A comet is a small, icy celestial body that orbits around the sun. Its long tail of gas and dust 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.
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. The coma is the roughly spherical blob of gas that surrounds the nucleus of a comet. The coma is comprised of water vapor, carbon dioxide gas, and other neutral gases that have sublimed from the solid nucleus.