The sun is the source of all life on our planet. It is a huge fusion reactor that works by merging lighter atoms into heavier atoms (hydrogen into helium). This can happen only with the very high temperatures and pressure that are in the nucleus of the sun. By weight, the sun is 70% hydrogen, 28% helium, 0.5% carbon, 0.5% nitrogen, 0.5% oxygen, and 0.5% other elements.