A nebula is an interstellar cloud of gas or dust. There are several fundamental types of nebulae.
- Reflection nebulae – These nebulae relfect the light of a nearby star or stars. The nebulosity around the Pleiades (M 45) is an example of a reflection nebula.
- Emission nebulae – These nebulae consist of gas that glow due to obsorbtion of energy from nearby stars. The Ring Nebula (M 57), a planetary nebula, is an example of an emission nebula.
- Dark nebulae – These nebulae are dense enough to block light from a source further away and as such are seen in silhouette. The Horsehead Nebula (Barnard 33) is a famous example of a dark nebula. It blocks the light of the emission nebula IC 434.