What core condition must be met to achieve the sustained state of a main-sequence star?
Answer
Initiation of sustained hydrogen fusion
The transition from a pre-main-sequence object (like a protostar or T Tauri star) to a true, long-lived main-sequence star is fundamentally defined by a specific thermonuclear event within its core. The object must attain a core temperature of approximately 15 million Kelvin. At this critical threshold, hydrogen nuclei begin fusing into helium at a rate sufficient to generate a massive outward pressure. This outward pressure perfectly balances the inward force of gravity, achieving a state known as hydrostatic equilibrium, which allows the star to shine steadily for billions of years, representing about 90% of its total active existence.
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