Comets are temporary visitors of the inner regions of our solar system. Being a mixture of rocks, water and frozen gases, they usualy develop a bright ( greenish/blue)coma and a tail as soon as the surface warms up enough so the volatile elements start to evaporate. A comet usualy has two types of tails, one consisting of dust left behind in the trail of the comet and illuminated by sunlight, and the other one made up by ions and which always point straight away from the sun, following the solar wind. The ion trail has a bluish color and is most of the time to weak to be seen with the naked eyes and shows up only on photographies.
This photo of Comet C/2001 Q4(NEAT) had been taken one day before its closest approach to the sun. It shows a big coma, rather short dust trail and a longer but very faint ion trail.
Remarkable also is the high speed of this comet, visible on the star trails. The serie of exposures had been taken within 18 minutes.