Polars (pronounced "pole-ars", like pulsars) are white dwarf stars in accreting binary star systems that emit a large amount of polarised light, where the polarization is caused by their strong magnetic fields. The magnetic fields are also responsible for their dynamical behavior; they have such strong magnetic fields (from 10 to 230 megagauss) that the rotation rate of the white dwarf is tidally locked to the orbital period of the binary. The true polars, also known as AM Herculis stars, accrete matter from the secondary star overfilling its Roche lobe. But instead of forming an accretion disk, matter flows from the secondary along the magnetic field lines of the white dwarf, directly onto the magnetic poles of the white dwarf. From there, the matter spreads out over the surface of the star, increasing the mass of the white dwarf over time.

As a class, the polars all have very short orbital periods -- most below two hours, and all below four hours. In these systems, the "magnetic radius" rμ, where the magnetic force equals the ram pressure of infalling gas, is a significant fraction of the binary separation. So what happens is that gas flowing off of the secondary star flows into a circular orbit around the white dwarf, but at a few hundred thousand kilometers from the white dwarf, it gets channeled into these magnetic streams.

When matter falls onto the white dwarf, it really falls hard. White dwarfs have the entire mass of a star packed into a sphere a few thousand miles across, so the force of gravity at the surface is a thousand times that on the surface of the Sun, and hundreds of thousands of times stronger than the force of gravity on Earth. So when matter falls along the magnetic field lines, it is moving very quickly. The matter falls in blobs, rather than a continuous flow; the little blobs fall apart and slow down in the upper atmosphere of the white dwarf where they emit lots of X-rays and optical light. The bigger blobs actually make it to the surface where they impact moving at about 1 percent of the speed of light. The gas releases all of this kinetic energy as heat, and emits blackbody radiation at a temperature of several hundred million kelvins. So the spectra of the AM Herculis stars are chock full of interesting features that change drastically over just a few minutes. With all this activity, the polars are quite variable stars, and their brightnesses can change by a few magnitudes over the course of the orbital period.

Although the polars don't necessarily have outbursts like the dwarf, recurrent, or classical novae, they may eventually undergo a nova explosion on the surface of the star, or explode as type I supernova if they accrete enough mass to surpass the Chandrasekhar limit. No one really knows if any of the known polars are close to doing this, however, so don't hold your breath -- it could take millions of years.

The polars have poor cousins called intermediate polars, or DQ Herculis stars, which (probably) have weaker magnetic fields (5 megagauss or less). These stars also funnel matter down onto the white dwarf via the magnetic field lines, but the infalling matter has enough angular momentum to form a proper accretion disk within the system. The magnetic field then sucks matter from the inner edge of the accretion disk down to the star. So in the intermediate polars, you can have a bright hotspot where matter falls from the companion star onto the disk, but you don't have a boundary layer in the inner disk to generate heat and light. The intermediate polars are asynchronous rotators -- the white dwarf rotates faster than the binary star orbit so they aren't tidally locked. Again, this is probably because the magnetic field is weaker, so the interaction of the white dwarf's field with that of the secondary star doesn't produce strong torque.

There are a few dozen known polars in the Milky Way. Many of these were found by X-ray observatories like the ROSAT satellite, mainly because of the strong X-ray emission they can give out. However, many were known from optical observations simply because they were so prominently variable, and the strong magnetic fields were easily detected with polarimetry. AM Herculis, the class prototype, can be observed with a moderate-sized backyard telescope (8-12 inches/20-30 cm). It is a magnitude 12.5 object, located in the constellation Hercules (α 18h 16m 13.4s, δ +49° 52' 3.1').