The Zeeman effect in astrophysics

George E. Hale discovered the Zeeman effect in sunspot spectra in 1908, and derived the laws of sunspot polarity. Recent observations of the much weaker (1 to 50 gauss) magnetic fields of the solar photosphere, made with the solar magnetograph, are described. The bearing of these results on circulation in the sun and their consequences for the topology of the sun's magnetic field are mentioned, with reference to the solar cycle and to reversal of the sun's polar field.The problem of measuring the magnetic field of stars is outlined, with reference to instrumental requirements. Spectrographs and crystals are described, together with techniques of measurement of magnetic fields in sharp-line stellar spectra. Results of a systematic program of observations with the 100-inch and 200-inch telescopes are reviewed. All stellar magnetic fields are variable; some are periodic. The strongest yet measured is 34 000 gauss. The magnetic variations are usually related to variations in light, color, line width, and line intensity. Chemical abundance anomalies are common in magnetic stars, and this has led to much work on nuclear reactions in stellar atmospheres, but the phenomena have not been satisfactorily explained. The possible importance of element concentration through paramagnetic migration in strong magnetic field gradients, enhanced by optical pumping, is pointed out.