What phase does a star enter when it begins hydrogen fusion?

A star enters the main sequence phase when it begins hydrogen fusion in its core. This phase is characterized by the process where hydrogen atoms are converted into helium through nuclear fusion, releasing a substantial amount of energy that produces the light and heat we associate with stars. In this phase, the star reaches a state of hydrostatic equilibrium, balancing the gravitational force pulling inward with the thermal pressure from the energy produced by fusion. The main sequence phase is the longest and most stable part of a star's life cycle, lasting millions to billions of years, depending on the star's mass.

The other phases mentioned do not correspond to the onset of hydrogen fusion. For example, in the red giant phase, a star has already exhausted hydrogen in its core and has moved on to fuse helium and other heavier elements. The protostar phase represents the period before fusion starts when the star is still forming and accumulating mass. The white dwarf phase occurs after a star has expended its nuclear fuel and shed its outer layers, leading to a dense, hot remnant. Thus, the correct identification of the main sequence phase ties directly to the initiation of hydrogen fusion in a star's lifecycle.