A change to the energy of an electron within an atom or a molecule in which the electron gains sufficient energy to escape. The electron goes from being bound to being free, and leaves behind an ion; hence this is another name for ionization. The energy for the change may come from a photon, resulting in the absorption bands known as ionization edges in stellar spectra, or from collisions with other atoms or particles (collisional ionization). If the energy comes from another excited electron within the atom, the process is known as autoionization.