Explanation:

Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town.

These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets.
When a comet's orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets.
The sun's heat causes the comet's ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers.
The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the sun, sometimes forming a long, bright tail. Comets actually have two tails - a dust tail and an ion (gas) tail.