Alpha process elements (or
alpha elements) are so-called since their most abundant isotopes are integer multiples of four – the mass of the helium nucleus (the
alpha particle). These isotopes are called
alpha nuclides. of the relative energy output () of
proton–proton (),
CNO, and
triple- fusion processes at different temperatures (). The dashed line shows the combined energy generation of the and CNO processes within a star. • The stable alpha elements are:
C,
O,
Ne,
Mg,
Si, and
S. • The elements
Ar and
Ca are
"observationally stable". They are synthesized by alpha capture prior to the
silicon fusing stage, that leads to •
Si and
Ca are purely alpha process elements. •
Mg can be separately consumed by
proton capture reactions. The status of oxygen (
O) is contested – some authors consider it an alpha element, while others do not.
O is surely an alpha element in low-
metallicity Population II stars: It is produced in
Type II supernovae, and its enhancement is well correlated with an enhancement of other alpha process elements. Sometimes
C and
N are considered alpha process elements since, like
O, they are synthesized in nuclear alpha-capture reactions, but their status is ambiguous: Each of the three elements is produced (and consumed) by the
CNO cycle, which can proceed at temperatures far lower than those where the alpha-ladder processes start producing significant amounts of alpha elements (including
C,
N, &
O). So just the presence of
C,
N, or
O in a star does not a clearly indicate that the alpha process is actually underway – hence reluctance of some astronomers to (unconditionally) call these three "alpha elements". == Production in stars ==