The tail is usually made up of a pair of tails, one of gas and one of dust. Early Astronomers thought comets were big balls of gas. Newton explained that a comet's tail was caused by the sun. The tail is nothing but a fire vapour which comes from the head of the comet by the suns heat.
Scientists think comets are made up from materials like magnesium, aluminum and silicon.
People think there is rock and other material left over when a solar system formed about five billion years ago. Then the solar system was nothing but a big spinning cloud of dust and gas. The material in the middle of the cloud made a sun, further out the planets and there moons formed. The comets cloud lies beyond the planets, between Earth and the nearest stars. It was a vast globe of waste matter.