A helium flash is a thermally unstable ignition of helium in an electron-degenerate stellar core. A nondegenerate intermediate-mass core can expand and regulate ignition, so a usual isolated star of five solar masses begins core helium burning without this flash.
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A helium flash is a crucial astrophysical event that occurs in the later stages of a star's life, particularly in low- to intermediate-mass stars (like those in the range of about 0.8 to 8 solar masses). This phenomenon happens when the core of a red giant, primarily composed of helium, becomes sufficiently hot and dense to ignite helium fusion after a period of hydrogen burning in the star's earlier life stages.