Introduction

• Recording
• PowerPoint presentation. In Czech only.
• PowerPoint presentation from the previous year (corresponds to the recording). In Czech only.
• Hand written notes.

• Convection zone starts at ~500 Mm from the centre, where the opacity increases due to the partial recombination of heavy ions (iron, nickel, etc.). Opacity increases due to the bound-free transitions. The regime is nothing what can be achieved in the laboratory experiments. Low viscosity, large density, large temparature gradient.
• Most of the convection zone is mildly stratified. Within top-most 20 Mm the density increases from the top by 5.5 orders, pressure by 7 orders. Similar stratification is withing the rest of the convection zone. Hence the dynamics of the surface is crucial.
• Description: mixing-length theory. Based on the motion of the overheated element with respect to the stratified background. Estimates for convective velocities etc. are possible. Mixing-legth is proportional to the pressure scale height by definition. The constant of proportionality is the mixing-length alpha. In the given model it plays a role of the universal constant.
• Under the surface the radiative losses play a role, hence the gradient is more than adiabatic, hence the superadiabatic zone. The treatment is again parametric and a bit different.
• Surface spatial spectrum of convection has a continuous profile with two peaks, corresponding to granulation and supergranulation.
• The typical cell size should increase with depth from the surface.
• Granules represent the thermal dissipation scale. Their size is about 1 Mm, lifetime of a few minutes. They look like true convective cells.
• Supergranules have sizes about 30 Mm and lifetimes of about 24 hours. They exhibit a cellular pattern covering the whole Sun, however they do not behave like true convective cells. The origin of the supergranulation is still being debated.
• Other modes: mesogranules, giant cells, are controversial. There are papers announcing their existence, however they are not widely accepted. In some cases, there seem to be good reasons for those "modes" to exist.
• Entropy consideration: the convection is driven from the surface, where the entropy deficite (by radiation) is much larger than the entropy replenishment at the bottom by recombination. The rising plasma further gains entropy by drag and mixing. Most of the buyoancy work is done by downflows.
• Mixing-length has issues, such as an unrealistic energy content transported up and down. Alternatives exist, however not widely accepted.
• The description of the convection is by far the weakest part of the theory of stellar structure and evolution.