Figure 9.9: Nuclear fusion is a nuclear reaction in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles (neutrons and/or protons). The proton-proton chain dominates in stars the size of our sun or smaller, while the carbon-nitrogen- oxygen cycle dominates in stars heavier than the sun. A supernova explosion can produce enough energy to fuse nuclei into elements heavier than iron.
Figure 9.10a: Some hundred thousand years after the Big Bang, protons and electrons cooled down enough to settle into atoms of hydrogen and helium. After millions of years stars began to form from collapsing clouds of hydrogen. Fusion converts the core’s hydrogen into helium. In stars larger than eight times the mass of our sun, helium fuses to carbon, carbon and helium into oxygen, and two carbon atoms into magnesium. Through such processes, successively heavier elements, up to iron, are formed. Since iron fusion uses energy instead of producing it, the outward pressure that until then has counterbalanced the inward pressure of gravity, diminishes. The star’s days are numbered. Gravity takes over, and it rapidly implodes until a point where it explodes, blowing the rest of the star to bits in a supernova explosion. The elements which make up our planet and our bodies were originally made in a star. We are the children of stars!