The nuclear furnace is also the source of a star's stability.

The early life of a star is dominated by GRAVITY which causes all the layers of the gas cloud to contract toward the center.

The contraction causes increased temperature and density in the stellar interior.

If nuclear reactions did not occur the star would continue to collapse in on itself. However, at high enough temperatures, nuclear fusion reactions commence and increase the interior pressure.

The star reaches an equilibrium state ("hydrostatic equilibrium") in which the outward force caused by changes in pressure (the pressure gradient) is exactly balanced by the inward force of gravity.

A star spends most of its life burning hydrogen into helium in this stable configuration ("the main sequence" in the Hertzsprung - Russell diagram).