational interactions with other celestial bodies.