The detection of starspots using polarisation measurements

This animation explains how a spot on the surface of a distant star can be detected and measured using polarisation. As the star rotates the spectrum of the light coming from the region of a spot is split because of the magnetic field, by the Zeeman Effect, and also shifted by the Doppler Effect. In addition a significant circularly polarised signal can be detected in parts of the spectrum. This information can be used by astronomers to detect and characterise starspots, and even make maps of the stellar surfaces.