Polarization is an important phenomenon in
astronomy. The polarization of starlight was first observed by the
astronomers William Hiltner and John S. Hall in 1949. Subsequently,
Jesse Greenstein and Leverett Davis, Jr. developed theories allowing the use of polarization data to trace interstellar magnetic fields. Though the integrated
thermal radiation of
stars is not usually appreciably polarized at source, scattering by
interstellar dust can impose polarization on starlight over long distances. Net polarization at the source can occur if the
photosphere itself is asymmetric, due to limb polarization. Plane polarization of starlight generated at the star itself is observed for
Ap stars (peculiar A type stars).
[1] Both
circular and
linear polarization of light from the Sun has been measured. Circular polarization is mainly due to transmission and absorption effects in strongly magnetic regions of the Sun's surface. Another mechanism that gives rise to circular polarization is the so-called alignment-to-orientation mechanism. Continuum light is linearly polarized at different locations across the face of the Sun (limb polarization) though taken as a whole, this polarization cancels. Linear polarization in spectral lines is usually created by anisotropic scattering of photons on atoms and ions which can themselves be polarized by this interaction. The linearly polarized spectrum of the Sun is often called the
second solar spectrum. Atomic polarization can be modified in weak magnetic fields by the
Hanle effect. As a result, polarization of the scattered photons is also modified providing a diagnostics tool for understanding
stellar magnetic fields.