Why are ionization nebulae found near hot massive stars?

Ionization nebulae are found near hot massive stars because these stars emit intense ultraviolet radiation that ionizes the surrounding hydrogen gas. The high-energy photons from the stars strip electrons from hydrogen atoms, creating glowing regions of ionized gas. This process not only illuminates the nebulae, giving them their characteristic colors, but also contributes to star formation as the dense regions within the nebulae can collapse under gravity. Thus, the presence of hot massive stars is crucial for the formation and maintenance of ionization nebulae.