Comets are named for the Latin aster kometes which means "long haired stars". They are small objects, their actual size ranging from 5 to 20 kilometers in diameter. They are mainly composed of dust and ice. As they near the Sun the surface vaporizes to form a gaseous head and an unusually long tail. They are believed to originate and evolve from dirty, icy objects that were part of the primordial debris thrown outward into interstellar space when the Solar system was formed. This source region is called the Oort Cloud in honor of Jan Hendrik Oort, a Dutch astronomer, who proposed its existence in 1950.
Comets consist of four parts.
1. The nucleus, a few kilometers in diameter,
and composed of rocky or metallic material and solid ice made up of water,
methane, ammonia, and carbon dioxide, (The comet proper)
2. The head or coma, which surrounds the Nucleus
several hundred kilometers in diameter, formed from the nucleus as it approaches
within five astronomical units from the Sun,
3. The long ( millions of kilometers in length) voluminous,
and magnificent tail composed of a combination of ionized molecules and
dust, and
4. A spherical hydrogen cloud surrounding the coma,
formed by dissociation of water molecules in the nucleus.
Their orbit is very highly elliptical. Their greatest distance from the Sun is 50,000 AU. which was thought to be the limit of the Sun's gravitational influence. But recently astrophysicists made computer simulation that the outer limit is between 80,000 and 100,000 AU. where it is balanced by the Milky Way. Sir Edmund Halley (1656 - 1742), a British Astronomer was the first to suggest and predict the periodic appearance of the same comet. He observed the comet bearing his name in 1682, and predict its return in 76 years. It has appeared every 76 years, the last two appearances being in 1910 and 1986.
For life and details of Sir Edmund Halley, click below
For pictures of comets Click below
Comets
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