"The helium burning
sets in when the temperature in the contracting core reaches about 80 million degrees Kelvin.
This is because they can become hot enough (around 100x[10.sup.6] K) for helium burning
to commence before the core becomes degenerate.
"The number one unknown in nuclear astrophysics [has been] the process of helium burning
," says Moshe Gai of Yale University,
Helium burning can now take place by converting three helium nuclei into carbon (and oxygen) via the triple alpha process.
Stars of much lower mass than the Sun can't reach temperatures high enough to commence helium burning and may not even produce PN but just slowly cool down.
A similar sequence in Wolf-Rayet stars can also produce fluorine at the start of helium burning
. The star's wind then blows the element into space before the temperature rises enough for the helium to destroy it.
In stars that start out with less than about 2.3 solar masses, core helium burning
begins abruptly, engendering a brief thermonuclear runaway, or core helium flash.
We followed the Sun as it became a red giant (undergoing considerable mass loss); as it encountered a violent helium core flash; as it underwent quiescent helium burning
in its core; as it became a red giant for the second time while traversing the asymptotic giant branch (AGB) |on the Hertzsprung-Russell diagram~, where it encountered violent repetitive helium shell flashes and again suffered considerable mass loss; and as it left the AGB .