Let us consider a star fusing hydrogen into helium when the star has too low a mass to ignite helium fusion.
Red giants are also less dense than their main-sequence counterparts, meaning that helium fusion occurs in a domain at lower pressure than hydrogen fusion and produces more heat.
The only thing he left out was a suggestion that, at some time in the distant future, our descendants will figure out a way to replenish the Sun's core with fresh hydrogen at just the right rate to prevent Old Sol from ever needing to initiate helium fusion
In the last 500,000 years, helium fusion
Inside these slightly enriched new stars, temperatures could reach 100,000,000[degrees]C or more, and stable helium fusion
could take place.
Helium fusion within the core does provide a stable source of energy, but compared to hydrogen it's only about 1/10 as efficient.
Therefore, at any given time either hydrogen or helium fusion dominates.
An aging red giant undergoes repetitive bursts of helium fusion
in a narrow shell surrounding its carbon-oxygen core.
At the center of the star is a small but very dense core composed almost entirely of carbon and oxygen, the end products of helium fusion.
For about a century the star derives its energy from helium fusion.