How is the glow observed in a nebula produced without internal nuclear fusion?

The glow is caused by ionization from nearby stars or reflected light.

Nebulae do not generate light in the same manner as stars; they do not possess a central fusion reactor to produce energy. Instead, their visible appearance is largely due to external factors. When young, hot stars form within or near a nebula, their intense radiation ionizes the surrounding gas, causing it to emit light. Alternatively, the dust and gas within the nebula can reflect the light of nearby, existing stars. This interplay between the radiation from stars and the diffuse gas of the nebula creates the colorful, vibrant structures seen in astronomical images, which are essentially the nebula reacting to the energy emitted by nearby stellar objects.