Polar (cataclysmic variable)

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A Polar (also AM Herculis Star) is a type of cataclysmic variable binary star system with a very strong magnetic field.

In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star in the form of an accretion disk. In polar systems, the magnetic field of the white dwarf is too strong for an accretion disk to form. Infalling gas will form an accretion stream, following the dipole magnetic field lines of the white dwarf.

Typical magnetic field strengths of polar systems are 10 million to 80 million gauss (1000–8000 teslas). The star AR UMa has the strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss (23 kT).

Polars derive their name from the linearly and circularly polarized light that they produce. Polarization is important to the study of polars, and information about the geometry of the binary star system can be found by studying the polarization.