Thermonuclear Reactions
Nuclear reactions, highly energetic processes involving subatomic particles found within the nuclei of atoms, exist in two general forms. Fission reactions involve the splitting of atomic nuclei into smaller subatomic particles, in the process releasing relatively large amounts of energy. Fission chain reactions are responsible for the massive explosions of the atomic bombs that were employed during World War II. The second variety, fusion reactions, accomplish just the opposite.
Fusion reactions create larger atoms from the nuclei of smaller atoms. Here, when the nuclei of two or more atoms collide with sufficient force, they fuse to form a single, larger nucleus. During the fusion process, part of the mass of the fused nucleus is converted into energy. The energy produced is then released as heat, light, and various forms of radiation. Such nuclear reactions require extremely high temperatures to induce the necessary collisions. Critical temperatures for fusion range from 50 to 400 million degrees Celsius (122 to 752 million degrees Fahrenheit). Because such immense amounts of heat energy are required, fusion reactions are also called thermonuclear reactions. Thermonuclear chain reactions in turn release massive amounts of energy once started, and are utilized by thermonuclear bombs (also known as hydrogen bombs). Thermonuclear warheads have destructive potentials many thousands of times greater than their fission counterparts.
As imposing as man-made thermonuclear reactions are, they are merely tiny reproductions of much larger and more significant thermonuclear processes found naturally throughout the universe. The stars themselves are formed and fueled by thermonuclear reactions. Stars begin their lives as massive, nebulous interstellar clouds of gas, consisting mostly of hydrogen. As gravity condenses the hydrogen, the clouds become more and more dense. In the process, the condensed matter becomes hotter over time, reaching many millions of degrees. Eventually, the heat energy becomes great enough to initiate thermonuclear reactions, and a new star is born.
Once created, the condensed gaseous matter continues to fuel the thermonuclear reactions of stars. For example, the massive amounts of solar energy released by the Sun are produced by thermonuclear fusion of its gaseous contents. In the 1930s, it was discovered that the Sun derives its energy from the fusion of hydrogen atoms, in a process called the proton-proton reaction. This thermonuclear process occurs in relatively cool stars. The proton-proton cycle involves the fusion of four hydrogen nuclei (protons) to form a single helium nucleus. Therefore, life as it exists on earth is made possible by only by thermonuclear reactions. A different kind of thermonuclear reaction, called the carbon-nitrogen cycle, creates most of the energy radiated by stars much hotter than the Sun. The carbon-nitrogen cycle involves isotopes of the elements hydrogen, carbon, nitrogen, and oxygen that are formed during a complex chain of fusion events, each releasing energy. As long as the gaseous elemental fuel for their fusion reactions remains plentiful, the stars in the cosmos continue to shine with thermonuclear brilliance.
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