ABSTRACT
In the case of the proton cycle in stars, this barrier is penetrated by tunneling, allowing the process to proceed at lower temperatures than that which would be required at pressures attainable artificially on the earth. Generally, when dealing with elements lighter than iron, the lower the ratio of atomic mass to mass number, the heavier the nucleus will be. This is known as mass defect. Fusion of lighter nuclei into heavier nuclei leads to loss of mass when the energy of binding is removed (this energy carries away the lost mass), even though no nucleons are lost. When the nucleons group together to form a nucleus, they lose a small amount of mass, that is, there is mass defect. This lost mass is present in the released energy in accordance with E = Δmc2, where Δm is the loss of mass [51]. In nuclear reactions, the energy that must be radiated or otherwise removed as binding energy may be in the form of electromagnetic waves, such as gamma radiation, or as heat. Again, however, no mass deficit can in theory appear until this radiation has been emitted and is no longer part of the system. This energy is a measure of the forces that hold the nucleons together, and it represents energy which must be supplied from the environment if the nucleus is to be broken up. It is known as binding energy, and the mass defect is a measure of the binding energy because it simply represents the mass of the energy which has been lost to the environment after binding. Most of the excess binding energy is released as kinetic energy of the resulting particles. When these particles are slowed, this energy is available to do work or be converted to electromagnetic radiation or heat. Fusion reactions power the stars and produce virtually all elements in a process called nucleosynthesis. The fusion of lighter elements in stars releases energy (and the mass that always accompanies it). The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. There are extreme astrophysical events that can lead to short periods of fusion with heavier nuclei. This is the process
that gives rise to nucleosynthesis, the creation of the heavy elements during events like supernovas. 3.2 Hydrogen Burning in Stars In the nebular hypothesis, the majority of the mass of the dust cloud collects at the center. The intense gravitational forces present ultimately lead to nuclear fusion taking place. As most of the matter initially present in the nebula is hydrogen, the process of hydrogen burning takes place. The general scheme of the hydrogen burning is shown in Fig. 3.1. The main solar nuclear fusion process is so called the proton-proton chain reaction that is the dominant fusion process in the sun. This phenomenon is possible due to tunneling. There are a number of stages through which nuclear fusion in the sun occurs, in a proton-proton chain reaction. Through this process, hydrogen fuel is progressively converted into helium, along with release of energy in the process.
