As the hydrogen shell burning produce 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 become degenerate prior to helium fusion . Finally , when the temperature increase sufficiently , helium fusion begin explosively in what is called a helium flash , and the star rapidly shrink in radius , increase its surface temperature , and moves to the horizontal branch of the HR diagram . For more massive stars , helium core fusion start before the core become degenerate , and the star spend some time in the red clump , slowly burning helium , before the outer convective envelope collapses and the star then move to the horizontal branch .