NEBULA

A nebula (pl. nebulae) is an interstellar cloud of gas and/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.

These nebulae are usually blue in color.

• 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.

These nebulae are usually red in color.

• 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.