Abstract
Magnetic activity similar to that of the Sun is observed on a variety of cool stars with external convection envelopes. Stellar rotation coupled with convective motions generate strong magnetic fields in the stellar interior and produce a multitude of magnetic phenomena including starspots in the photosphere, chromospheric plages, coronal loops, UV, X-ray, and radio emission and flares. Here I review the phenomenon of starspots on dierent types of cool stars, observational tools and diagnostic techniques for studying starspots as well as starspot properties including their temperatures, areas, magnetic field strengths, lifetimes, active latitudes and longitudes, etc. Evolution of starspots on various time scales allows us to investigate stellar dierential rotation, activity cycles, and global magnetic fields. Together these constitute the basis for our understanding of stellar and solar dynamos and provide valuable constraints for theoretical models.
Highlights
The multitude of activity phenomena on the Sun are related to magnetic fields which are generated by cyclonic turbulence in the outer convection zone and penetrate the solar atmosphere forming sunspots, plages, network, etc
A remarkable progress in observational facilities and numerical techniques for studying starspots achieved for the last two decades allows for a deep insight into the nature of starspots and underlying magnetic fields
In addition to traditional photometric and spectroscopic observations, advanced instrumentation for high-precision spectropolarimetry andinterferometry together with foreseen space missions constitute a powerful arsenal for studying starspots and promise for ground-braking discoveries in stellar magnetism
Summary
The multitude of activity phenomena on the Sun are related to magnetic fields which are generated by cyclonic turbulence in the outer convection zone and penetrate the solar atmosphere forming sunspots, plages, network, etc. A detailed study of solar activity phenomena reveals the structure of underlying magnetic fields and provides valuable constraints for solar dynamo theory. These same activity phenomena are observed on cool stars with outer convection zones. Light-curve variations due to starspots and other magnetic phenomena were discovered on various types of stars It was suggested by Skumanich (1972) that rotation plays a crucial role in the generation of stellar activity. Orbital period variations suggest long-term changes of the magnetic field distribution in stellar interiors (Hall, 1991b; Rodono et al, 1995; Lanza et al, 1998a) Such observations provide valuable constraints for stellar and solar dynamo models. The present review covers main tendencies in starspot research for the last two decades
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