What provides the powerful, outward-directed thermal pressure that finally counteracts the inward crush of gravity at fusion ignition?
Answer
The energy released by fusion in the core
The contraction phase is fundamentally halted when the core reaches the ignition temperature for thermonuclear reactions. At this point, the fusion of hydrogen into helium begins releasing enormous amounts of energy within the core itself. This newly generated energy manifests as a powerful, outward-directed thermal pressure. This fusion-powered pressure is finally robust enough to successfully counteract and balance the sustained, crushing inward force exerted by the star's entire mass, thereby stopping the large-scale gravitational shrinking that characterized the protostar stage.
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