Instability regions in the upper HR diagram

The following instability regions for blueward evolving-supergiants are outlined and compared. (1) Areas in the Hertzsprung-Russell (HR) diagram where stars are dynamically unstable. (2) Areas where the effective acceleration in the upper part of the photospheres is negative, hence directed outward. (3) Areas where the sonic points of the stellar winds (where nu (wind) = nu (sound)) are situated inside the photospheres, at a level deeper than tau (Ross) = 0.01. We compare the results with the positions of actual stars in the HR diagram and we find evidence that the recent strong contraction of the yellow hypergiant HR 8752 was initiated in a period during which [g(eff)] < 0, whereupon the star became dynamically unstable. The instability and extreme shells around IRC + 10420 are suggested to be related to three factors: [g(eff)] < 0; the sonic point is situated inside the photosphere; and the star is dynamically unstable.