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What Are the Three Main Parts of a Comet System?

People of ancient times fostered many misconceptions about comets and considered them to be beyond comprehension and unpredictable. In some countries, the appearance of comets supposedly foretold the coming of catastrophic events such as deaths of monarchs, pestilence or natural disasters. In 1577, Tycho Brahe, a Danish astronomer, attempted to shed some light on the origin of these mystifying celestial bodies. Another astronomer, Edmond Halley, consolidated his knowledge with Isaac Newton's theories on planetary motion to further illuminate the nature of comets. Halley, extrapolating from past accounts and his own observations, was able to forecast the periodic appearances of a certain comet --- now named Halley's Comet. With advancing developments in the telescope and understanding of mathematical concepts, astronomers have amassed a considerable body of facts and figures concerning the origins, periodicity and structure of these once mysterious objects.

Origins
- Comet originate from beyond the confines of the solar system.
  
  Comets orbit in elongated circular figures called ellipses at different angles around the Sun. Comets originate in one of two regions in the solar system: the Oort Cloud and the Kuiper Belt. The Oort Cloud, a region containing rocky material and other particles, envelops the solar system. Within the Oort Cloud, comets form from accumulated debris. A comet begins its journey around the sun with the aid of a passing star whose gravity yanks at the piece of debris, dragging it into orbit. The Kuiper Belt, which lies closer to the solar system, stretches beyond the orbit of the planet Neptune. Some astronomers believe periodic comets, those that reappear in the sky every few hundred years or less, could have originated from the closer Kuiper Belt.
Nucleus
- The nucleus, or solid portion, of a comet can acquire different nonsymmetrical shapes, ranging from lopsided or elongated to roughly spherical. Close encounters with comet nuclei have revealed that they contain mixtures of various frozen substances, including water, carbon dioxide and ammonia. Comet nuclei also consist of darker masses such as silicates, compounds made of silicon and oxygen. When comets approach the sun, these layers of ice and dirt melt away and disperse through space in a process called "outgassing."
Coma
- While advancing toward the sun, the comet's nucleus forms a glowing head, the coma. The sun's rays heat up the nucleus, causing the frozen material to thaw. The melted matter seems to glow as it escapes from the nucleus' gravity into space. Some comets exhibit spectacular heads, measuring over 10,000 km across, comparable to the diameter of Earth.
Tail
- A comet's tail is arguably the most definitive feature of a comet.
  
  Human eyes recognize the passing of a comet by the sight of its most definitive feature: a brilliant tail that can stretch for millions of kilometers. Comets actually possess two tails of differing colors, with one usually more visible than the other. The blue tail made of charged gas particles points away from the sun and stems from contact with the solar wind, minute substances the sun radiates outward into space. The yellowish dust tail also leads away from the sun, but displays an arc-like shape. A comet loses much of its mass because of the matter propelled from its tails.
The Meteor Shower Connection
- Meteor showers occur annually, with each originating from a particular comet.
  
  Surprisingly, comets, not random asteroids, give rise to another celestial phenomenon: meteor showers. As a comet passes through the solar system, the particles in its tail contact Earth's atmosphere, causing the objects to blaze and fall toward the planet. Because astronomers have learned to calculate a comet's path and position, meteor shower occurrences have become predictable. Meteor showers occur periodically at particular times throughout the year. Astronomers have attributed certain meteor shower events to specific comets. To illustrate, the Perseid meteor showers in August originate in Comet Swift-Tuttle whereas Halley's Comet generates the Orionid meteor showers in October.

Astronomy