As the inert helium core contracts under gravity, it grows hotter and denser. In stars up to about 2 solar masses, the core becomes so compressed that helium ignition happens all at once in a violent event called the helium flash. In seconds, the core temperature jumps to over 100 million Kelvin and helium begins fusing into carbon.

The helium flash is invisible from outside because the energy is absorbed by the outer layers. After the flash, the star stabilizes briefly on the horizontal branch, fusing helium in its core and hydrogen in a shell.