The end of a red giant star like our Sun begins suddenly. As helium accumulates at its center, it becomes electron-degenerate. When the core temperature reaches about 100 million degrees, helium starts fusing into carbon through the triple-alpha process. This generates immense heat, but unlike normal matter, electron-degenerate helium doesn't expand when heated. The self-regulating mechanism found in main-sequence stars is absent here.
As the temperature rises, helium fusion accelerates exponentially. Within minutes, about 6% of the helium core (around 40% of a solar mass) is fused into carbon. This event, known as the helium flash, releases energy equivalent to what our current Sun generates in 200 million years.
Despite its power, the helium flash has little visible impact on the red giant. Most of the energy is absorbed by the core's expansion out of its degenerate state. Observers wouldn't notice much change in the star's appearance.
After the flash, the core expands and cools significantly, causing a dramatic drop in the hydrogen-burning shell's pressure and energy output. The red giant's size and brightness decrease to less than 2% of their former values, resulting in a smaller, orange-yellow star.
In its last 140 million years, the Sun will have a double energy source: a carbon-oxygen core surrounded by helium and hydrogen burning shells. Helium fusion produces less energy per kilogram than hydrogen, so the Sun remains primarily a hydrogen reactor.
On the "helium main sequence," the Sun will maintain relative stability for a while. However, after about 100 million years, it will again become a red giant. Without a carbon flash equivalent to stop it, the Sun's core will continue to compress and grow denser while its outer layers expand further.
In its final stages, the Sun will experience multiple fuel ignitions and disruptions in fusion processes. Its outer layers will be expelled in four or five massive bursts, forming a planetary nebula. About 45% of the Sun's mass will escape, leaving behind a white-hot core.
This exposed core will emit more x-rays than visible light, with a luminosity of 4,000 times that of the current Sun. After a few thousand years, the nebula will disperse, and the Sun will become a white dwarf, slowly cooling over billions of years.
Even though the helium flash is a spectacular event by galactic standards, it is hardly noticeable from an observer's perspective. The Sun's journey ends quietly as a white dwarf, still glowing faintly due to its immense density and residual heat.