Astronomical Polarimetry

Polarimetry is a powerful tool also in astronomy, where observations are limited by the extreme distance of most targets. The pioneering polarimetric measurements that Bernard Lyot began making in the 1920s ushered in the age of modern astronomical polarimetry. Although the benefits delivered by studying the polarization of astronomical phenomena were undeniable, polarimetry remained a niche and underutilized technique in astrophysical research. However, the last decade has seen a renaissance of polarimetry in astrophysics, with instruments from the gamma-ray to radio regimes coming online and making exciting discoveries. In this chapter, I highlight the most impactful instruments and observations of the last 10 years and attempt to identify some key themes that have emerged. This overview includes work from most areas of astrophysics and some findings from our Solar System. Beyond our scope is the field of heliophysics and most of cosmology, where the polarization of the cosmic microwave background is of critical importance and the focus of much research.