Question

When a Comet is near the sun, what happens to the nucleus

Answer 1

it becomes gas because when the comet nears the sun, the sun freezes the comet causing it to become gas.

Answer 2

When the comet gets closer to the sun, the comet starts getting warmer, the ice heats up and becomes a gas. The dusty gas is coming from the nucleus and it forms a hot cloud around the comet called the coma.