Abstrakt
Most of the stars, where starspots were detected, are so distant that it is yet impossible to spatially resolve them. However, we can find a star that can be spatially resolved: the Sun.
In some cases, the study of the Sun allow us extrapolate its properties to other stars. In this talk we will review the research that signifies the importance of the vertical component of the magnetic field to inhibit convective motions and thus create umbrae on the solar surface.
The discovery of a constant vertical magnetic field on the boundary between umbra and penumbra in sunspots (Jurčák, J. 2011) triggered further research of sunspots and other magnetic structures, such as pores. Statistical analysis led to the Jurčák's criterion.
This criterion states that the boundary between the umbra and the penumbra in a stable sunspot is characterized by a constant vertical magnetic field meaning that, when the vertical magnetic field is stronger than a critical value, the umbral convection mode occurs. However, magnetic structures with the vertical magnetic field weaker than such critical value are unstable against more vigorous modes of magnetoconvection and prone to vanish.
We want to present the Jurčák's criterion to the stellar community to ponder its implications and importance for starpots.