What mechanism maintains balance within a star existing on the Main Sequence?

Answer

Hydrostatic equilibrium balancing gravity and thermal pressure.

The Main Sequence is defined as the longest and most stable phase in a star's existence, powered by core hydrogen fusion. This stability is achieved through a precise, dynamic balance known as hydrostatic equilibrium. This equilibrium requires the relentless, inward crushing force exerted by the star's own massive gravity to be perfectly countered by the immense, outward thermal pressure generated by the thermonuclear reactions occurring in the stellar core. If gravity slightly overcomes pressure, the star contracts and heats up, increasing fusion until pressure reasserts itself; conversely, if pressure dominates, the star expands, cooling the core until gravity causes a contraction, thus maintaining a near-constant size and luminosity over billions of years.

What mechanism maintains balance within a star existing on the Main Sequence?

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