A cloud-like region of gas or dust in outer space: If a nebula is bright, it shines either by its own light (emission nebula) or by reflected light (reflections nebula), or it simply absorbs light falling onto it if it is a dark nebula.

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.

The cloud of expanding gas surrounding a star that has blown off its outer layers, possibly in a nova stage: A planetary nebula is a shell of gas thrown off by a star at the end of its life.

Planetary nebulae have nothing to do with planets. They were named by William Herschel, who thought their rounded shape resembled the disk of a planet.

After a star, such as the Sun, has expanded to become a red giant, its outer layers are ejected into space to form a planetary nebula, leaving the core as a white dwarf at the center.

A nebula that shines as the result of the scattering of the light of a star or group of stars nearby: A reflection nebula reflects starlight and looks blue in photographs.

Such scattering is usually caused by dust within the nebula.