For stars similar to the Sun, what critical physical mechanism supports the helium core prior to the explosive onset of helium fusion known as the helium flash?
Answer
Electron degeneracy pressure, which is independent of temperature
Sun-like stars (those below roughly $2.25$ solar masses) experience a unique event when their contracting helium cores finally reach the temperature necessary for helium fusion. Because these cores are supported against collapse not primarily by normal thermal pressure but by electron degeneracy pressure—a quantum mechanical effect dependent only on density—the core temperature can rise drastically without a corresponding increase in outward thermal pressure to counteract gravity. This lack of thermal opposition causes the temperature to spike rapidly until fusion ignites suddenly, resulting in the explosive helium flash.
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