What happens when the outer layers of Cepheids are too cold or too hot?
When the outer layers of a Cepheid are too cold, the layer in which the gas must be ionized will be too small to lift the top layers. On the other hand, if the outer layers of the star are very cold, the process of pulsating doesn't take place as well. This is because in this case the layer in which the gas must be ionized is very close to the surface and, therefore, can't create an apparent pulse.