What exactly causes helium flashes in stars at the point when they begin to burn helium rather than hydrogen?
A helium flash occurs because the core of the star is in what is known as a “degenerate” state. This means that the core has contracted so much that the pressure of the electron shells of the atoms making up the core prevent the core from contracting further. Under normal gas conditions (i.e. NOT a degenerate state), an increase in the temperature of the core would cause an increase in core pressure resulting in the core expanding and the temperature then dropping. This state is known as hydrostatic equilibrium. With a degenerate core, the temperature increases but the pressure doesn’t. This extra energy ignites the helium creating run-away nuclear reactions. This is what is referred to as a “helium flash.” For more details, check out: http://www.physics.gmu.edu/classinfo/astr103/CourseNotes/Text/Lec05/Lec05_pt5_txt_stellarPostMSEvol.htm Hope this helps.
