Geology Reference
In-Depth Information
Table 11.1
Nuclear fusion stages in stellar nucleosynthesis
are more stable than half-filled ones. This additional
stability manifests itself in the form of more compact
orbitals and a smaller nucleus, which reduces the
nuclear 'cross-section' (or 'target size') upon which the
probability of collision depends, thereby depressing
the rate of the fusion or neutron-capture reactions that
consume the nuclide, and allowing its abundance to
build up. Even-number values of
Z
and
N
account for
the greatest number of stable nuclides, lending the
nuclide chart a 'staircase' appearance in which even
values of
Z
form the treads and even values of
N
form
the steps (Box 10.1). Conversely, nuclei having odd val-
ues of
N
or
Z
do not enjoy this additional stability, have
larger collision cross-sections, and are more suscept-
ible to consumption through fusion or neutron capture
reactions or radioactive decay.
Stage
Maximum temperature
Range of nuclei produced
1
10
7
K
H
→
He
2
10
8
K
He
→
C, O, etc.
3
5 × 10
8
K
C, O
→
Si
4
5 × 10
9
K
Si
→
Fe
peak' in the abundance curve noted above (Figure 11.2).
Beyond this point, however, further fusion is impeded
because the core temperatures required to overcome
electrostatic repulsion between nuclei with such large
positive charges exceed those of even the hottest stars.
The synthesis of heavier nuclides requires a different
process that is not impeded by nuclear charge.
The manufacture of nuclides heavier than iron pro-
ceeds instead by the addition of
neutrons
, neutral
nuclear particles that experience no repulsion by the
target nucleus. Many reactions in stars produce neu-
trons, particularly in the later stages of stellar evolu-
tion. The nuclides to the right of iron in Figure 11.2 are
thus believed to be products of cumulative
neutron cap-
ture
. Neutrons are absorbed by a nucleus, increasing
the
N
value until an unstable, neutron-rich isotope has
been produced, which transmutes by
β
-decay into an
isotope of the next element up
(Z
increases by one,
N
falls by one; Box 10.1). The repeated operation of this
process can produce all of the heavier nuclides, given a
sufficiently high neutron flux (Rauscher and Patkós,
2011). If the rate of neutron capture is
slow
relative to
the relevant β-decay rates, the nucleosynthesis path-
way lies close to the main band of stable nuclides
shown in Figure 10.1.1; this 's-process' (s standing for
'slow') leads as far as bismuth (Bi,
Z
= 83, the nuclide
above Pb in Figure 10.1.1). The manufacture of heavier
elements like U and Th requires higher neutron fluxes,
as discussed under 'Supernovae' below.
The abundance peak around iron in Figure 11.2 (item
(e) on p. 211) suggests that neutron capture reactions
consume iron-group nuclei more slowly than fusion
reactions produce them.
Why are nuclides that have even values of
Z
- or
N,
for that matter - more abundant than those with odd
values (item (d) on p. 210)? Protons and neutrons
reside in
orbitals
inside the nucleus, just as electrons
do outside it. According to nuclear wave mechanics,
filled orbitals containing two protons or two neutrons
Supernovae
When a smaller star (<
M
⊙
.) reaches the end of its life, it
can progress into a 'white dwarf' phase and quietly
fade away. But theory suggests that massive stars
(>2
M
⊙
.) follow a different path, leading to a catastrophic
collapse of the core, the shock wave from which causes
a colossal stellar explosion (Bethe and Brown, 1985).
Such
supernovae
are characterized, for a brief period, by
energy output of staggering intensity: the luminosity
from a single exploding star can rise briefly to levels
typical of a whole galaxy (~10
11
stars), lasting for a few
Earth days or weeks. The huge quantities of energy
transferred to the zones of the star immediately sur-
rounding the core cause a large proportion of the star's
mass to be expelled at high velocity (~10
7
m s
-1
). The
expanding Crab Nebula is thought to be the remnant
of a supernova observed by Chinese astronomers in
AD 1054. A supernova was actually observed in the
Large Magellanic Cloud on 23 February 1987.
Supernovae contribute to nucleosynthesis in two
important ways:
(a) The neutron flux becomes exceedingly high during
a supernova, prompting a burst of very rapid
neutron-capture ('r-process') reactions leading
to U and Th, and even beyond (to heavy unstable
nuclides such as plutonium, Pu).
(b) The products of stellar nucleosynthesis, confined
up to that moment within a star's interior, are flung
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