Even with the unaided eyes, faint hazy patches of light can be seen dotted around the night sky.

Through a telescope these patches are resolved into various kinds of diffuse nebulae, which are clouds of dust and gas that can be sen in the visible part of the spectrum.

Nebulae are classified in three basic types:

1. Emission nebulae; ultraviolet radiation from nearby bright blue stars excites hydrogen atoms in the gas of an emission nebula.
2. Dark nebulae; an absorption, or dark, nebula is seen as a dark patch, sometimes surrounded by a halo of light. The light from stars behind the nebula is either absorbed or scattered by the nebular material.
3. Reflection nebulae; the dust particles in the cloud reflect and scatter the light from stars that are not hot enough to make the nebula itself emit light.

A diffuse or dispersed cloud of interstellar dust or gas or both that is seen in silhouette as it absorbs light from behind: An absorption nebula is also called a "dark nebula".

An immense cloud in interstellar space which, as a result of ionized gas within it, shines by its own light: An emission nebula is a gas cloud that receives energy from a hot star, allowing it to give off radiation in emission lines, such as those of hydrogen.

The characteristic reddish radiation of many emission nebulae is mostly from the hydrogen-alpha line.