HR DIAGRAM

A comet is a small, icy celestial body that orbits around the sun. It is made up of a nucleus, a gaseous coma and a long tail. The tail develops when the comet is near the Sun. Its long ion tail of always points away from the sun, because of the force of the solar wind. The tail can be up to 250 million km long, and is most of what we see. Comets are only visible when they're near the sun in their highly eccentric orbits.There are five parts to a comet1. Nucleus: The nucleus is the frozen center of a comet's head. It is composed of ice, gas, and dust. The nucleus contains most of the comet's mass but is very small2. Coma: The coma is the roughly spherical blob of gas that surrounds the nucleus of a comet; it is about a million km across. The coma is comprised of water vapor, carbon dioxide gas, ammonia, dust, and neutral gases that have sublimed from the solid nucleus. The coma and the nucleus form the head of a comet.3. Ion Tail: A tail of charged gases always faces away from the sun because the solar wind pushes it away. When the comet is approaching the Sun, the ion tail trails the comet: when the comet is leaving of the Sun, the ion tail leads. The tail fades as the comet moves far from the Sun. The ion tail can be well over 100 million km long.4. Dust Tail: The dust tail is a long, wide tailcomposed of microscopic dust particles that are buffeted by photons emitted from the Sun; this tail curves slightly due to the comet's motion. The tail fades as the comet moves far from the Sun.5. Hydrogen Envelope: Hydrogen gas surrounds the coma of the comet and trails along for millions of miles. The hydrogen envelope is about 10 million km across at the nucleus of the comet and about 100 million km long. It is bigger when the comet is near the Sun.Comets orbit the Sun in highly elliptical orbits. Their velocity increases greatly when they are near the Sun and slows down at the far reaches of the orbit. Some comets crash into the Sun or get so close that they burn up; these comets are called sungrazers. The solar wind pushes the tail away from the Sun.