The spectrum produced by a particular ion depends on properties of the plasma such as the temperature, density and ionization state, as well as properties of the ion, such as the energy level structure. The energy level structure, determined from quantum mechanics, is specific to each ion and the electrons orbiting the positive nucleus are confined to these specific energies. Electrons gain or lose energy as they jump between the levels, absorbing or emitting a photon at that energy difference. Thus each ion presents a unique pattern of spectral lines that we can observe in astrophysical spectra.

Large atomic and molecular databases containing information on millions of spectral features are used by computer codes to calculate the atomic processes that determine the ionization state, energy level excitation, and spectral properties. The computed spectra are then compared with observations to determine the temperature, density and elemental abundances. Astronomers in the SSP division contribute to the compilation, evaluation, calculation of atomic data needed for astrophysics, as well as the spectral modeling codes that produce the synthetic spectra. This work is necessary to make optimal use of astronomical data: even in the solar spectrum, we may be missing half the spectral lines.