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The big bang which created the universe only created the elements Hydrogen H and Helium He and possibly a very small amount of Lithium Li However a glance at the periodic table of the elements shows that today some 15 billion years after the big bang there are at least 108 known elements Every atom of every element heavier than Li has been produced since the big bang The factories which make these elements are stars Nucleosynthesis or the synthesis of nuclei is the process by which stars which start out consisting mostly of H and He produce all other elements The key is nuclear fusion in which small nuclei are joined together to form a larger nucleus This contrasts with nuclear fission in which a large nucleus breaks apart to form two smaller nuclei Fusion requires an extremely large amount of energy see fig 1 and can typically only take place in the centers of stars FIGURE 1 a Low energy proton is strongly repelled by the 7Be nucleusb High energy proton moves so fast that it can strike the 7Be nucleus Once the proton touches the nucleus it has a chance to stick If the proton sticks the 7Be becomes a 8B nucleusc 8B is radioactive and changes into 8Be plus a positron b and a neutrino n 8Be is itself radioactive and almost immediately breaks into two 4He nuclei Protons repel each other This repulsion becomes stronger as the protons get closer together just like when you try to stick two magnets together north to north or south to south Try this As you push the magnets closer together it becomes harder to do However if the protons can actually touch each other they have a chance to stick together This is because of the strong nuclear force which attracts nucleons protons or neutrons together and is much stronger at close range than the electromagnetic force repulsion
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