Mixing length theory

Summary

Mixing length theory is the standard simple model of convection inside stars. It pictures convection as blobs of gas that rise or sink over a characteristic distance, the mixing length. Then the blobs dissolve into their surroundings and give up their excess heat. The mixing length is taken proportional to the pressure scale height, with a free factor $\alpha$ of order one that is fitted to the Sun. The model turns the Schwarzschild criterion into a formula for the heat flux carried by convection. It goes back to the mixing length idea of Prandtl in the theory of turbulence.