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Gaetano Valentini
ENERGY PRODUCTION IN THE UNIVERSE:
STELLAR NUCLEAR REACTIONS |
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The stellar energy production
is one the most important issues in the present astrophysics.
To understand the various celestial phenomena, the phases of
the stellar evolution, the chemical content in the
interstellar medium and in the planetary systems and,
finally, the appearance of life on Earth is necessary to
have a deep knowledge on the physics of the stellar energy
production.
A star bears from the gas contraction of an interstellar cloud,
when the gravitational energy wins the thermal particle
energy.
This contraction is in action until the equilibrium
condition is reached: as demonstrated by Virial theorem: |
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In particular,
to support the star’s spherical symmetry is necessary that
pressure radial forces, which favour expansion, and
gravitational radial forces, which favour contraction, are
equal: |
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This equilibrium is one of
the major characteristics during stellar evolution and it
will be finally broken only during the possible supernova
explosive phase.
Moreover, the Virial theorem indicates also the relation
between gravitational energy and thermal energy: an increase
of the gravity causes a growth of the gas temperature and
then of the pressure: |
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In other words, half of the
gravitational energy emitted during the contraction must
increase the thermal content of the protostellar structure.
In the interstellar gas cloud the temperature progressively
rises, favouring the nuclei ionization, until the nuclear
reactions become efficient.
Then the energy production starts, with the energy being
transferred through transport mechanisms (conduction and
convection) in the outer stellar region: a star is born. |
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