In variable stars like Cepheids, what triggers the violent expansion phase?

Answer

A layer of gas trapping heat, becoming temporarily opaque, and causing trapped heat to expand the layer violently

The pulsation mechanism involves specific gas layers trapping radiation until they heat up and become opaque, causing violent expansion; as they cool and become transparent, gravity pulls them back inward to restart the cycle.

In variable stars like Cepheids, what triggers the violent expansion phase?
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