|MadSci Network: Earth Sciences|
All of the elements heavier (more massive) than hydrogen are formed within stars, such as our sun, by a somewhat complicated process called nucleosynthesis.
Nucleosynthesis is a process of building more massive atomic nuclei by combining certain basic components - principally protons, neutrons, and alpha particles. Protons and neutrons, as you are probably aware, are the fundamental particles that make up the nuclei of all atoms, and therefore of all chemical elements. Protons are positively charged particles and neutrons are uncharged particles. Alpha particles are the nuclei of helium, the next element heavier than hydrogen, and consist of two protons and two neutrons.
This process is also the principal source of the energy that makes stars shine. When two or more of these particles combine to form a heavier nucleus the product has slightly less mass than that of its components. This "missing mass" has been converted to energy, which heats the star. Remember Einstein's formula E = mc2, which shows that energy and mass are interchangeable?
The great majority of a star's life is spent combining (or "burning") protons (the nuclei of hydrogen), neutrons, and alpha particles in several steps to produce the nuclei of the lighter elements. The force that drives this process is simple gravity, but in the core of larger stars this can squeeze material together to create densities more than 10,000 that of water and temperatures greater than 100 million °C. This process produces most of the elements up to about the mass of iron.
Uranium is the heaviest naturally occurring element and cannot be formed by a simple combining of lighter particles in the "stellar kitchen" model described above. The nucleus of uranium has far more neutrons in it than protons and can only be created in an environment of tremendous neutron flux - a supernova explosion!
Uranium and the other very heavy elements are produced by the same process that scatters them and the rest of their star throughout a vast region of their galaxy, to be swept up and used as raw material in forming the next stage of stars and planets - one of the basic operations of cosmology.
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