Polarization in astronomy
Polarizationis an important phenomenon in astronomy. The polarization of starlightwas first observed by the astronomers William Hiltnerand John S. Hallin 1949. Subsequently, Jesse Greensteinand Leverett Davis, Jr.developed theories allowing the use of polarization data to trace interstellar magnetic fields. Although the thermal radiationof stars in not appreciably polarized at source, its scattering by interstellar dustimposes polarization on starlight over long distances. Circular polarization of light from the Sun has, however, been measured; it is due to transmission and absorption effects in strongly magnetic regions of the Sun's surface. Plane polarization of starlight generated at the star itself is also observed for Ap stars (peculiar A type stars) [http://www.edpsciences.org/articles/eas/abs/2003/04/eas0928/eas0928.html]
Polarization is also present in radiation from coherent astronomical sources (e.g. hydroxyl or methanol masers), and incoherent sources such as the large radio lobes in active galaxies, and
pulsarradio radiation (which may, it is speculated, sometimes be coherent). Apart from providing information on sources of radiation and scattering, polarization also probes the interstellar magnetic fieldin our Galaxy as well as in radio galaxies via Faraday rotation. In some cases it can be difficult to determine how much of the Faraday rotation is in the external source and how much is local to our own Galaxy, but in many cases it is possible to find another distant source nearby in the sky; thus by comparing the candidate source and the reference source, the results can be untangled.
The polarization of the cosmic microwave background is also being used to study the physics of the very early universe.
* [http://www.planck.fr/article263.html Discovery by Hiltner and Hall, analysis by Greenstein]
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