The formation of sunspots and starspots

The spatial scale of large solar active regions and many of their observed features indicate that they originate from the emergence of a coherent magnetic structure of rather well‐ordered toroidal magnetic flux in the solar interior. Numerical simulations of the instability of magnetic flux tubes and their rise through the convection zone are consistent with the basic observed properties of sunspot groups like low emergence latitudes, tilt angles with respect to the east‐west direction, and proper motions. The success of this approach in the case of the Sun motivates its application to other magnetically active stars. The effect of the Coriolis force on rising magnetic flux tubes provides an explanation for the existence of high‐latitude spots on rapidly rotating active stars. Detailed models have been developed for the distribution of flux emergence latitudes depending on the rotation rate and internal structure of cool stars in various evolutionary stages. The models for very young (T‐Tauri‐like) stars exhibit extended latitude ranges of flux emergence, including the poles. In the case of rapidly rotating main‐sequence stars, the flux tubes appear in mid to high latitudes; neither equatorial nor truly polar flux emergence is found. The trapping of flux tubes in giants with a sufficiently small (relative) core size suggests an explanation for the strong decline of X‐ray emission of cool giants across the ‘coronal dividing line’ in the Hertzsprung‐Russell diagram. We give an overview of the solar and stellar results and discuss the benefits and the limitations of applying the solar paradigm to active stars.