Fusion of hydrogen to helium

High-temperature hydrogen plasmas are nsed in experiments to produce energy by thermonuclear fusion of hydrogen to helium this is the same reaction by which the energy of the Sun and other stars is generated. 

With respect to the production of energy, the fusion of hydrogen to helium is of special interest ... 

Stars are fueled by nuclear fusion, which we discuss in more detail in Chapter 19. Under the conditions within the core of a star, hydrogen nuclei can combine (or fuse) to form helium. Fusion gives off enormous quantities of energy, which is why stars emit so much heat and light. The fusion of hydrogen to helium can fuel a star for billions of years. 

Bob s description of the energy-producing fusion of hydrogen to produce helium is correct, but a simplification of the actual process. The proton-proton reaction is actually a chain of thermonuclear reactions that are the main energy sources for the Sun and cool, Main Sequence stars.13 In the first step of the reaction, two hydrogen nuclei H (shown as two protons, each symbolized by + in the rectangle of figure 7.10) combine to form deuterium 2H. One of the... 

The 14N atom is the fourth mostabundantnucleus that is produced by nucleosynthesis in the stars (after l60,12C and 20Ne). Its nucleosynthesis occurs primarily in stars of insufficient mass to become supernovae, especially in AGB stars (see Glossary). It is the natural byproduct of the fusion of hydrogen into helium in the carbon-nitrogen cycle. The 14N is created by proton-capture reactions transmuting the initial C and O within the star to 14N during the CN cycle. Production from intial C and O makes 14N a secondary nucleus, since it can be made only in proportion to the amount of C... 

Note that it is possible to obtain the net reaction that occurs during fusion by deleting species that occur on both sides of the above three equations. Before doing so, however, notice that the last step (Step 3) can occur only after steps 1 and 2 have first occurred twice. That is, two helium-3 nuclei are required in order for Step 3 to occur. Therefore, it is necessary for Step 2 (and, of course, Step 1) to have taken place twice to produce that many helium-3 nuclei. The net reaction for the fusion of hydrogen into helium, then, can be calculated as follows ... 

Only 150 million km away (considered close in astronomical terms), the Sun provides the energy needed to support life on Earth. The Sun makes its energy through the fusion of hydrogen to make helium. Scientists have determined that the core of the Sun is composed of approximately 50% heUum, leaving enough hydrogen for the Sim to burn for another 5 billion years. 

The fusion of hydrogen to form helium is the dominant nuclear reaction for most of a star s lifetime. Toward the end of its life, the hydrogen in the star s core is exhausted and the helium atoms fuse to form progressively heavier elements. A select few stars end their lives in dramatic supernova explosions such as the one shown in the chapter opening photograph. The nuclear reactions that occur when a star goes supernova are responsible for the existence of all naturally occurring elements heavier than nickel. 

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