Ionization is the process whereby an atom receives
enough energy input (either via a collsion with
another atom or by absorbing a photon) that the
electron is stripped from the nucleus.  That
electron can later recombine with the nucleus
and as it falls to the ground state, photons at
discrete energies are emitted.  These are called
emission lines.  Some of the images in these
gallery are narrowband images that focus on the
emission lines of oxygen as they are formed in
this recombination process.