What balance stops the contraction process once nuclear fusion begins?
Answer
Hydrostatic equilibrium
Once sustained nuclear fusion ignites in the core, the object undergoes a fundamental change in its structural dynamics. The massive outpouring of energy generated by the fusion of hydrogen into helium creates a powerful, sustained outward pressure. This outward pressure perfectly counteracts the constant, relentless inward crushing force exerted by the star's immense gravity. This precise and stable balance between the inward gravitational force and the outward thermal pressure from fusion energy is termed hydrostatic equilibrium. Achieving this state signals the end of the contraction phase and marks the official entry of the object onto the Main Sequence.
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