What is Nuclear Fusion?
- A reaction in which two or more lighter nuclei fuse to form a heavy nucleus and a large amount of energy is given out is called Nuclear fusion reaction.
- Fusion is the process that powers active or main-sequence stars or other high magnitude stars. The Sun is a main-sequence star and thus generated its energy by nuclear fusion of hydrogen into helium. In its core, the sun fuses 500 million metric tons of hydrogen each second into Helium.
- It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen. When accelerated to high enough speeds, nuclei can overcome this electrostatic repulsion and be brought close enough such that the attractive nuclear force is greater than the repulsive Coulomb force. The strong grow rapidly once the nuclei are close enough, and the fusing nucleons can essentially "fall" into each other and the result is fusion and net energy produced. The fusion of lighter nuclei, which creates a heavier nucleus and often a free neutron or proton, generally releases more energy than it takes to force the nuclei together. This is an exothermic process that can produce self-sustaining reactions.
Nuclear Fusion in Stars and its constitution-
An important fusion process is a stellar nucleosynthesis that powers stars including the Sun. In the 20th century, it was recognized that the energy released from nuclear fusion reactions accounted for the longevity of stellar heat and light. The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides that energy and synthesizes new nuclei as a byproduct of the fusion process. Different reaction chains are involved, depending on the mass of the star (and therefore the pressure and temperature in its core).
Around 1920, Arthur Eddington anticipated the discovery and mechanism of nuclear fusion processes in stars, in his paper The Internal Constitution of the Stars. At that time, the source of stellar energy was a complete mystery; Eddington correctly speculated that the source was the fusion of hydrogen into helium, liberating enormous energy according to Einstein's energy-mass equation E = mc2. This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed of hydrogen, had not yet been discovered. Eddington's paper, based on knowledge at the time, reasoned that -
1. The leading theory of stellar energy, the contraction hypothesis, should cause stars' rotation to visibly speed up due to conservation of angular momentum.
2. The only other known plausible source of energy was the conversion of matter to energy; Einstein had shown some years earlier that a small amount of matter was equivalent to a large amount of energy.
3. Francis William Aston had also recently shown that the mass of helium(He) atom was about 0.8% less than the mass of the four hydrogen atoms which would, combined, form a helium atom, suggesting that if such a combination could happen, it would release considerable energy as a byproduct.
4. If a star contained just 5% of fusible hydrogen, it would suffice to explain how stars got their energy. (We now know that most 'ordinary' stars contain far more than 5% hydrogen).
5. Further elements might also be fused, and other scientists had speculated that stars were the "crucible" in which light elements combined to create heavy elements, but without more accurate measurements of their atomic masses, nothing more could be said at the time.
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