Nuclear Fusion

Nuclear fusion is the process in which light nuclei combine to form a more tightly bound heavier nucleus, and energy is released during the process.

Because both nuclei carry a positive charge, they repel each other through electrostatic force. This repulsion is called the Coulomb barrier in the context of fusion.

Thermonuclear fusion is fusion initiated by extremely high temperature, which gives nuclei enough kinetic energy to approach one another closely.

• For light nuclei, binding energy per nucleon increases as mass number increases. 
• When light nuclei combine, the final nucleus becomes more stable. 
• The increase in binding energy appears as released energy. 

Simple Analogy

Fusion can be compared to small parts joining to form a stronger, more tightly locked structure; the final arrangement is more stable, so excess energy is released. This analogy helps explain why energy is released when light nuclei combine. 

Some fusion reactions mentioned in the source material are listed below.

In stars, not all particles have exactly the same energy. Even when the average thermal energy is lower, some particles in the high-energy tail of the distribution can participate in fusion. 

For this reason, thermonuclear fusion in the Sun can occur at a temperature of about \[1.5 \times 10^{7}\] K.

• The source material explains that the Sun’s energy is produced by fusion reactions involving hydrogen nuclei. 
• The main process is the proton–proton cycle, in which hydrogen nuclei ultimately combine to form helium. 

Proton–Proton Cycle Steps

1. Two protons combine to form deuterium, releasing a positron and a neutrino.
2. Deuterium combines with another proton to form helium-3 and gamma radiation.
3. Two helium-3 nuclei combine to form helium-4 and two protons.

Net Result

The net fusion process converts four hydrogen nuclei into one helium-4 nucleus with the release of about 26.7 MeV of energy. 

• As hydrogen in the core is gradually converted into helium, the star's core changes over time. 
• When hydrogen burning decreases, the core can cool and contract under gravity, causing the temperature to rise again. 
• At about \[10^{8}\] K, helium nuclei may fuse to form carbon nuclei. 
• This process shows how heavier elements can be produced in stars through successive fusion stages. 

• The source notes that the Sun is about $$5 \times 10^{9}$$ years old and can continue hydrogen burning for roughly another 5 billion years.

• After the hydrogen in its core is exhausted, the outer envelope of the Sun is expected to expand, and the Sun will evolve into a red giant.

Flow of Events

Hydrogen fusion in the core → helium accumulation → core contraction → rise in temperature → helium fusion → expansion of outer layers → red giant stage.