Answer:
1) In fusion reactions, two light atomic nuclei fuse together to form a heavier nucleus. In doing so, they release a comparatively large amount of energy that arises from the binding energy, creating an increase in temperature of the reactants.
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3) When a large, fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. However, one neutron does collide with an atom of uranium-235, which then splits and releases two neutrons and more binding energy.
4) Iron cannot release energy by fusion because it requires a larger input of energy than it releases. This energy blows the outer layers of the star off into space in a giant explosion called a supernova (plural: supernovae.)
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