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Thermonuclear Fusion
Thermo means “related to heat,” nuclear
indicates atomic nuclei, and fusion means
“joining together.” This is a process of nuclei
joining together into larger nuclei, and it
happens when they’re exposed to a very high
temperature.
This is what powers the Sun and most other
stars, and the Sun powers nearly everything
that’s important to humans. Although the
fusion processes in some stars involve many
elements, the Sun’s reactions are limited to the
fusion of protons into alpha particles.
Conditions in the Sun’s Core
This graph shows the temperature inside the
Sun as a function of depth, where 1.0 on the
horizontal axis is the surface, and 0 is the
center. The central temperature is 15 million
Kelvin, which is 27 million Fahrenheit. We know
this is true from study of the movement of the
Sun’s surface; waves traveling through the Sun
serve as probes of its interior.
In the extreme temperature environment in the
Sun’s core, there are no atoms. Nuclei and
electrons move much too fast for the electrons
to settle into orbitals. The average speed of a
proton at this temperature is about 1.4 million
mph. They slam into each other trillions of
times each second. Most of the time they
bounce off, because protons have a positive
electric charge, and two positively charged
particles repel each other through the EM force.
But on a rare occasion, two particularly fast
protons happen to hit head on, and they
approach so close to each other that the very
short ranged Strong Nuclear Force takes over,
and they bond together. One of the two quickly
changes into a neutron due to the influence of
the Weak Nuclear Force.
This composite particle can easily absorb
another proton (second stage of the process in
the above drawing). The resulting group of
three nucleons (labeled 3𝐻𝑒) eventually
combines with another one like itself, and the
final result is 4𝐻𝑒, a group of 2 protons and 2
neutrons, otherwise known as an alpha particle.
The nuclei of most of the helium atoms in a
helium balloon are these.
You might wonder how it can be known that
this process occurs inside the Sun. But
remember that these same particle groups exist
on Earth, and experimentation with them has
informed scientists about their behavior in such
conditions. The rates of all these reactions are
well known.
Also, in the last few decades, experiments have
detected neutrinos (the particle marked 𝛎 in the
diagram) emanating from the Sun, and they’ve
been measured at the rate predicted by
calculations of this process (the neutrino, along
with the electron and the two quarks that make
up nucleons, is one of the four known
elementary particles).
This process is exothermic, meaning that energy
is released. The amount of energy released
during the fusion of a pound of protons is about
45 million times greater than the energy
released when a pound of gasoline/air mixture
burns in the engine of your car. That’s because
this is a nuclear process while gasoline
combustion is a chemical process, and nuclear
processes involve far greater forces and
amounts of energy.
Nucleosynthesis
Calculations like the ones used to determine the
processes inside the Sun’s core have indicated
that additional processes occur inside stars with
higher core temperatures. The 4𝐻𝑒 (helium
nuclei) can combine to form carbon and oxygen
nuclei, and as a result these are the most
abundant elements behind hydrogen and
helium. The higher a star’s central
temperature, the more elements it can
produce, and about the first half of the periodic
table can be accounted for this way.
Star’s that have high central temperatures also
eventually explode, an event called a
supernova. This and other types of supernovae
can account for the second half of the periodic
table.
In fact, the abundances of elements in the Solar
System (shown below) agree well with the
relative amounts produced in these processes.
Thermonuclear Fusion & Power Production
Given that thermonuclear fusion releases so
much energy and that the whole universe runs
on it, why don’t we use it to produce electrical
energy? The answer is that no one has been
able to build a machine that can heat nuclei to
the required temperatures and still make
enough energy to make it a cost effective
energy source. But it is an area of active
research.