 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 HR 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 . 
 As the hydrogen shell burning <a href='uninflect' title='1'><mark>produces</mark></a> 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 <a href='uninflect' title='2'><mark>becomes</mark></a> degenerate prior to helium fusion . Finally , when the temperature <a href='uninflect' title='3'><mark>increases</mark></a> sufficiently , helium fusion <a href='uninflect' title='4'><mark>begins</mark></a> explosively in what is called a helium flash , and the star rapidly <a href='uninflect' title='5'><mark>shrinks</mark></a> in radius , <a href='uninflect' title='6'><mark>increases</mark></a> its surface temperature , and moves to the horizontal branch of the HR diagram . For more massive stars , helium core fusion <a href='uninflect' title='7'><mark>starts</mark></a> before the core <a href='uninflect' title='8'><mark>becomes</mark></a> degenerate , and the star <a href='uninflect' title='9'><mark>spends</mark></a> some time in the red clump , slowly burning helium , before the outer convective envelope collapses and the star then <a href='uninflect' title='10'><mark>moves</mark></a> to the horizontal branch . 
As the hydrogen shell burnin produce more helium , the core increases in mass and temperature . In a red bigass of up to 2 @.@ 25 M ☉ , da mass of tdahelium 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 thdaR diagram . For more massive stars , helium core fusion start before the core become degenerate , and thedaar spend some time in the red clump , slowly bburninhelium , b4 the outer convective envelope collapses and the star then move to the horizontal branch .