Maturity of A Star
Eventually, the core exhausts its supply of hydrogen, and moves off the main sequence (if it was there at all). Without the outward pressure generated by the fusion of hydrogen to counteract the force of gravity, it contracts until either electron degeneracy becomes sufficient to oppose gravity or the core becomes hot enough (around 100 megakelvins) for helium fusion to begin. Which of these happens first depends upon the star's mass.
Read more about this topic: Stellar Evolution
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