A nebula is an interstellar cloud of gas and dust. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebulae are powered by young, massive stars and emit their own light, reflection nebulae shine by reflecting light from nearby massive stars, and dark nebulae, as the name suggests, are dark and can only be seen when silhouetted against a bright background.

The Orion nebula is an emission nebula. Credit: AAO/David Malin

The Pleiades is a reflection nebula. Credit: AAO/ROE/David Malin

Barnard 68 is a dark nebula. Credit: ESO

Nebulae can also result from the end stages of stellar evolution. In this case they are present as either a planetary nebula or a supernova remnant depending on the mass of the dying star.