Comets

        Comets are loose collections of ice, dust, and rock particles, that have very long, narrow ellipses.  The comet has two tails, the gas tail (on top) and the dust tail (on bottom).  The brightest part of a comet is the comet's head, which is made of the nucleus and the coma.  The coma is only there when sublimation occurs as the comet gets close enough to the sun.  Gas and dust boil off the nucleus creating a fuzzy outer layer, called the coma.  The nucleus is like the inside of a comet and is made of ice and dust.  The Greek interpretation of comet means long-haired star.  Each of the comet's tails can e more than a mile long.  Comets are classified into two groups, short-term comets, and long-term comets.  To be classified into the short-term comet group, a comet's period of revolution had to be less than 200 years, and to be a long-term comet, a comet's period of revolution had to be more than 200 years.  The most famous comet, Halley's comet is a short-term comet.  It's period of revolution is 76 years.  Since it's last visit was in 1986, it will visit again in 2062.  Hale Bopp is a long-term comet.  Astronomers estimate it's period of revolution to be around 4000 years.  All comets originate in the Oort cloud or the Kuiper belt (see my post on the solar system).  The sizes of a comet vary-they can be as big as a mountain.