What causes an emission nebula?
An Emission nebula is a nebula that emits light.
An emission nebula is the stellar remnant consisting of ionized gases and a star typically a white dwarf in the middle. Think of it like a paper lantern where the colored paper is the ionized gas and the candle is the star. The high energy photons ionize the gases making them emit light in most parts of the electromagnetic spectrum.
An example of one is the Cats eye nebula, but it's really a planetary nebula. Which is basically looks like a shell of expanding gas emimating (originating) from the white dwarf at the center. This is produced by a Red Giant in its dying phase.