Subatomic Physics

586 Nuclear and Particle Astrophysics
Yp
___ D 0.24
H
0.23
7 Li/H p
0.27
0.26
0.25
10 −3
10 −4
10 −5
10 −9
5
2
10 −10
1
0.005
4 He
D/H p
3 He/H p
Baryon density ΩBh 0.01 0.02 0.03
2
CMB
BBN
2 3 4 5 6 7 8 9 10
Baryon-to-photon ratio η × 10−10 Figure 19.3: Abundances of 4 He, 2 H, 3 He, 7 Li as predicted
by the standard model of big bang nucleosynthesis. The
boxes correspond to the observed abundances (small boxes:
±1σ; large boxes:±2σ.) The vertical line indicates the cosmic
baryon density from CMBR. [From PDG.]
19.3 Stellar Energy and Nucleosynthesis
Further, the alpha-particle
capture reaction,
4 He 4 He −→ 8 Be, (19.4)
leads to the highly unstable
nuclide 8 Be which breaks up
immediately into two alpha
particles. When the temperature
of the universe dropped
to about 3×10 8 K, (kT ∼ 10 4
eV), approximately half an
hour after its birth, primordial
nuclear synthesis ceased
because the Coulomb barrier
prevented further nuclear reactions.
The abundances of
the elements formed in the
big bang were frozen, so that
the presently observed abundances
of the light elements d,
3 He, 4 He, and 7 Li still reflect
this stage.
And God said, Let there be light; and there was light.
Genesis
The mechanism of energy production in the sun is understood and has been
tested, and we shall discuss it as an example of stellar power sources. The construction
of a terrestrial fusion reactor is difficult. The main difficulty is containment:
A plasma with a temperature of about 10 8 K must be kept enclosed within a finite
volume. Solid walls cannot withstand such a temperature and magnetic or laser
confinement is used. The magnetic field volume must be relatively small (a few
m 3 ), or power and construction costs become prohibitive. Instabilities plague all
these confinement schemes, so the designer(s) of the Sun has chosen a simple but
robust scheme: The “container” is huge, with a radius of about 7 × 10 8 m, with
an outside temperature of about 6000 K, and with a central temperature of about
1.6 × 10 7 K, (kT ∼ 10 3 eV). Fusion reactions then proceed at a much lower rate
than that needed for terrestrial reactors. Nevertheless, total energy production is
large because the volume is huge.