How do emission and reflection nebulae differ?

Emission and reflection nebulae differ primarily in their interactions with light, their appearance, and the presence of young stars. Here are the key differences:

1. **Interactions with Light**:
- **Emission Nebulae**: These nebulae emit their own light. They are typically composed of hot gas that is ionized by ultraviolet radiation from nearby young, hot stars. The ionized gas emits light at various wavelengths, giving these nebulae their characteristic colors, such as red due to hydrogen emissions.
- **Reflection Nebulae**: These do not emit their own light but instead reflect light from nearby stars. The light is usually from stars that are not hot enough to ionize the gas, so the nebula appears blue (due to Rayleigh scattering, similar to how Earth's sky is blue).

2. **Hard Line Edges**:
- **Emission Nebulae** might have more diffuse borders as they are made up of glowing ionized gas, while **Reflection Nebulae** may appear to have sharper edges as they reflect starlight.

3. **The Presence of Young Stars**:
- **Emission Nebulae** are often associated with regions of active star formation. They typically contain young, hot stars that contribute to the ionization of the gas.
- **Reflection Nebulae** may or may not have young stars within them, but the light that they reflect is from nearby stars.

4. **Contains Remnants of Stars**:
- Neither type of nebula is specifically a remnant of stars; however, **emission nebulae** can sometimes occur in the remnants of supernova explosions, while **reflection nebulae** are usually found in regions where young stars are forming.

In summary, the key differentiator lies in how they interact with light: emission nebulae emit their own light due to ionization, while reflection nebulae reflect light from nearby stars.