magnetic starsStars that have detectable and often very large magnetic fields, up to a few tesla. Magnetic stars are found in spectral classes B to F, but most are peculiar A stars (Ap stars). Many of these stars show either periodic or irregular variations of the field together with a reversal of the polarity; others show irregular field variations but constant polarity. Magnetic variability is usually accompanied by very small changes in brightness. Magnetic stars are studied by measurements of the Zeeman splitting (see Zeeman effect) of spectral lines caused by a magnetic field; these give the average magnetic field over the entire disk. Periodic variations in the magnetic field could result from the magnetic axis of the star being tilted with respect to the rotational axis so that areas of different magnetic field strength are presented to the observer as the star rotates: this is the oblique rotator theory. Evidence for chromospheric activity and starspots on red dwarf stars and RS Canum Venaticorum binaries indicates that they have weaker magnetic fields, similar to the Sun's. A few white dwarfs have fields up to 105 tesla while some neutron stars detected as pulsars have fields of up to 109 tesla. See also magnetic cataclysmic variables; spectrum variables.
stars on whose surfaces there exist magnetic fields stronger than a few hundred gauss.
The magnetic fields of stars were first measured by the American astronomer H. W. Babcock in 1948 from the Zeeman splitting in the spectrum of a star. The strongest magnetic field measured thus far is for the star HD 215441 and is equal to 34,000 gauss. All known magnetic stars have an anomalous abundance of chemical elements in their atmospheres—a great excess of rare-earth elements (Eu, La) and an excess of elements of the iron group (Fe, Mn, Cr) and lighter elements (Si, Cl, P). On the basis of this characteristic, magnetic stars belong to the group of peculiar A-type stars.
The magnetic field intensity and the chemical composition of the atmospheres of magnetic stars, which is determined on the basis of the stars’ spectra, periodically vary. This is due to the rotation of the stars, which are characterized by a nonuniform distribution of the magnetic field and chemical composition on the surface.
On the Hertzsprung-Russell diagram, magnetic stars lie on the main sequence in the region of spectral classes from FO to B5 and constitute about 10 percent of all stars in these classes. The strong magnetic field of such stars may arise either during the star’s formation (compression of the partially ionized gas, which initially had a weak magnetic field, leads to intensification of the field) or by way of the hydromagnetic dynamic processes in a rotating star. The origin of the anomalies of chemical composition has not been determined.
REFERENCEEruptivnye zvezdy. Moscow, 1970. Chapter 7.
V. L. KHOKHLOVA