AM Herculis is the class prototype of the polars -- cataclysmic variables whose white dwarf primaries have strong magnetic fields. Cataclysmic variables are binary stars in which the primary is a white dwarf star that accretes matter from the secondary star in the binary pair. Matter falls off of the secondary and spirals down the gravitational potential well of the white dwarf, releasing light and heat as it does so. Most CVs form accretion disks around the white dwarf in which the material spirals onto the white dwarf rather than falling directly onto it. But in the polars, the white dwarf's magnetic field is so strong that it overcomes the angular momentum of the falling material, and the accreting matter falls onto the white dwarf along the magnetic field lines rather than along an elliptical orbit. Because of this, all of the matter is channeled onto the white dwarf's magnetic poles, and the impacting matter releases tremendous amounts of energy, including X-rays.
AM Herculis varies irregularly between its quiescent an active states. Its light curve is quite well sampled, and has been since its observation was first encouraged by the AAVSO in the 1970s.
Prepared By: Matthew Templeton