A huge, diffuse cloud of gas and dust in interstellar space. The gas is mostly hydrogen (H2).
Bright nebulae glow with light emitted by the gas of which they are composed (the emission nebulae) or by reflected starlight (the reflection nebulae) or both.
Dark nebulae consist of clouds of gas and dust that are not illuminated.
Protoplanetary nebulae are condensations of gas and dust in the process of forming a star and solar system
The archaic term Planetary nebulae refers to shells of gas ejected by exploding stars. Spiral nebulae was an Old Earth term for galaxies.