Astropolarimetry

a branch of practical astrophysics that is concerned with the application of polarimetry to radiation coming from celestial objects. Polarization measurements are accomplished by visual, photographic, and elec-trophotometric means after the radiation being studied passes through the analyzer, a birefringent crystal, or a Polaroid sheet polarizer. The visual method, because of its high resolution, is successfully employed in the investigation of polarization in different regions of the image of planets or comets by means of the polarimeters of Savart, Lyot, and others. The photographic method is used for measurements of polarization at individual points of the solar corona, galactic nebulae, and galaxies in which the light flux is too faint. The electrophotometric method is used chiefly for measurements of the polarization of starlight. In the photographic method one obtains pictures of the object at three angular positions of the analyzer and subsequently measures the density of the photographic image. In the electrophotometric method one measures the variations of light flux during the rapid rotation of the analyzer. The accuracy of the measurements of polarization of the light of bright objects reaches hundredths, and for dimmer objects, tenths of a percent.

Except for the light of the solar corona and certain nebulae, polarization of the light of celestial objects is slight and reaches only a few percent or fractions of a percent of the general radiation from the object under study. The polarization of light in gaseous nebulae indicates the nonthermal nature of the radiation (for example, in the Crab nebula, it is the Bremstrahlung of relativistic electrons), and in dust nebulae it indicates the scattering of light by dust particles. In the planets and the moon the polarization of light of individual formations permits one to draw conclusions concerning the nature of the surface of a planet and the presence of scattering particles in its atmosphere. The polarization of light of the solar corona is caused basically by the scattering of the sun’s light by free electrons. The polarization of the light of stars originates along the path of propagation of light waves from a star to the observer as a result of light scattering by nonspherical dust particles oriented by interstellar magnetic fields of the galaxy, which are homogeneous on a sufficiently large scale. The magnetic fields in the sun and stars are also detected and measured by polarimetric analysis of spectral lines on the theoretical basis of the Zeeman effect. The polarization of starlight can occur also in the vast stellar atmospheres and can vary over time.