Polarization is an important phenomenon in
astronomy.
== Stars ==
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]
== Sun ==
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.
== Other sources ==
Polarization is also present in radiation
from coherent astronomical sources due to
the Zeeman effect (e.g. hydroxyl or methanol
masers).
The large radio lobes in active galaxies,
and pulsar radio radiation (which may, it
is speculated, sometimes be coherent) also
show polarization.
Apart from providing information on sources
of radiation and scattering, polarization
also probes the interstellar magnetic field
in 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.
== Cosmic microwave background ==
The polarization of the cosmic microwave background
(CMB) is also being used to study the physics
of the very early universe.
CMB exhibits 2 components of polarization:
B-mode (divergence-free like magnetic field)
and E-mode (curl-free gradient-only like electric
field) polarization.
The BICEP2 telescope located at the South
Pole helped in the detection of B-mode polarization
in the CMB.
The polarization modes of the CMB may provide
more information about the influence of Gravitational
Waves on the development of the early universe.
It has been suggested that astronomical sources
of polarised light caused the chirality found
in biological molecules on Earth
