Stars with convective envelopes, like the Sun, are able to generate strong magnetic fields that stifle convection. In the outer layers of these stars, convection is the most efficient form of energy transport, so when it is inhibited by the magnetic fields, the region of the surface will have a cooler temperature and appear darker. These darker regions are starspots. The strong magnetic fields reconnect above the stellar surface. This sudden ejection of energy results in a flare. These are just two types of readily-observed stellar activity.
Imaging Stellar Surfaces
Imaging techniques provide an opportunity for understanding stellar surfaces. Indirect imaging can be accomplished with photometry and spectroscopy, but both methods are limited due to the degeneracies of the methods. Direct imaging with long-baseline optical interferometry allows for imaging without the degeneracies and uncertainties of the other methods by detecting the star as it is on the sky.
Interferometric image of the spotted star zeta Andromedae. More to come...
