They presented their conclusions at the American Astronomical Society meeting in Toronto on January 13.
Most stars emit unpolarized light, containing equal amounts of the two polarizations.
However, the light reaching us from stars behind a dust cloud becomes linearly polarized after passing through such a cloud. The cloud of aligned dust grains acts like a giant polaroid filter, preferentially dimming one polarization more than the other. For nearly 50 years, astrophysicists have struggled to understand what force could be responsible for the observed alignment of these tiny particles. Because the particles appeared to be aligned with their long axes perpendicular to the interstellar magnetic field, magnetic effects were often considered, but careful studies always found the magnetic forces to be too weak to accomplish the observed alignment. The dust particles in interstellar space are illuminated by light from the many stars in the galaxy.