Comets are mostly icy material that spend most of their time far from the sun.
This makes up the nucleus. When they approach the Sun, they partly evaporate, and the material boiled and blown off makes the tail, coma, and a cloud of invisible Hydrogen. Bluer color, straight Dust tail is dust particles that partially follow comet s orbit as they are blown back. Not all comets have both tails fully developed. Hydrogen cloud surrounds nucleus, coma.