Atomic Spectroscopy and Astrophysics

Most of our knowledge of the universe has come from the study of spectral lines emitted by astronomical objects. The wavelengths of the lines identify unequivocally the elements that are present, their states of ionization and the velocities with which they are moving. Thus the shifts to lower energy of the spectral line wavelengths for distant galaxies—the redshifts—demonstrate that the universe is expanding (see the article by Ralph Alpher and Robert Herman on page 24). The intensities of spectral lines give us information on the densities, temperatures and abundances of the elements in the environment in which the lines are emitted or absorbed. However, to determine these physical quantities we also need to know atomic and molecular parameters such as transition probabilities and oscillator strengths, so it is no accident that much of laboratory spectroscopy has come to be known as laboratory astrophysics. Millions of spectral lines are useful as diagnostic probes, and the assembly of reliable tables of atomic and molecular data is an essential part of astronomical spectroscopy.