When does helium fusion occur in an evolution of star??? Well, i just need help in this confusing part of a star's life. First of all, when the sun finishes the H-fuel at core, why doesn't it immediatly use the H-fuel on the surface.. And then another doubt is, what causes the expansion of the Star and when does the helium fusion occur.. Before or after becoming a red giant. My point of view is that after the H-fuel at core ends, the helium inside is at high pressure, causing the particles around the star exert a pressure on h at surface causing fusion at higher rates which leads to expansion ... But the other question is that if energy is so much , the g is also so much .. isn't it?? I just need good responses and not just links to wikipedias... Thanks in advance to whoever dominates.. I gotta tough exams within days!