The
nucleus is the solid, central part of a
comet, popularly termed a
dirty snowball or an
icy dirtball. A cometary nucleus is composed of
rock,
dust, and frozen
gases. When heated by the
Sun, the gases
sublimate and produce an
atmosphere surrounding the nucleus known as the
coma. The force exerted on the coma by the Sun's
radiation pressure and
solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an
albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.