The alpha process generally occurs in large quantities only if the star is sufficiently massive, ( being the mass of the sun); these stars contract as they age, increasing core temperature and density to high enough levels to enable the alpha process. Requirements increase with atomic mass, especially in later stages -- sometimes referred to as silicon burning -- and thus most commonly occur in supernovae. Type II supernovae mainly synthesize oxygen and the alpha-elements (Ne, Mg, Si, S, Ar, Ca, and Ti) while Type Ia supernovae mainly produce elements of the iron peak (Ti, V, Cr, Mn, Fe, Co, and Ni). Sufficiently massive stars can synthesize elements up to and including the iron peak solely from the hydrogen and helium that initially comprises the star.
Typically, the first stage of the alpha process (or alpha-capture) follows from the helium-burning stage of the star once helium becomes depleted; at this point, free  capture helium to produce . This process continues after the core finishes the helium burning phase as a shell around the core will continue burning helium and convecting into the core. The second stage (neon burning) starts as helium is freed by the photodisintegration of one  atom, allowing another to continue up the alpha ladder. Silicon burning is then later initiated through the photodisintegration of  in a similar fashion; after this point, the peak discussed previously is reached. The supernova shock wave produced by stellar collapse provides ideal conditions for these processes to briefly occur. 
During this terminal heating involving photodisintegration and rearrangement, nuclear particles are converted to their most stable forms during the supernova and subsequent ejection through, in part, alpha processes. Starting at  and above, all the product elements are radioactive and will therefore decay into a more stable isotope -- e.g.  is formed and decays into .