Comets- Comets are the objects which move in highly elongated orbits around the sun. Their central part is solid while outer part is made up of frozen gases like ammonia, Methane, water vapours etc. However it is not a star. Comets are heavenly bodies which revolve around the sun in highly elliptical path. Usually comets come no closer than a few million km to the sun. Their mass is very less. They many be even millionth part of the mass of earth. So far only comets could be seen. It is believed that they may be more than Arab in number. Most of the comets revolve much beyond the orbit of its outer most planet. Thus, generally, they are not visible from the earth. Normally when they are away from the sun, they are neither visible nor they come close to the sun, the frozen gases vapourize and start glowing. As the distance from the sun reduces, the vaporisation increases and hence the length of the tail increases. Due to the presence of this tail, it is also called a "Puchchal Tara". We can see comets when they are closer to the sun because in this position they are closet to the earth too.