Abstract On the basis of the hypothesis that the highly luminous peculiar stars are gravitationally contracting after they have exhausted their hydrogen, we consider here the stability of the upper atmosphere, which is assumed to consist of pure helium, against the radiation pressure. The criterion curves which indicate the deviation from the stable distribution are plotted in the (log of surface gravity, effective temperature) diagram. In transforming the observed (bolometric magnitude and effective temperature), into (surface gravity and effective temperature), we assumed thatSalpeter reactions set in when the central temperature of contracting stars reaches the required degree. Most of the nuclei of planetary nebulae fall in the region of radiative instability, which may suggest that the streaming out atmospheres or nebulous shells can be attributed to radiation pressure.