Polarization in astronomy

Polarization is a vital tool in the field of astronomy. It refers to how electromagnetic radiation, such as light or microwaves, is organized. Scientists use polarization to detect many different astronomical phenomena. It can reveal the presence of interstellar dust in deep space. It can also show the location and strength of magnetic fields. By studying these patterns, researchers can learn about the physics of the early universe.

Polarization occurs through several distinct physical mechanisms. One common method is scattering by interstellar dust. As starlight travels long distances, this dust can impose polarization on the light. Another method is the Zeeman effect. This effect causes polarization in radiation from coherent astronomical sources, such as methanol masers. In the Sun, circular polarization happens because of transmission and absorption effects. These effects occur in regions with very strong magnetic fields. There is also an "alignment-to-orientation mechanism" that creates circular polarization.

Astronomers study different types of polarization depending on the source. In stars, net polarization can happen at the source itself. This occurs if the photosphere, or the star's outer layer, is asymmetric. This specific type of light is known as limb polarization. Some peculiar A-type stars, called Ap stars, show plane polarization from the star itself. The Sun provides a complex example of these variations. The Sun shows both circular and linear polarization. The linearly polarized spectrum of the Sun is often called the second solar spectrum.

History shows how our understanding of these patterns has grown. In 1949, astronomers William Hiltner and John S. Hall first observed the polarization of starlight. This was a major step for the field. Later, Jesse Greenstein and Leverett Davis, Jr. developed important theories. Their work allowed scientists to use polarization data to trace interstellar magnetic fields. These discoveries turned light patterns into a map for the galaxy.

Scientists use specific effects to measure magnetic fields with precision. The Hanle effect modifies atomic polarization in weak magnetic fields. This modification changes the polarization of scattered photons. This provides a diagnostic tool to understand stellar magnetic fields. Another tool is Faraday rotation. This process probes magnetic fields in our galaxy and in distant radio galaxies. If it is hard to tell where rotation happens, scientists use a reference source. They compare a candidate source to a nearby distant source to untangle the results.

Polarization also exists in very large or very energetic objects. Large radio lobes in active galaxies show polarization. Pulsar radio radiation also exhibits these patterns. Even quasars show this behavior, such as the quasar 3C 286. This specific quasar has been measured with ALMA.

One of the most important areas of study is the cosmic microwave background, or CMB. This is the microwave energy from the primordial universe. The CMB has two components of polarization. These are called E-mode and B-mode polarization. E-mode is a curl-free gradient, while B-mode is divergence-free like a magnetic field. The BICEP2 telescope at the South Pole once claimed to detect B-mode polarization. However, that specific result was later retracted. Studying these modes might reveal the influence of gravitational waves on the early universe. Some scientists even suggest these light sources caused the chirality found in biological molecules on Earth.