Comets are basically giant balls of water ice, gas, dust and small rock-like particles. They formed in the outer portions of the solar system at the same time the solar system was born. They normally orbit far from the Sun, but their orbits can be perturbed by planets and asteroids and they can be sent into the inner solar system. When they near the Sun, the solar heat causes the frozen ice in the comet's nucleus to sublimate and turn directly into gas, bypassing the liquid stage. As the material is ejected it forms a gigantic cloud around the nucleus which we see as the coma. It then separates into the blue ion gas tail, and the dust tail. Some of the molecules in the ion tail are broken down by sunlight and become electrically charged ions, hence the name ion tail. The ion tail is then blown away from the sun by the solar wind and it points directly away from the sun. This tail gets its blue color and is visible because the ions glow as they fluoresce, much the same as a fluorescent light. The dust tail is made of dust grains of different sizes, including silicates and "organic" molecules with carbon atoms. It is illuminated by direct sunlight. The dust grains follow behind the comet in its orbit around the sun, and usually the dust tail curves gracefully behind the comet. If the Earth happens to pass through a stream of these dust particles that were left behind in orbit around the Sun, a meteor shower can take place. Image Data
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