If the temperature is raised, the average speed of the molecules is increased, and the lines are still broader. Thus, measurement of the width of certain spectrum lines gives an indication of the temperature of the source, such as the sun. In many cases, the interior of a source is at a higher temperature than the exterior. An emission spectrum of broad lines will then arise from the interior, and an absorption spectrum will be produced in the exterior; the exterior, however, being cooler, produces narrower lines, and the result for each line is a bright region with a dark centre. This phenomenon is called self-reversal.
Related to the Doppler effect is the Mössbauer effect, the discovery of which was announced in 1958 by the German physicist Rudolf Ludwig Mössbauer. In a Mössbauer-effect experiment, the recoil-free emission and absorption of gamma rays from one nucleus to another is measured. For absorption to occur, the energy spectrum of gamma rays from the emitter must nearly match the spectrum of possible energies of excitation in the absorber. The slightest change in the motion of the absorber relative to the emitter causes the apparent energy of gamma rays “seen” by the absorber to change. By moving the source or the absorber, scientists may sort out the energies of the gamma rays with high precision. This information is valuable in studies of the electronic and magnetic fields at the nuclei of a solid. The effect also provides an accurate picture of relative motion for use in applications such as the docking of space vehicles. High-resolution spectroscopy is employed in nuclear physics to study the influence of nuclear size and shape on outer atomic structure. Also, when a light source is placed in magnetic or electric fields, spectral lines are often split or widened, thus revealing important information about the atomic structure of the source, or about the fields, not otherwise available. The Dutch physicist Pieter Zeeman discovered in 1896 that, when a ray of light from a source placed in a magnetic field is examined spectroscopically, the spectral line is widened, or even doubled. This phenomenon was named Zeeman effect. The so-called Stark effect was named after the German physicist Johannes Stark, who succeeded in splitting spectral lines into several components with a strong electric field in 1913.