Editing Stellar nucleosynthesis (section)

===Helium fusion===
{{Main|Triple-alpha process|Alpha process}}
Main sequence stars accumulate helium in their cores as a result of hydrogen fusion, but the core does not become hot enough to initiate helium fusion.  Helium fusion first begins when a star leaves the [[red giant branch]] after accumulating sufficient helium in its core to ignite it. In stars around the mass of the Sun, this begins at the tip of the red giant branch with a [[helium flash]] from a [[Degenerate matter|degenerate]] helium core, and the star moves to the [[horizontal branch]] where it burns helium in its core. More massive stars ignite helium in their core without a flash and execute a [[blue loop]] before reaching the [[asymptotic giant branch]]. Such a star initially moves away from the AGB toward bluer colours, then loops back again to what is called the [[Hayashi track]]. An important consequence of blue loops is that they give rise to classical [[Cepheid variable]]s, of central importance in determining distances in the [[Milky Way]] and to nearby galaxies.<ref>Karttunen, H., Kröger, P., Oja, H., Poutanen, M., & Donner, K. J., eds., ''Fundamental Astronomy'' ([[Berlin]]/[[Heidelberg]]: [[Springer Science+Business Media|Springer]], 1987), [https://books.google.com/books?id=DjeVdb0sLEAC&pg=PA250&redir_esc=y#v=onepage&q&f=false p. 250].</ref>{{rp|250}} Despite the name, stars on a blue loop from the red giant branch are typically not blue in colour but are rather yellow giants, possibly Cepheid variables. They fuse helium until the core is largely [[carbon]] and [[oxygen]]. The most massive stars become supergiants when they leave the main sequence and quickly start helium fusion as they become [[red supergiant]]s. After the helium is exhausted in the core of a star, helium fusion will continue in a shell around the carbon–oxygen core.<ref name=jones2009/><ref name=deloore_doom1992/>

In all cases, helium is fused to carbon via the triple-alpha process, i.e., three helium nuclei are transformed into carbon via [[Beryllium-8|<sup>8</sup>Be]].<ref>Rehder, D., ''Chemistry in Space: From Interstellar Matter to the Origin of Life'' ([[Weinheim]]: [[Wiley-VCH]], 2010), [https://books.google.com/books?id=baI91e8lgm0C&pg=PT30&redir_esc=y#v=onepage&q&f=false p. 30].</ref>{{rp|30}} This can then form oxygen, neon, and heavier elements via the alpha process. In this way, the alpha process preferentially produces elements with even numbers of protons by the capture of helium nuclei. Elements with odd numbers of protons are formed by other fusion pathways.<ref>[[Michael Perryman|Perryman, M.]], ''The Exoplanet Handbook'' (Cambridge: Cambridge University Press, 2011), [https://books.google.com/books?id=ngtmDwAAQBAJ&pg=PA398&redir_esc=y#v=onepage&q&f=false p. 398].</ref>{{rp|398}}