Of all objects that lighten the night sky, comets are the rarest. They are glowing spheres with long streaming tails of light. They actually consist of only dust and frozen gas. There may be thousands and thousands of them that orbit the sun. As a comet comes near the sun, only the bright head is visible at first. As it comes closer, the comet develops a tail which may be even million miles long. Dust and gas constitute the tail.
But where do they come from? It is quite uncertain. There is a theory that suggests comets are actually the leftovers from the formation of our solar system. It is also said that billions of comets are circling at the outer reaches of the system. Accidently they sometimes get diverted from their orbit and start to move in towards the sun.
The orbits of visible comets are very long eclipses. Some of them swing out beyond the farthest planet, and then travel back to sun, coming even closer to sun than mercury.
There are comets that return again and again on a predictable schedule. Others are seen once or twice than vanish forever. They may remain visible for weeks or months at a time. After a comet passes away, the bits of debris that it leaves behind causes what are called shooting stars or meteors. Unlike comets, they streak across the sky vanishing within moments.