The region to the right is forbidden because it can be shown that a star in the region must have a temperature gradient of: d ln ⁡ ⁡ T d ln ⁡ ⁡ P > 0.4 where d ln ⁡ ⁡ T d ln ⁡ ⁡ P = 0.4 for a monatomic ideal gas undergoing adiabatic expansion or contraction. A temperature gradient greater than 0.4 is therefore called superadiabatic.