Comets are like "dirty snowballs" that have orbited the Sun since our Solar System formed, billions of years ago. A comet is really just a round chunk of ice and dust. The ball-like part of the comet is called the nucleus, and its long "tail" is called the coma. The coma forms as tiny bits of the nucleus break down and fall off. As solar winds sweep away this dust the whole comet appears to glow.

Image credits: main image, A Star Cluster Through Hale-Bopp's Tail, A. Dimai, R. Volcan, D. Ghirardo (Col Druscie Obs., ACC).