Answer:
The luminosity of a star remains approximately constant during its time on the main sequence. There is a slight increase over time, which is, as you guessed, related to the changing composition of the star due to fusion.
When a star's luminosity and temperature begin to change, the point that represents the star on the H–R diagram moves away from the zero-age main sequence. ... If the rate of fusion goes up, the rate at which energy is being generated also increases, and the luminosity of the star gradually rises.
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