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Young World
WORLD OF SCIENCE
Gigantic snow balls
DR. T. V. PADMA
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Comets are made up of dirt, dust and frozen chemicals mixed with some rocks and minerals.
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Chinese astronomers were probably the first to record the appearance of a comet, more than 4,000 years ago. The early Chinese called comets broom stars, and thought Gods used comets to sweep away evil. By contrast, for many years, Europeans considered comets harbingers of doom. Today, comets are known to be gigantic snow balls made up of dirt, dust, and frozen chemicals (primarily carbon dioxide, methane, and water) mixed in with some rocks and minerals.
The icy, hard part of a comet, called the nucleus, can be upto 1,00,000 km in diameter. When a comet nears the sun, frozen chemicals turn into gas, and dust is also released. This forms a halo around the nucleus called a coma. When a comet passes near the sun, its glowing coma may be visible on earth, if the coma is large enough. Coma can light up very quickly — making it seem like a comet suddenly appeared in the sky from nowhere. Solar wind (charged particles that stream out of the sun) can blow gas and dust away from the coma — forming one or more ‘tails’. Tails may stream in front or behind a comet, but always point away from the sun. Tails can be as much as 100 million miles long.
Oort cloud
Astronomers think billions of comets might be orbiting the sun, three trillions of miles (5.5 trillion km) away from it, in a swarm called the Oort cloud. There is also another swarm of comets called the Kuiper belt, just beyond Neptune. Rarely, the gravity of a passing star pulls a comet out of the Oort cloud and flings it on a path towards the sun. Comets from the Kuiper belt can be hurled towards the sun by the gravity of a planet. Every year, a few hundred comets may be thrown towards the sun — but most are too small to be seen from Earth with the naked eye.
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