This part of the comet glows as it gets closer to the sun and can be as large as 100,000 miles across, or half the distance from the Earth to the moon. The largest coma ever recorded by astronomers was even larger than the sun. As the comet is heated by the sun, evaporating gas is forced outward and forms the third part of the comet, the tail . Actually, many comets have two tails, one made of gas and one of dust. A gas tail appears narrow and straight.
Dust tails are wider with more of a curve.
Tails made out of both gas and dust tend to have streaks and spirals. Just like some comets have two tails, some comets have no tail at all, only a nucleus and a fuzzy coma. A tail acts like a rocket engine propelling a comet along its orbit and is sometimes millions of miles long.