If you imagine a comet as a dirty iceberg in orbit around a sun, as it gets closer to the sun it makes sense that at some point the ice will melt. When it melts it becomes like water in a vacuume, it essentially boils off its higher pressure gases and this boiling action would likely cause ultra small droplets of water to spray all over the place. Once separated from the body of the comet they are going to drift away at a rate equal to the force which sprayed them (the boiling action) and at a similar rotation to the comets rotation. This is going to make the nucleus of the comet be at or near the center of the co-orbiting cloud of droplets, gasses and dust. At some point they (the water droplets) will be disbursed enough and be far enough away from the sun to freeze into ice crystals. At this point they are going to refract light just like the ice fog we see in Alaska on very cold nights when the weather conditions are right. Here in Alaska when this occurs, it doesn't matter whether you are flying above town, or walking on the ground, you see an effect similar to what you see when watching a comet. There appears to be a tail of light coming off the back side of street lights. The catch is there is no source of light aiming that direction. With a comet the tail is always going away from the bright side of the comet, but there is no source of light coming off the back side of the comet, and because of the shadow which is cast by the comet itself, there can be no way for sunlight to reflect off of matter which some claim is trailing off behind the comet. Thus the tail of a comet is just the refraction of light at just the right angle to make it apear as if there is a tail on the comet.

Dan