How nebula expand or contraction?

Nebulae expand or contract due to the balance between gravitational forces and internal pressure. When a star forms within a nebula, nuclear fusion generates energy, creating outward pressure that can cause expansion. Conversely, when a star exhausts its fuel and collapses, gravitational forces can lead to the contraction of the surrounding nebula. Additionally, supernova explosions can compress nearby gas, triggering new star formation and altering the nebula's structure.