The protostar's heat, and therefore brightness, is derived from the increased energy of motion of its particles due to gravitational collapse. By the end of a few more million years spent as a protostar however, things slowly change. At this time the protostar is said to approach the main sequence stage. The outward pressure force is balanced by the inward pull of gravity. Our Sun is a middle-age, main sequence star, having age of 4.5 billion years.
Some hydrogen still exists around the helium core, in a spherical shell.
This is so because on the outer part of the initially all hydrogen core, the temperatures are relatively lower than in the inner core, and the fusion of hydrogen does not occur at the same rapid rate as the inner core's hydrogen. As a result of the exhausted hydrogen supplies, fewer fusion reactions occur, producing less energy and consequently less radiation pressure. But remember, the radiation pressure is needed to produce the outward pressure force that balances the inward pull of gravity. With a smaller outward radiation pressure force, the interior of the star begins contracting under the influence of a stronger force of gravity. This contraction keeps the temperature high enough to maintain the fusion of hydrogen in the spherical shell around the helium core, and at the same time produces even higher temperatures in the inner core required for the fusion of helium.