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2. Stellar thermonuclear reactions

At normal temperatures the particles in a gas are neutral. But at temperatures above a few electron volts (eV), the individual particles tend to separate into their constituent elements (ions and electrons) and the gas gets transformed into a mixture of charged particles, that is, a plasma.
About 99% of the matter making up the Universe consists of plasma - also called the “fourth state of matter”. It is the main constituent of the sun and stars. The sun’s interior temperature is 14 million degrees and here the reaction resulting from the fusion of hydrogen nuclei is responsible for most of the energy which reaches us on earth in the form of heat and light (and solar neutrinos).

solar fusion reaction

In the hotter or more massive stars, other reactions predominate. At around 15-20 million degrees of temperature, these reactions are based on the carbon cycle, in which carbon-12 acts as a catalyst to fuse 4 protons to a nucleus of He4, two positrons, two neutrinos, and a gamma, producing 26.63 MeV of energy (5% of which associated with the neutrinos produced).
Stellar evolution depends on the fusion energy and on gravitational energy. In a young star, consisting mostly of hydrogen atoms, gravitational energy is dominant, the star contracts, and its temperature and density increase until fusion reactions become significant enough for energy to be released. The gravitational and nuclear stages evolve sequentially at ever higher temperatures and densities and the burning nuclei become increasingly charged until they react into iron nuclei with maximum binding energy. At this point, the nuclear reactions begin to absorb energy rather than produce it.

Photograph of solar flaresPhotograph of solar flares


To obtain a fusion reaction, the hydrogen plasma has to be confined in a limited space. In the sun this situation is brought about by enormous gravitational forces. The fusion process in the sun occurs extremely slowly, which is why it has kept on shining for billions of years.



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