Through the evolution of the star the only force opposing the gravitational collapse is the pressure of the stuff the central blob is made of; this pressure is initially very small compared to the pull of gravity. This means that the blob will contract until pressures and temperatures at the center are so high that nuclear reactions turn on. At this point the energy release from the fusion reactions heats up the stellar material, this in its turn increases the pressure and the contraction stops. As mentioned above, the main reaction occurring at this stage consume Hydrogen and produce Helium: the star ``burns'' Hydrogen into Helium. This goes on for a long time: if the star is light (as our sun) it proceeds for about 10 billion years, much heavier stars use up Hydrogen much faster (for the heaviest ones it takes `only' 1 million years).