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comet 
Small icy body that partially evaporates as it approaches the Sun; made up of a head with a solid core and tails composed of gas and dust.
coma
Cloud of gas and dust particles emitted by the expulsion of gas from the nucleus when a comet approaches the Sun.
nucleus
Central part of the comet; composed mainly of ice and rocky matter.
head
Part made up of the nucleus and the coma.
dust tail
Visible tail formed by dust particles pushed out of the coma by pressure from the Sun’s rays; can reach over 10 million kilometers in length.
ion tail
Almost invisible tail formed by the gas of the coma pushed back by the solar wind; can reach several hundreds of millions of kilometers in length.
