Final answer:
Main-sequence stars, like the sun, maintain a stable size by balancing the inward force of gravity with the outward pressure caused by nuclear fusion reactions in their core.
Step-by-step explanation:
A main-sequence star, like the sun, maintains a stable size due to a balance between the inward force of gravity and the outward pressure caused by nuclear fusion reactions in its core.
The sun's core, where nuclear fusion occurs, generates an enormous amount of energy by converting hydrogen nuclei into helium. This energy is released in the form of photons, which travel outwards, exerting pressure on the layers above.
Gravity, on the other hand, pulls the layers of the star inward. The equilibrium between the outward pressure and inward gravitational force keeps the star stable in size and prevents it from collapsing.
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