helium flash
At the top of the red-giant branch, a low-mass star like the Sun finally faces a buried problem: its core is a dense ball of helium ash, hot but not yet hot enough to burn helium itself. Gravity keeps squeezing, the core keeps heating, and then, all at once, helium fusion ignites in a runaway burst — the helium flash. For a few brief minutes, the core releases energy at a rate rivaling an entire galaxy of stars, yet almost none of it ever reaches the surface. It is one of the most violent events in a quiet star's life, and you would never see it from outside.
Why a flash and not a gentle lighting? In a low-mass red giant the helium core has become degenerate — packed so tightly that its pressure comes from the quantum behavior of electrons, not from heat. Normally a star is self-regulating: heat it and it expands and cools. But a degenerate gas does not expand when heated, so when helium ignites, the temperature soars without the safety valve of expansion, which makes fusion go faster, which raises the temperature further — a thermonuclear runaway. It only stops when the core finally gets so hot that the gas stops being degenerate and can expand, settling into steady helium burning.
The helium flash marks a turning point: the star leaves the red-giant branch and settles onto the horizontal branch, now burning helium in its core and hydrogen in a shell. Crucially, the flash is hidden — the explosive energy goes into lifting the degeneracy and rearranging the core, not into blowing the star apart, so from outside the star quietly dims and shrinks over the following years. Only stars below about 2 solar masses have a flash; heavier stars ignite helium smoothly and gently because their cores never became degenerate.
In about 6 billion years the Sun, perched at the tip of the red-giant branch, will undergo its own helium flash. For a few minutes its core will blaze with the power of billions of Suns — yet a distant observer would notice only a slow fade over the next 100,000 years as the star reorganizes itself.
An immense burst of fusion buried in the core, invisible from the surface.
The helium flash does not blow the star up. Despite its galaxy-rivaling power, the energy is absorbed in lifting the core's degeneracy; the star survives and brightens only quietly. Stars above ~2 solar masses skip the flash entirely.