What is a comet?

Comet is a kind of star in astronomy. Comet is a celestial body orbiting the sun when it enters the solar system, and its brightness and shape will change with the distance from the sun. It has a unique appearance like a cloud. Comets are divided into three parts: nucleus, coma and tail. The nucleus of a comet is composed of ice. When the comet approaches the star, the comet material sublimates, forming a hazy coma and a tail composed of thin material flow around the ice core. Due to the pressure of the solar wind, the tail always points away from the sun, forming a long tail. The tail of a comet is generally tens of millions of kilometers long, and the longest can reach hundreds of millions of kilometers. Comets are shaped like brooms, so they are usually called broom stars. The orbits of comets are mostly parabolic or hyperbolic, and a few are elliptical. At present, more than 1700 comets have been found orbiting the sun. The famous Halley's Comet orbits the sun for 76 years.