Comet, sky night's beauty.
The comets have a nucleus that may reach several kilometers in diameter. The composition varies, and although most of them are rocky there are lot of them too that are carrying metals, water vapour and hydrocyanic acid. Their orbits are very eccentric, and some of them might travel in parabolic or hyperbolic orbits. Due to its eccentric orbit they go away from the Sun billions of kilometers to come back years later to a lesser distance than is Mercury. Being at that point near the sun is when the solar wind evaporates part of the materials that compose it, causing the formation of a gaseous envelope surrounding the bright comet's nucleus that is called coma. If the heat and the solar wind evaporates sufficient material, then form what is called the tail of the comet, being able to reach millions of kilometers in length and make it as magnificient, and also extends its tail in the opposite direction to the Sun. As the comet goes away on his journey fron the Sun, is diminishing its tail until being cut to the core, giving conclude its spectacle.
When the comet returned after a complete turn around its orbit and is coming back to the Sun, after hundreds, thousands or millions of years, beginns a new show.
As we have said, each time that the comet approaches to the Sun, theirs compounds evaporate, leading to that over the years the comet losses part of its mass.