At which stage of a star's life cycle does it begin to fuse hydrogen into helium?

In the life cycle of a star, the process of hydrogen fusion into helium begins during the main sequence stage. At this point, a star has reached a stable state where the gravitational forces pulling inward are balanced by the outward pressure generated from nuclear fusion in its core. The star primarily converts hydrogen into helium through a series of nuclear reactions, producing energy that causes it to shine.

During the protostar phase, the star is still in the process of forming and has not yet initiated nuclear fusion. In later stages, such as the red giant phase, the star moves on to fuse helium and other heavier elements as it exhausts its hydrogen supply. The white dwarf stage occurs after a star has completed its active life, where it no longer undergoes fusion and is in the process of cooling and fading. Thus, the main sequence stage is crucial for a star's development, as it marks the transition to sustained hydrogen fusion.