What physical state defines a star achieving main sequence status?
Answer
Hydrostatic equilibrium
A star reaches the main sequence when it establishes a dynamic balance known as hydrostatic equilibrium. This equilibrium is a precise, continuous counteraction between two massive forces acting upon the star's structure. On one side is the crushing, inward force exerted by the star's own immense gravity, constantly trying to compress the stellar material toward the center. Opposing this is the powerful outward pressure generated by the heat resulting from sustained nuclear fusion reactions occurring within the core. As long as there is sufficient hydrogen fuel in the core to maintain this outward pressure, the star remains stable in the main sequence phase.
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