why does comet have a tail
As seen through a telescope, a comet has a ?head? and a ?tail.? The head is a large cloud of glowing gases called the ?coma? of the comet. The coma may be more than 1,609,300 kilometers across. Its gases are so light that the ?wind? from the sun blows them. The tail of a comet forms when gases are blown back by the solar wind. As a comet approaches the sun, its tail grows larger and larger because the pressure of the solar wind is increasing. As the comet moves away from the sun into the coldness of space, the pressure of the solar wind continues to blow against the gases. For this reason the tail of a comet always points away from the sun. A small, shining point of light can sometimes be seen in the center of the coma. The point of light is called the ?nucleus? of the comet. Astronomers think that the nucleus is like an enormous, dirty snowball?a mixture of ice and dust particles forming a ball about 1 kilometer in diameter. In their trip around the sun, most comets travel very elongated orbits. That is, the path they take resembles the shape of a long, fat cigar. A comet may take thousands of years to complete one trip, About three or four times a century, a comet passes so close to the sun that its bright, glowing tail can easily be seen. We see a comet only when one happens to pass close to the sun. Then the heat of the sun changes the ice in the nucleus into gas. Radiation from the sun passing through these gases ionizes them and causes the gases to glow with light.