Why does the Helium Flash occur suddenly and explosively rather than gradually in a degenerate core?

The degenerate core cannot regulate temperature through expansion, so fusion skyrockets once the threshold is met

The defining characteristic of the Helium Flash is its explosive nature, which is a direct consequence of the core's state of electron degeneracy. In a normal, non-degenerate star, an increase in fusion rate would cause the core to expand, which in turn would lower the density and temperature, acting as a natural 'thermostat' to slow down the reaction. However, because the degenerate core resists expansion—it cannot 'turn down the thermostat'—once the critical temperature of 100 million Kelvin is reached, the rate of helium fusion increases uncontrollably and almost instantaneously. This results in an enormous burst of energy that briefly destabilizes the core structure until it heats up enough for normal thermal pressure to take over.