In stellar evolution, what begins when fusion of hydrogen into helium starts?

The phase of stellar evolution that begins when hydrogen fusion into helium starts is the main-sequence star phase. During this stage, a star fuses hydrogen nuclei into helium, which produces energy that counteracts gravitational collapse, allowing the star to achieve a stable state. This hydrogen fusion occurs in the core of the star, and it is characterized by a balance between gravity trying to pull the star inward and the pressure from fusion pushing outward.

In this main-sequence stage, the star remains in this stable state for the majority of its life, which can last billions of years, depending on the star's mass. The temperature and pressure in the core are sufficient to sustain nuclear fusion, which is the dominant process powering the star.

The other stages mentioned, such as red supergiants, white dwarfs, and neutron stars, occur later in the lifecycle of a star, following the main sequence. Red supergiants represent a later state that occurs after a star exhausts its hydrogen fuel and begins fusing heavier elements. White dwarfs and neutron stars are the remnants of stars after they have undergone significant changes, including supernova explosions or other cataclysmic events in their lifecycle. Thus, the initiation of hydrogen fusion directly marks the entrance into the