In pyrotechnics a comet is a block of pyrotechnic composition. They are very similar to stars but are usually much larger, however there is no official size where a comet is called a comet rather than a star. Comets are sometimes donut-shaped to increase surface area and decrease burn time. They can either be fired alone or as rising tails, which are comets that are attached to the outside of an aerial shell. They burn and emit sparks as the shell is rising, leaving a trail in the sky, hence the name. Comets are usually manufactured using the same method as pumped stars.
The best compositions to use are long tailed glitter, charcoal streamer, or flitter, made from a potassium nitrate based, easily ignitable composition. Attaching a flat based pumped star to a round hemisphere is not an easy task. However, a good way to overcome this problem is, instead of pumping the star on a flat surface, pump it directly on either the finished shell or a spare shell hemisphere. This will concave the base of the comet to match the exact shape of the aerial shell. Please note that a comet that has been pumped on a 2" aerial shell will not fit snug on a 4" aerial shell etc.