Scientists think a comet is probably made of a great swarm of bits of solid matter, combined with gases. Its first appearance is seen as a tiny point of light, though it may be thousands of miles in diameter. This point of light is “the head,” or nucleus, of the comet. Where this matter originally came from is what is still a mystery. As the comet approaches the sun, a tail usually appears behind it. The tail consists of very thin gases and fine particles of matter that are shot off from the comet’s nucleus when it comes under the influence of the sun. Surrounding the nucleus of the comet is a third portion, known as “coma.” It is a glowing cloud of matter that sometimes reaches a diameter of 150,000 miles, or even more. Comet tails are very different in shape and size. Some are short and stubby. Others are long and slender. They are usually atleast 5,000,000 miles in length. Sometimes they are almost 100,000,000 miles long.