Emission Nebula (thing)

This is the visible photon emission of ionized protons and electrons recombining into hydrogen. The emitted light is generally red in colour.

The large clouds of ionized gas that form these nebulae are created by electron-stripping UV rays from a nearby hot star.

For more of this kind of fun, check out APOD.

This is not to be confused with en Reflection Nebula, which is a cloud of dust.