What distinguishes the visibility mechanism of an emission nebula from a reflection nebula?
Emission nebulae glow because the gas itself is excited to emit light, while reflection nebulae's visibility relies on external starlight scattering off surrounding dust particles.
The fundamental difference between these two types lies in the source of the observed light. An emission nebula generates its own light because the gas atoms are energized—ionized—by nearby massive stars, causing them to release photons upon recombination. In contrast, a reflection nebula does not generate its own light; its visibility is entirely dependent on the proximity and brightness of an external star whose light strikes the dust grains and is subsequently scattered toward the observer. The emission nebula is an active light source due to its internal atomic processes, whereas the reflection nebula is a passive reflector dependent on an external illuminator.
