What part of a comet is the bright cloud of gas and dust that surrounds its nucleus?

The bright cloud of gas and dust that surrounds a comet's nucleus is known as the coma. This feature forms when a comet approaches the Sun, causing the frozen gases within the nucleus to vaporize. As these gases and dust are released, they create a surrounding envelope of material that can span thousands of kilometers. The coma is often illuminated by sunlight, giving it a bright appearance that distinguishes it from other parts of the comet.

The nucleus, on the other hand, is the solid, central part of the comet composed of ice, dust, and rocky material. The tail of a comet, which is also a significant characteristic, forms as solar wind and radiation pressure push the particles away from the coma, creating a streaming tail that always points away from the Sun. The term "core" generally refers to the solid, dense center of a celestial body, but it is not a specific term used in the context of comets in the way that coma is. Therefore, the coma is the appropriate term for the bright cloud encircling the nucleus of a comet.