Nebulae are interstellar clouds of gas and dust. The word nebula is derived from the Latin word for mist or cloud, and historically has been used to refer to almost any diffuse astronomical object. In 1781 French astronomer Charles Messier (1730-1817) published a catalog of 103 nebular objects. Of these, only 11 were true nebulae, the rest were ultimately discovered to be distant galaxies.

There are three general types of nebula: dark, reflection, and emission nebulae, of which the latter may be subdivided into H II regions, planetary nebulae, and supernova remnants. Emission nebulae are also known as gaseous nebulae. The main difference between these are the manner in which they are illuminated, and whether gas or dust is more prominent.

Emission nebulae are visible as a result of light emitted by atoms and ions which have been excited by nearby hot stars. Electrons bound to hydrogen atoms are ejected by...