Emission and reflection nebulae primarily differ in the presence of young stars.
Emission nebulae are regions where gas is ionized by the high-energy radiation from young, hot stars, causing them to emit their own light. In contrast, reflection nebulae do not emit their own light but instead reflect the light of nearby stars. These differences often correlate with the presence of young stars in emission nebulae while reflection nebulae may contain older stars whose light is simply scattered.