Emission nebulae (also called "H II regions" ) are clouds of
gas in which Hydrogen atoms (at least) are ionized by ultraviolet radiation from nearby stars. Only very hot stars (T > 30,000 K) produce
enough UV radiation to ionize a significant number of Hydrogen atoms. Any excess energy beyond that needed to ionize H goes into kinetic
energy for the electrons ejected from the H atoms. This energy is distributed to other atoms in the gas by collisions. As the ions
de-excite to lower energy levels they emit their characteristic spectral lines. The most prominent one in the spectrum above is the red line
of H which gives emission nebulae their red color. Other lines, particularly from He (red & yellow lines) and ionized O (green lines), also
appear if the exciting stars are extremely hot.
Spectrum of Emission Nebula (Eta Carina)