Sunspot showing partition into umbra, penumbra, and quiet sun as well as the magneto-convective stability of their sub-surface structure. Top: bolometric intensity I_c. Bottom: map of the GT stability shows the smaller of the LHSs of Eqs. (2) and (3) in a piece-wise logarithmic and linear colour scale from −1 to +1. The green lines are as in Fig. 1. The magenta contour marks the plasma β = 8πp/|B|² = 1 surfaces. The red line marks the outer edge of the density ρ < 3 × 10⁻⁹ g cm⁻³ region. The time evolution of this figure is available as animation online.