Polar (cataclysmic variable star)

A Polar (also AM Herculis Star) is a highly magnetic type of cataclysmic variable binary star system. Like other cataclysmic variables (CVs), polars contain two stars: an accreting white dwarf (WD), and a low-mass donor star (usually a red dwarf) which is transferring mass to the WD as a result of the WD's gravitational pull. (In technical terms, the donor star overfills its Roche lobe.) Polars are distinguished from other CVs by the presence of a very strong magnetic field in the WD. Typical magnetic field strengths of polar systems are 10 million to 80 million gauss (1000–8000 teslas). The WD in the polar AN Ursae Majoris has the strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss (23 kT). The presence of such a strong magnetic field gives rise to a number of fundamental differences between polars and non-magnetic CVs.