The nucleus is the solid part, made of rock and/or ice. This is the only part that exists when the comet is not passing close to the sun.
The coma is the nebulous envelope around the nucleus. It's made of ice, dust and gases. The word 'coma' is Latin for "hair".
The tail is made up of fragments streaming from the nucleus, away from the Sun. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. Separate tails are formed of dust and gases. The two tails are visible for different reasons: the dust reflects sunlight directly, and the gases glow from ionisation.
© Haydn Thompson 2017