How do reflection nebulae differ from emission nebulae?

The primary difference between reflection and emission nebulae lies in how they produce the light that makes them visible to observers.

• Reflection Nebulae: These are clouds of interstellar dust that do not emit their own light. Instead, they become visible by reflecting the light of a nearby star. The stars illuminating these nebulae are typically not hot enough to ionize the surrounding gas. Because they rely on reflection, their spectra mimic the absorption lines of the stars that illuminate them.12
• Emission Nebulae: These are clouds of gas that emit their own light. This occurs when ultraviolet radiation from a nearby, very hot star (typically exceeding 20,000 K) ionizes the hydrogen gas. When the resulting free electrons and nuclei recombine, the atoms drop to lower energy levels and release visible light. Consequently, these nebulae display their own characteristic emission lines.132

In short, while reflection nebulae act as cosmic mirrors for nearby starlight, emission nebulae are energized by high-temperature stars to produce their own glow.132