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').