This is known in the literature as the “instability strip” and is almost, but not quite, parallel to the luminosity axis
on the H-R diagram. In normal stars, any compression of the star, and the associated rise in temperature,
results in a decrease in opacity; the resulting escape of photons produces expansion and cooling. For stars in the
instability strip, a layer of partially ionized He close to the surface causes opacity to rise instead of falling with an
increase in temperature, producing a degree of positive feedback and consequently oscillations. The instability
strip has a finite width, which causes a small degree of dispersion in period–luminosity correlations among
Cepheids.