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Which two competing effects determine the size of a star?

June 1, 2025 by rawan239

Which two competing effects determine the size of a star?

Nuclear fusion and electrostatic effects

Nuclear fusion and magnetic effects

Nuclear fusion and gravitational effects

Gravitational and electromagnetic effects

This question was previously asked in

UPSC Geoscientist – 2022

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The size of a stable star is determined by the balance between the inward force of gravity and the outward pressure generated by nuclear fusion in its core.

Gravity constantly pulls the star’s mass inward, tending to collapse it. Nuclear fusion reactions in the core, primarily converting hydrogen to helium, release enormous amounts of energy in the form of photons (radiation) and particles. This energy creates a strong outward pressure (radiation pressure and thermal pressure) that counteracts gravity. A star remains stable, maintaining a relatively constant size, when these two opposing forces are in equilibrium, known as hydrostatic equilibrium.

While other effects like magnetic fields can influence stellar activity and structure in specific regions or phases, the primary factors determining the overall size and stability of a star throughout its main sequence life are the balance between self-gravity and the pressure from nuclear fusion.

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