Why does the helium flash occur in stars around one solar mass?
Answer
The onset of fusion while supported by temperature-independent electron degeneracy pressure.
The helium flash is a runaway nuclear event unique to lower-mass stars that develop degenerate cores. Since electron degeneracy pressure supports the core against gravity, it is temperature-independent; thus, when the core finally reaches $100$ million Kelvin and helium fusion begins, the resulting thermal energy output does not immediately cause the core to expand and cool itself (as happens on the main sequence). This lack of immediate self-regulation causes a massive, near-instantaneous surge of energy release as the entire core ignites almost simultaneously, leading to the explosive event known as the helium flash.
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