The part we can see of the head of the comet is the coma . The coma of a comet can be very big, between the size of Jupiter and the Sun when a comet is very close to the Sun. The gas and dust that make up the coma all come from the tiny nucleus. The particles of gas and dust must be spread very, very far apart to fill up such a large volume. Attach the yarn to the cotton ball with glue or tape.
Comets really have two visible tails.
The second is the blue ion tail. For older students who have been introduced to both types of comet tails , add a second blue piece of yarn or ribbon measured out to about the same length as the white tail. Compare your scale model comet with object which represents the Earth. Even though the solid nucleus is very small, the coma and tail are much larger than the Earth. The tails of a comet lengthen as a comet gets close to the Sun, and shorten as it moves away from the Sun.