 As the hydrogen shell burning produces more helium , the core increases in mass and temperature . In a red giant of up to 2 @.@ 25 M ☉ , the mass of the helium core becomes degenerate prior to helium fusion . Finally , when the temperature increases sufficiently , helium fusion begins explosively in what is called a helium flash , and the star rapidly shrinks in radius , increases its surface temperature , and moves to the horizontal branch of the <unk> diagram . For more massive stars , helium core fusion starts before the core becomes degenerate , and the star spends some time in the red clump , slowly burning helium , before the outer convective envelope collapses and the star then moves to the horizontal branch . 
