What is the last stage in the evolution of the Sun before it turns into a planetary nebula?

A star of average size, such as our Sun, burns through its fuel more slowly than a star of high mass. Its lifetime is estimated to be around 10 billion years. Currently, our Sun is in the main sequence stage, where it is converting hydrogen into helium. As a result, the core is gradually filling with helium; eventually, the core will collapse and the outer layers will expand.

Because of the extreme pressure at the core, the temperatures inside the core are high enough to start the burning of helium, but the Sun will also continue to burn hydrogen in a shell that surrounds the core.

There will come a time when the Sun burns all of its helium into carbon. At that point, the pressure from nuclear fusion will cause the outer layers to expand to such an extent that the star is scattered into space as a planetary nebula with a white dwarf at its center, and the core will collapse due to inward acting gravity.

The red giant phase is the final stage of the Sun's evolution before it becomes a planetary nebula.