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Gaetano Valentini

 

ENERGY PRODUCTION IN THE UNIVERSE:

STELLAR NUCLEAR REACTIONS

 

 

 

 

 

 

    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:

 

 

   In particular, to support the starís spherical symmetry is necessary that pressure radial forces, which favour expansion, and gravitational radial forces, which favour contraction, are equal:

 

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:

 

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.