One of the two nuclear fusion reactions that convert helium into heavier elements. *(also see [[Triple-Alpha Process]])* After the hydrogen fusion ([[Proton-Proton Chain|pp chain]], [[Carbon-Nitrogen-Oxygen Cycle|CNO cycle]]) and helium fusion ([[Triple-Alpha Process|triple-alpha process]]) processes have produced enough carbon, the alpha ladder can begin and the production of heavier elements can take place. Each step in the ladder only uses a $\ce{^{4}He}$ nuclei and the product of the previous reaction. $ \begin{alignat}{4} &\ce{^{12}_{6}C} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{16}_{8}O} &\ + \ &\gamma &\hspace{1cm} &E = 7.16 \; {\rm MeV} \\ &\ce{^{16}_{8}O} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{20}_{10}Ne} &\ + \ &\gamma &\hspace{1cm} &E = 4.73 \; {\rm MeV} \\ &\ce{^{20}_{10}Ne} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{24}_{12}Mg} &\ + \ &\gamma &\hspace{1cm} &E = 9.32 \; {\rm MeV} \\ &\ce{^{24}_{12}Mg} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{28}_{14}Si} &\ + \ &\gamma &\hspace{1cm} &E = 9.98 \; {\rm MeV} \\ &\ce{^{28}_{14}Si} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{32}_{16}S} &\ + \ &\gamma &\hspace{1cm} &E = 6.95 \; {\rm MeV} \\ &\ce{^{32}_{16}S} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{36}_{18}Ar} &\ + \ &\gamma &\hspace{1cm} &E = 6.64 \; {\rm MeV} \\ &\ce{^{36}_{18}Ar} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{40}_{20}Ca} &\ + \ &\gamma &\hspace{1cm} &E = 7.04 \; {\rm MeV} \\ &\ce{^{40}_{20}Ca} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{44}_{22}Ti} &\ + \ &\gamma &\hspace{1cm} &E = 5.13 \; {\rm MeV} \\ &\ce{^{44}_{22}Ti} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{48}_{24}Cr} &\ + \ &\gamma &\hspace{1cm} &E = 7.70 \; {\rm MeV} \\ &\ce{^{48}_{24}Cr} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{52}_{26}Fe} &\ + \ &\gamma &\hspace{1cm} &E = 7.94 \; {\rm MeV} \\ &\ce{^{52}_{26}Fe} &\ + \ &\ce{^{4}_{2}He} &\ \longrightarrow \ &\ce{^{56}_{28}Ni} &\ + \ &\gamma &\hspace{1cm} &E = 8.00 \; {\rm MeV} \end{alignat} $ Many of these reactions can occur simultaneously during the evolution of a large star. While the heavier elements sink and the later-stage reactions take place in the core, the prior-stage reactions can still happen in the outer layers of the star. (i.e. shell burning) Although the chain could theoretically continue, steps after nickel-56 are much less exothermic and the temperature is so high that [[Photodisintegration]] prevents further progress. > [!note] Energy Output > > The alpha ladder has a very low reaction rate, given the temperatures and densities necessary for them to occur in stars. For elements heavier than neon ($Z \gtrsim 10$), there is an even lower reaction rate due to the increased Coulomb Barrier; therefore, oxygen and carbon are the main **"ash"** of helium-4 burning.