The understanding that fusion reactions are responsible for energy production in stars brought the accompanying realization that such reactions might be a source of energy for human needs. Since the temperatures needed for fusion are in the millions of degrees, fusion reactions are also known as thermonuclear ( “thermo ” for heat) reactions. Thereafter, gravity keeps a star together and fusion keeps it hot. The reason that fusion reactions can occur in stars is that when they first form by gravitational compression of drifting gas, the temperatures in their centers are raised high enough to bring about fusion. The only net difference between this reaction and Bethe ’s carbon cycle is the amount of energy involved in the overall set of reactions. The net result of this sequence of reactions would be the combining of four protons (hydrogen nuclei) to form a single helium –4 nucleus. Finally, the helium –3 nucleus could fuse with a fourth proton to form a helium –4 nucleus. That deuteron could, then, fuse with a third proton to form a helium –3 nucleus. Another approach, for example, would be one in which two protons fuse to form a deuteron. The Bethe carbon –cycle is not the only nuclear fusion reaction possible. Bethe was able to show that the total amount of energy released by this sequence of reactions was comparable to that which is actually observed in stars. Net result of which is that four protons disappear and are replaced by one helium atom.īethe chose this sequence of reactions because it requires less energy than does the direct fusion of four protons and, thus, is more likely to take place in a star. The reaction then continues through a series of five more steps, the In 1939, the German –American physicist Hans Bethe worked out the mathematics of fusion energy release, in which a proton first fuses with a carbon atom to form a nitrogen atom. Certainly no familiar type of chemical reaction, such as oxidation, could possibly explain the vast amounts of energy released by even the smallest star over billions of years. Naturally occurring fusionĪs early as the 1930s, a number of physicists had considered the possibility that nuclear fusion reactions might be the mechanism by which energy is generated in stars. For example, in order to fuse two protons with each other, enough energy must be provided to overcome the force of repulsion between the two positively charged particles. That means that fusion reactions always require very large amounts of energy in order to overcome the force of repulsion between two like –charged particles. Except for the neutron, all of these particles carry at least one positive electrical charge. The particles most commonly involved in nuclear fusion reactions include the proton, neutron, deuteron, a triton (a proton combined with two neutrons), a helium –3 nucleus (two protons combined with a neutron), and a helium –4 nucleus (two protons combined with two neutrons). Nuclear fusion reactions are like nuclear fission reactions, therefore, in the respect that some quantity of mass is transformed into energy. This “loss ” of mass is expressed in the form of 2.23 MeV (million electron volts) of kinetic energy of the deuteron and other particles and as other forms of energy produced during the reaction. When a proton and neutron combine, for example, the mass of the resulting deuteron is 0.00239 atomic mass unit less than the total mass of the proton and neutron combined. This missing mass corresponds to energy released in the reaction. In general, the mass of the heavier nucleus thus produced is less than the total mass of the two lighter nuclei. As an example, a proton (the nucleus of a hydrogen atom) and a neutron will, under the proper circumstances, combine to form a deuteron (the nucleus of an atom of deuterium, an isotope of hydrogen). Nuclear fusion is the process by which two light atomic nuclei combine to form a heavier atomic nucleus.
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