What does the head of the comet consist of what happens when it is heated?

As the head of the comet is heated up, the ice surrounding the nucleus undergoes sublimation and  produces a bright cloud which envelops the nucleus. This cloud is commonly referred to as the coma and is composed of ammonia, carbon dioxide, dust, and water vapor. Together with the nucleus, these two make up the head of a comet. 

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -