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.

Number of Comets: 750 identified, about 10 trillion others
Where: in the Oort Cloud
Types: Short-period and long-period comets
Typical size: 6 miles wide
Composition: lumps of ice, dust, and rock
Significance: give astronomers clues to how the solar system formed

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