Fusion

Process in which two lighter atomic nuclei combine to form a heavier atomic nucleusCore of an atom, where nearly the entire mass and all positive charge is concentrated. It consists of protons and neutrons.. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion reactions involving light elements release large amounts of energy. The mass of the resulting nucleus is less than the combined masses of the two original nuclei. The difference in mass, Δm (mass defect), is released as energy:

Fusion of elements up to ⁵⁶Fe results in the release of energy. Thermonuclear fusion powers stars such as our own sunOur parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region. Image source: http://eclipse99.nasa.gov/pages/SunActiv.html. The core is the hot, dense central region in which the. Main sequence stars are dominated by hydrogen burningProcesses by which hydrogen (1H) is fused into helium (4He) with in a star. The five possible fusion paths can be divided into two sets of processes: the Proton-Proton (PP) process, which depends only on the amount of H and He in the star, and the CNO cycle (carbon-nitrogen-oxygen), which fusion reactions. In red giants, He is converted into C by the triple-alpha processProduction of 12C by fusion of 3 4He nuclei (α particles). Helium produced in hydrogen burning cannot undergo fusion reactions because of a stability bottleneck. The two most likely reactions for 4He fusion are: However, both products are unstable, and decay before they can undergo any further reactions. Only at. In highly evolved high-mass stars, fusion reactions synthesize a succession of elements up to Fe by helium captureFormation of heavy elements by the capture of helium nuclei. For example, at ~6 x 108 K, carbon can form heavier elements by fusion with other carbon nuclei: However, fusion is more likely to occur by helium capture, which requires less energy and lower temperatures (~2 x 108 K): Similarly,.

For the elements heavier than ⁵⁶Fe, scientists hypothesize there are other formation mechanisms including for example the merger of neutron stars that may be responsible for the formation of the lighter heavy elements.