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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).
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 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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