Consider a star with a superadiabatic gradient. Imagine a parcel of gas that starts at radial position r, but moves upwards to r + dr in a sufficiently short time that it exchanges negligible heat with its surroundings—in other words, the process is adiabatic. The pressure of the surroundings, as well as that of the parcel, decreases by some amount dP. The parcel's temperature changes by d T = 0.4 T P d P . The temperature of the surroundings also decreases, but by some amount dT′ that is greater than dT. The parcel therefore ends up being hotter than its surroundings. Since the ideal gas law can be written P = ρ ρ R T μ μ , a higher temperature implies a lower density at the same pressure. The parcel is therefore also less dense than its surroundings. This will cause it to rise even more, and the parcel will become even less dense than its new surroundings.