Hi im new here but have been studying similar stuff, if the nuclear fusion reactions occur more frequently due to higher temperatures and pressure then the radiation pressure exceeds that of the gravitational pressure, depending on the stars mass when this happens a red giant (3 solar masses or below) or super red giant (greater than 3 solar masses) occurs. Its red due to the surface being cooler, a helium flash occurs. This is because the nuclear fusion reactions are still occuring at the central core and the as rate increase the pressure causes the outer core to be ejected in the helium flash as a planatary nebula. The central core then becomes a white dwarf and can go no further i.e a neutron star or black hole due to fermi pressure.
However only in larger stars do supernovae occur, the white dwarfs formed over come the fermi pressure as the gravitational forces are large enough, electrons combine with protons and the core is left with only neutrons. The outer shells surrounding the core then collapse violently. This causes the "explosion" left behind is a neutron star or if further collapse occurs a black hole,
hope i helped a bit ! cheers
Josh.