An
emission nebula is a
nebula formed of
ionized gases that emit light of various colors. The most common source of
ionization is high-energy
photons emitted from a nearby hot
star. Among the several different types of emission nebulae are
H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and
planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.