Helium flash

Stars between around 0.5 and 2.5 solar masses happily 'burn' hydrogen for most of their lives. Once one runs out of hydrogen, it suddenly isn't producing enough energy in its core to support its outer layers, and the star collapses under gravity. The first thing that happens here is that the density increases, so the star starts fusing hydrogen in layers outside the core, which causes the outer layers to expand outwards. The star becomes a red giant, but its core and inner layers continue to collapse under gravity. The two main forces that prevent any star from collapsing inward are thermal pressure (fusion inside emits radiation and prevents nearby layers from encroaching) and degeneracy pressure (a quantum effect - no two fermions (like electrons or hydrogen atoms) can occupy the same quantum state, so the star can't collapse any further than that). Since the core is now fusing much less hydrogen than before, the thermal pressure is dwarfed by degeneracy pressure. The pressure is high enough to initiate helium fusion, which increases the temperature. But because the core is primarily supported by degeneracy pressure, this increase in temperature does almost nothing to expand (and thus cool) the helium-fusing portion of the star. So the helium fusion becomes a runaway reaction in what's called the helium flash. This phase of the star's life only ends when the temperature finally rises enough to overwhelm degeneracy pressure. At this point, the star expands and cools a bit. Helium continues to burn, but in a more regulated manner, and thermal balance reigns once more.

Stars with less mass don't heat up enough to go through helium burning, and instead become helium white dwarfs. Stars with more mass are able to start helium burning before the core becomes degenerate, so there's no runaway helium flash. These eventually become neutron stars and supernovae - a topic for another day.