An emission nebula is a type of nebula that emits its own light, due to the presence of ultraviolet radiation from very hot young stars inside. In Star Trek, the Cardassian Amleth system is lsupposed to be located within an emission nebula. In the Star Trek Voyager episide "Lifesigns", Tom Paris suggested that the USS Voyager travel through an emission nebula to save time. Cloaking devices are apparently useless inside emission nebulae.

In real astrophysics, an emission nebula is a nebula that displays an emission spectrum because of energy that has been absorbed from one or more hot, luminous stars and reemitted by the nebular gas at specific wavelengths. An emission nebula is a cloud of ionized gas (i.e. a plasma) emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby hot star.

This is different from a reflection nebula, where the light from the nebula is simply reflected light from the central star. The difference becomes clear from looking at the spectrum of the nebula and comparing it to that of the stars providing the initial energy. If it is a reflection nebula, then the spectra will match; if it is an emission nebula, it will show emission lines of its own.

The Crab Nebula, an emission nebula'

The Merope Reflection Nebula