Most celestial bodies emit light which is unpolarized. The few exceptions include certain white dwarf and peculiar A stars, sources of synchrotron radiation such as the Crab Nebula, and reflection nebulae. Nevertheless the light from many ordinary stars contains a measurable degree of linear polarization. The amount of polarization is greatest for stars which are the most reddened, so it is a natural conclusion that the polarization arises during the pas¬sage of the starlight through the interstellar medium. The measured polarization is very approximately one per cent for every magnitude of visual extinction (Av). Consequently very few bright stars have polarizations of more than a few per cent. The amount of polarization varies with wavelength and has a maximum value near X = 0.55 ?m.

Several thousand stars have had their polarizations measured. These stars are plotted in galactic coordinates. It may be seen that there is a tendency for stars close to each other to be polarized in the same direction, and for stars at low galactic latitudes to be polarized parallel to the galactic plane. The reason for these patterns is probably the interstellar magnetic field. The field acts on those grains which have an elongated rather than a spherical shape, and tends to make them all line up in the same direction. It is this alignment of dust grains that is the cause of interstellar polarization.

Circular polarization has also been measured hi a few stars. This is a much smaller effect than linear polarization, and is therefore much more difficult to measure. The largest known value of inter¬stellar circular polarization is 0.06 per cent in the star VI Cygni 12.

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