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Fusion

What makes the Sun tick?

Fusion is the combining of two nuclei to form a new species, accompanied by the release of energy.

Fusion energy is a much-vaunted possible alternative means of generating electrical power, but the technology still illudes us. However, the fact remains that most of the universe is driven by fusion power: in stars.

Our Sun is mainly hydrogen:

ElementPercentage composition
Hydrogen73.46%
Helium24.85%
Oxygen0.77%
Carbon0.29%
Iron0.16%
Neon0.12%
Nitrogen0.09%
Silicon0.07%
Magnesium0.05%
Sulphur0.04%

The Sun has been burning hydrogen fuel for about 6 billion years, and is about half way through this phase of its life cycle. When it has depleted a critical amount of its hydrogen, its radiation pressure/gravity equilibrium will be lost. It will initially expand to one AU (distance Earth-Sun) radius, losing a third of its mass, before collapsing. At this stage, the star will continue with helium fusing, resulting in the fusing of carbon and oxygen. The exact evolutionary path of a star depends on its mass.

Sun profile

Hydrogen on Earth is mainly the $^1_1H$ isotope, meaning it has only one proton in its nucleus, with a single electron. There are rarer isotopes of $^2_1H$, deuterium, and $^3_1H$, tritium. The nucleus of a helium-4 atom is known as an alpha-particle.

Under the conditions of extreme pressure and temperature of the Sun, the following fusion reaction occurs:

$$4^1_1H {→} ^4_2He + 2^0_1e + 2^0_1ν_e + ^0_0γ$$

The combination of two nuclei with low binding energy will release energy and produce a nucleus with higher binding energy.

Content © Andrew Bone. All rights reserved. Created : March 1, 2014 Last updated :May 18, 2016

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