Magnetic white dwarfs

For around 20% of white dwarfs, a magnetic field can be detected. These range from a few kiloGauss (few 0.1 Tesla) all the way up to 1 GigaGauss (100,000 Tesla). How these fields are formed is still a matter of debate with suggestions of Fossil-fields from the main-sequence, dynamo generation during the giant star phases, dynamo generation in the disks of accreting white dwarfs, or though the process of interior crystallisation (or maybe some combination of several of these). Magnetic fields can detected either through circular polarisation of light from the white dwarf, or (more commonly these days), via the Zeeman effect that typically causes spectral lines to be split into three closely spaced lines, where the degree of the splitting corresponds to the field strength. The most magnetic white dwarfs known, have fields that cannot be generated in an Earth-based lab and so provide a unique way to learn about how atoms behave when subjected to extreme magnetism.