Subatomic Physics

590 Nuclear and Particle Astrophysics
This sequence can be repeated up the ladder of elements. In addition (α, n) (α, p)
and reactions with incoming neutrons and protons can form the elements that lie
in between the alpha-like nuclides. When α-burning becomes insuficient the star
compresses due to the gravitational pull and heats up until carbon burning occurs.
Figure 19.4: Products of carbon burning in an exploding
star. Circles represent solar-system abundances, calculated
abundances are shown as crosses. Solid lines connect all
stable isotopes of a given element. The assumed peak temperature
is 2 × 10 9 K, the density 10 5 g/cm 3 .[AfterW.D.
ArnettandD.D.Clayton,Nature 227, 780 (1970).]
Such reactions,
12 C 12 C −→ 20 Ne α
−→ 23 Na p (19.12)
−→ 23 Mg n
require temperatures higher
than about 10 9 K(kT ∼ 10 5
eV). Such temperatures occur
only in very heavy stars
and carbon burning thus is believed
to occur predominantly
in massive, also sometimes
called exploding stars. If it is
assumed that the temperature
in exploding stars is about
2 × 10 9 K, the abundance
of elements produced appears
to agree closely with observation,
as is shown in Fig. 19.4.
The exact path that nucleosynthesis takes depends on the initial conditions and
on whether there can be additional fresh hydrogen coming, e.g. from a companion
young star, but in general the production proceeds toward the more stable nuclei,
ending up in Fe.
As the formation of elements reaches iron, a new aspect appears. As Fig. 16.1
shows, the binding energy per nucleon reaches a maximum at the iron group. Beyond
these elements, the binding energy per nucleon decreases. Hence the iron
group cannot serve as fuel, and burning must cease once iron has been formed.
This feature explains why elements centered around Fe are more abundant than
others.
Most elements beyond the iron group are formed mainly through neutron capture
reactions. There are two processes, a slow one, called s, and a rapid one, called r.
The capture processes depend critically on the neutron flux. Figure 19.5 shows how
the two processes may generate heavier nuclei with different relative abundances.
With the beta decays of unstable nuclei, the proton number Z increases by one (as
shown by the diagonal lines in Fig. 19.5.) In the s-process that takes place in stars
like red giants, neutron captures are not very frequent because the fluxes are low.
So once a capture occurs there is plenty of time for the beta decay to happen before