Emission and reflection nebulae differ primarily in their interaction with light. Emission nebulae glow due to the ionization of their gas by the high-energy radiation from nearby young stars, which excites the gas and causes it to emit light of various colors. In contrast, reflection nebulae do not emit their own light but rather reflect the light of nearby stars, making them appear bluish due to the scattering of light by dust.
To summarize, the key difference lies in: emission nebulae emit light due to ionization by young stars, while reflection nebulae reflect light from nearby stars.
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