Comets: The Long-Haired Stars

In General...

Comets are icy bodies that travel around the sun in long,elliptical orbits. A comet contains a solid nucleus, a cloudy atmosphere called a coma, hydrogen cloud, and one or two tails. The nucleus is made up of various ices with rocky dust particles attached to those ices. When a comet approaches the sun, most of the surface ice is vaporized. Gases and particles that were formerly stuck to the nucleus fly away to the sun, creating the coma and tails.

Most comets can be seen only with telescopes. Others can be seen with the unaided eye but only when the comet is closest to the sun. they can be seen because of the reflection of sunlight from the dust in their coma and two tails. Gases also release energy absorbed from the sun causing them to shine.

Comets are told apart by observing positions when they are near the Earth. Long-period comets, those with periods of more than 200 years, have orbits that are not fixed to the Solar System's plane. They are believed to have come from far away because they have periods longer than 100,000 years. Their reservoir is the Oort comet cloud, named after the Dutch astronomer Jan Oort, who hypothesized it. The cloud is 1,000 times farther away than Pluto's orbit and most clouds in it will probably never come close to earth. Only a small number are disturbed and come plummeting toward us. Short-period comets are mostly confined to the Solar System's general plane and orbital direction. Their home the Kuiper belt, identified by a Dutch-American astronomer Gerard Kuiper, which extends the planetary disk to great astronomers to the Sun.

Structure

Comets are made up of four basic components:

The Nucleus. The source of all cometary gas and dust is the nucleus. The nucleus appears as a "dirty snowball". It is irregular in shape and ranges in diameter from a few hundred meters to 10km. The weight of the mass is thought to be split equally between ices and dust. Coma. The coma of a comet is a spherical envelope of gas and dust that surrounds the nucleus. It extends from 100,000 to 1,000,000km from the nucleus and flows away at a speed of 0.5km per second. The gas of the coma drags dust particles away from the nucleus. Comas do not appear until comets come to within about 3 AU of the sun. Hydrogen cloud. The hydrogen cloud contains all of the hydrogen that a comet produces. This escaping material does not and cannot begin in the nucleus. It probably comes from the dissociation of th hydroxyl radical, OH, by sunlight. Tails. Basically, there are two distinct kinds of tails: one containing dust and the other containing plasma (ions and electrons). The dust tail seems yellow because the light reaching us from it is sunlight. The plasma tail resembles a blue color because radiation emitted by florescing ions of carbon monoxide within the tail peaks at about 4200 angstroms. Dust and plasma tails can be found solo or paired together in any given comet.

Historical Observations and Beliefs...

In ancient times, without streetlights or pollution, comets were easily seen by everyone. Many people thought that they were omens and used them to predict floods, famine, pestilence, or the death of kings. Aristotle thought that comets were not part of the heavens but only earthly items ignited that transfered heat to the upper atmosphere. His ideas prevailed until the 14th century A.D. when a Danish nobleman named Tycho Brahe established proof that comets are heavenly bodies. He proved this by showing that a particular comet was at least four times farther away from the moon.

In modern times, comets are now known as part of the material of the proplanetary cloud from which the planets were formed. The planets formed from a collection of gas, ice, rock, and dust. Much of the ice and dust became part of the giant planets-Jupiter, Saturn, Uranus and Neptune. The comets remained as leftover bits of ice and dust. The study of the comets gives us insight on the conditions on which the solar system was formed, the primordial matter from which the planets and the earth were built, and the mode of condensation of this matter.

This page was written by Jade Enrique in the astronomy class of BCC/Broward County in July 1998. I would like to acknoweledge the NASA astronomy site for my information and Nova High School for the opportunity to use the computers at thier school.