Source: Origins, A NOVA Presentation: "Back to the Beginning"
Fusion, which occurs when atomic nuclei combine to form new elements, is extremely powerful. All the stars in the universe, including the Sun, are nuclear furnaces fueled by fusion. Through fusion, stars are responsible for forming all the naturally occurring elements heavier than hydrogen and helium -- before stellar nucleosynthesis, only the lightest elements existed in the universe. This video segment adapted from NOVA illustrates the critical role that stars play in creating the elements.
All matter is made up of atoms -- elements comprised of smaller particles such as protons, neutrons, and electrons. The number of protons within the nucleus -- the central component of the atom -- determines the type of element. An element can have different forms, called isotopes, based on the number of neutrons in the nucleus. For example, an ordinary hydrogen nucleus contains just one proton. But deuterium, an isotope of hydrogen, has one proton and one neutron in its nucleus.
The entire universe shares a common set of elements. In the very early universe, the only elements were hydrogen and helium. But since the formation of stars, lighter elements within the stars began fusing to create heavier elements, producing all the other naturally occurring elements. Under the extremely high temperatures and pressures within the core of stars, atoms collide at high enough speeds to overcome the usual electromagnetic repulsion of nuclei, allowing nuclear fusion to occur.
All stars live by fusing hydrogen into helium. In the first step of the process, two hydrogen atoms fuse to form deuterium. In the next step, another hydrogen atom fuses with the deuterium, creating a rare isotope of helium that has two protons and one neutron in its nucleus. In the third step, two of the rare helium atoms fuse to create a single normal helium atom and two hydrogen atoms. The fusion pathway described above requires six hydrogen atoms to create one helium atom -- however, there are two hydrogen atoms left over at the end of the process. The net result is that it takes four hydrogen atoms to make one helium atom. The energy that fuels a star is a result of the difference in mass between the original four hydrogen atoms and the resulting helium atom. Following Einstein's mass-energy relationship, E=mc2, the missing mass is converted to energy.
At even higher temperatures and pressures, heavier elements are able to form. Many are made from a process called "helium capture," in which a heavier element fuses with a helium atom. For example, helium fuses with carbon to make oxygen, and helium fuses with oxygen to make neon. Heavier nuclei can also fuse with each other, such as when carbon and oxygen fuse to make silicon or two silicon atoms fuse to make iron. Eventually, the interior of a massive star begins to resemble an onion, with different elements being created in different layers. However, elements heavier than iron are only produced in the extraordinary conditions created by the collapse and explosion of a star -- a supernova.
Learn in this NOVA classroom activity about the elements and their roles in the universe.
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