What is the helium flash?
The helium flash happens when the helium core of a star gets hot enough to start helium fusion. After a star finishes burning the hydrogen in its core, its core contracts, and it then starts burning hydrogen in a shell around its core (and the outer layers of the star inflate, turning the star into a giant). The hydrogen continues to add helium “ash” to the core, and eventually it contracts and heats enough to ignite helium – this tends to happen quite rapidly, hence the term “helium flash”.
