What is the structure of a comet? What does its core consist of?

The main part of the comet is the solid core. Its diameter is usually from 1 to 10 km. The core consists of ice, frozen gases and solid particles of some other substances.
As the comet approaches the Sun, the core heats up, and its substances begin to evaporate. A gas shell forms around the core, and then a long tail appears. The tail of a comet can stretch for millions of kilometers! It is always directed away from the Sun and consists of gases and fine dust. When a comet moves away from the Sun, its tail and gas shell gradually disappear.

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