THE NUCLEUS:
The nucleus is the concentrated part of the comet, with an average diameter of 16 kilometers; however, the diameter can range from 1-100 kilometers. Fred Whipple, an american astronomer proposed the idea that the nucleus was a combination of ices and dust. The nucleus is composed of a combination of frozen compounds, including water (80%), carbon monoxide (15%), ammonia, methanol and methane, all frozen and mixed with dust. [The percentage is made according to Halley's Comet.]

THE COMA:
The coma is a bright, cloudy halo which surrounds the nucleus when the nucleus sublimates leaving this atmosphere. The coma is spherical with the average diameter of nearly a million miles. The coma is very luminous because the gaseous atmosphere becomes excited by the sunlight and protons of the solar wind.[ side note: an electron goes from a ground state to an excited state when a source of energy excites it, allowing it to go to a higher energy level]. The brightness is a result from the transitions of electrons in two different energy levels. The sizes of comas can vary because the rate of sublimation of the gases increase as it is closer to the sun; therefore, depending upon its position in its orbit, the coma size varies. a second factor is the comet's composition. Depending upon each material, the temperature of sublimation differs.


COMET TAILS:

A.) The Ionic Tail
The ionic tail is also known as the type I tail, and is a gas trail, dominated by the color blue. The blue color is explained by the fact that the most abundant ion, CO+ gives off around 400 to 460 nm of the color. The tail gets this name because it is made up of ions. The tail is normally up to 100,000 kilometers wide and tens of millions of kilometers long. It is straight and in the opposite of the sun's direction.
The ions are existent due to the ionization of the gases of solar ultraviolet radiation or due to solar wind. A solar wind is a wind of charged particles, going at the speed of 400 kilometers per second. Ionization is the making of ions, when a proton removes an electron from its position on an atom.

B.) Type II (Dust Tail)

The dust tail has a yellowish color and extends over millions of kilometers with dust particles, with sizes between 0.1 and 10 millimeters. The tail's appearance seems as if it has a curved shape.
The yellow color is explained by the fact that the dust is made up of silicates (mostly sand) and only reflects off the sun, which is yellowish-white. The appearance of the dust tail has a slight curvature because each grain of dust has a different ellipse which varies slightly from the others.

Credits: By Vincent Deblauwe and Guillaume Polet
Credits: World Book Encyclopedia 1997 version (ci-cz pages 867-868) by Donald K. Yeomans