What process causes the Sun's energy output to gradually increase even during the main sequence?
Answer
Core compression raising temperature and density, accelerating fusion rate
Despite being in a stable phase, the Sun's energy production is not perfectly constant; it is slowly accelerating. This happens because as hydrogen is continuously fused into helium ash in the core, the inert helium ash increases the core's mass density, allowing gravity to win a slight battle against the outward fusion pressure. This resulting gravitational compression causes the temperature and density within the core to rise significantly. This increase in core temperature directly leads to an accelerated rate of nuclear fusion, manifesting as a gradual, steady increase in the Sun's overall brightness and heat output.
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