What is the heart of a comet called?

The heart of a comet is a mass of dust and frozen gas called the nucleus. Some comets have a rocky core, while others are made up mostly of frozen water and compounds like ammonia, carbon dioxide and methane.

As comets approach closer to the Sun, the frozen materials that make up the nucleus begin to boil off. As more of the nucleus is heated, particles and gases that have been released form a cloud known as the coma. Once comets have traveled further into the solar system, they begin to form a tail due to pressure from the solar wind.