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The reaction He³ + He³ -> He^4 + 2 H¹ is unusual in that it increases the hydrogen abundance and lowers the mean molecular weight (the mean weight per particle in the plasma). This generates a region that is unstable and leads to buoyancy.
In this simulation, the hydrogen burning shell is shown by a contour of the unstable isotope N¹³ (t 1/2 Å 600 sec), so over the time of this simulation, this region is decaying and being continuously regenerated over the course of the movie. Its stability shows that the heart of the hydrogen burning shell that powers the red giant is operating on the CNO cycle (creating the N¹³), and is stable. The temperature in a region just outside of that is sufficient for the He³ to burn and produce a region that is just slightly enhanced in hydrogen. These regions are shown as red contours. The contour cut is defined such that only a tiny a tiny increase in the hydrogen is needed to show the region. The hydrogen rich clouds grow and slowly rise.
At the end of this clip, a cloud deck has formed, and is rotated to show it from all sides. The cloud deck is 85,000 km in diameter so the upward motion of about 140 meters per second may be difficult to see.