Virtually all the Hydrogen originally in the inner core has by now been fused into Helium, which has been settling toward the center and now occupies almost the entire inner core. The outer part of the inner core is still fusing the last of the Hydrogen into Helium, and it feels this extra heat put out by the Helium. Its fusion rate rises accordingly, enough to increase the luminosity of the star dramatically, and enough to force the star to find a new equilibrium size. The star thus begins its transition into a Red Giant. The inert Helium core has continued to shrink and become hotter and denser by now almost 1,000 times more dense than the inner core was on Day 1.
Helium fusion begins, with a great amount of energy released.
Normally, the core would expand from this increased energy, reducing the temperature. But because the Helium core is degenerate, its size remains the same, so its temperature increases catastrophically. With a rapidly increasing temperature, Helium fusion takes off at a feverish pace, producing a dramatic surge in energy called the Helium Flash, in which a large fraction of the Helium is almost instantly fused into Carbon. The energy released in the Helium Flash finally overcomes the electron degeneracy, and the inner core can finally expand and reduce its temperature. The star has finally stabilized.