Also known as Zeeman splitting, the broadening or splitting of a spectral line into several polarized components when the source is in a strong magnetic field; it is named after the Dutch physicist Pieter Zeeman (1865-1943). Because the amount of Zeeman splitting and of polarization depend on the magnetic field strength, this effect provides a power tool for investigating cosmic magnetic fields. It was first used to map magnetic fields on the Sun, especially in and around sunspots where the local field strength can be as high as 0.4 tesla – thousands of times stronger than Earth's magnetic field. Across the Sun as a whole, and in the case of most other stars and cosmic objects, the field strengths are much lower and the spectral lines are broadened rather than actually split by the Zeeman effect. However, weak fields can still be studied by measuring the polarization of the line wings – the parts of a spectral line well away from the central peak. Zeeman splitting is observed in the case of some magnetic variables which are associated with intense magnetic fields.
