The Comet
The comet is any of a class of small celestial objects orbiting the Sun and developing diffuse gaseous envelopes and often long luminous tails when near the Sun. The comet makes a transient appearance in the sky and is often said to have a 'hairy' tail. The word comes from the Greek kilometres, meaning 'hairy one', a description that fits the bright comets noticed by the ancients. Despite their name, many comets do not develop tails. Moreover, comets are not surrounded by nebulosity during most of their lifetime. The only permanent feature of a comet is its nucleus, which is a small body that may be seen as a stellar image in large telescopes when tail and nebulosity do not exist, particularly when the comet is still far away from the Sun. A comet's orbit is more eccentric, therefore, its distance to the Sun varies considerably. Its material is more volatile. When far from the Sun, however, a comet remains in its pristine state for aeons without losing any volatile components because of the deep cold of space.