The most visible and perhaps beautiful parts of a comet are its two tails: the gas tail and the dust tail. In fact, the tails gave comets their name. The word "comet" comes from the Latin words aster kometes, which means "long-haired star".

The gas tail of a comet is very straight and narrow. It is composed of ionized gas molecules. This tail is visible because the ionized gas molecules glow. The gas tail forms because gas molecules in the comet’s head are blown outward by the solar wind. The solar wind is the fast moving cloud of plasma which constantly shoots out of the Sun in all directions. The gas tail thus always points away from the sun.

The dust tail consists of dust particles, which are also blown outward from the comet’s head. This tail has a broader, more curved shape than the gas tail. The dust tail is visible because the dust particles shine in sunlight. Sunlight also forms the dust tail. The pressure of sunlight striking the dust particles literally blows them away from the head of the comet.

Tail of Halley's Comet