interstellar polarizationThe partial polarization of starlight caused by dust in the interstellar medium. Since only nonspherical particles with a nonrandom orientation can polarize light, the dust grains must be elongated and are thought to be partly aligned in a weak galactic magnetic field.
Interstellar polarization may be due to reflection, absorption or emission. Reflection polarization is independent of grain alignment and is perpendicular to the line of sight from the reflecting dust to the light source; it is often used to determine the position of a hidden infrared source illuminating the dust. Emission and absorption polarization do depend on the grain alignment, which can be achieved with a very weak magnetic field. Since the grains tend to align perpendicular to the field, observations of dust polarization at infrared, submillimeter, and millimeter wavelengths can be used to probe the magnetic-field direction in increasingly dense regions of the dust clouds. Observations of absorption polarization require a bright (unpolarized) background source (usually a star), and hence are most commonly performed at IR wavelengths, and only at a few positions across the face of the cloud; emission polarization can be observed from the entire extent of the dust cloud.