The energy generation rate in the core of a star balances the gravitational force of the material in the star. A more massive star generates more energy, thereby holding itself up.
The energy generation rate depends on the temperature and density in the region where fusion occurs. Where the temperature and density are higher, fusion proceeds more furiously, both because the colliding nuclei are moving faster and because the higher density increases the likelihood that a collision will occur.
If the energy generation rate is too high, then the luminosity produced in the core Lc is too great, and the core expands, subsequently cooling, and thus restoring the proper rate of energy production.
Likewise, if the the energy generation rate is too low, then not enough luminosity is produced to balance the inward pull of gravity. Therefore, the core contracts, therby heating up so that the reaction rate is restored to its equilibrium level.
For more on research in controlled thermonuclear fusion, checkout this web site.
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