Polarization in astronomy
Encyclopedia
Light Polarization is an important phenomenon in astronomy
. The polarization of starlight was first observed by the astronomer
s 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 star
s 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).http://www.edpsciences.org/articles/eas/abs/2003/04/eas0928/eas0928.html
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 field
s.
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 pulsar
radio 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 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.
The polarization of the cosmic microwave background
is also being used to study the physics of the very early universe.
It has been suggested that astronomical sources of polarised light caused the chirality
found in biological molecules on Earth.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
. The polarization of starlight was first observed by the astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s 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
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s 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
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
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).http://www.edpsciences.org/articles/eas/abs/2003/04/eas0928/eas0928.html
Both circular
Circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....
and linear polarization
Linear polarization
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation...
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
Hanle effect
The Hanle effect is a reduction in the polarisaton of light when the atoms emitting the light are subject to a magnetic field in a particular direction, and when they have themselves been excited by polarised light....
. As a result, polarization of the scattered photons is also modified providing a diagnostics tool for understanding stellar magnetic field
Stellar magnetic field
A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively...
s.
Polarization is also present in radiation from coherent astronomical sources (e.g. hydroxyl or methanol masers
Astrophysical maser
An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum...
), and incoherent sources such as the large radio lobes in active galaxies, and pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
radio 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 field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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.
The polarization of the cosmic microwave background
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
is also being used to study the physics of the very early universe.
It has been suggested that astronomical sources of polarised light caused the chirality
Chirality
Chirality is a property of asymmetry important in several branches of science. It may refer to:* Chirality , a property of molecules having a non-superimposable mirror image...
found in biological molecules on Earth.