The space between stars is not a perfect vacuum. It contains gas molecules and dust grains, although they are few and far between by any terrestrial standard. In the presence of a magnetic field, many types of interstellar dust grains line up in a way that is reminiscent of iron filings near a bar magnet. When light from a star passes through a region of space with magnetically-aligned dust grains (though in this case the short axis of the dust grains aligns with the local magnetic field), light with the electric field vector perpendicular to the long axis of the grains is less likely to be absorbed by the grains than light whose electric field vector is parallel to the long axis of the grains. This causes the light passing through such regions of space to become slightly polarized, and the polarization of starlight is something we can measure easily here on Earth. In this way, the strength and orientation of invisible interstellar or circumstellar magnetic fields can be determined at a distance.
Various astrophysical processes result in polarized electromagnetic radiation. The differential absorption already mentioned polarizes the light from all stars to one degree or another. Only the Sun—which is vastly nearer—offers us almost completely unpolarized light. Scattering of light off of interstellar clouds and planetary surfaces also results in polarization. Finally, both synchrotron and cyclotron emission produce a characteristic polarization.
The polarization of starlight can be measured by the use of a polarimeter attached to the telescope. Unlike standard photometry, polarization is simpler to measure with ground-based telescopes because the measurements are relative rather than absolute and, under normal circumstances, the Earth’s atmosphere does not affect the polarization state of incoming light. Care must be taken, however, to ensure that the telescope itself does not create instrumental polarization due to oblique reflections. Placing the polarimeter at the unfolded Cassegrain focus is one desirable configuration (Hough 2006).
Hough, J. 2006, A&G, 47, 3.31