ORIGIN AND ABUNDANCE OF THE ELEMENTS

The synthesis of elements needs nuclear reactions, of which the most important type is the fusion of two light nuclei to prepare one of higher mass and charge. The attractive strong interaction, which holds neutrons and protons together, operates only over very short distances (around 10-15 m) and is opposed at longer size by the electrostatic repulsion between positively charged protons. To get two nuclei close enough together for fusion needs enormously high energies, which are usually found only at high temperatures (above 107 K) in the interior of stars. Under such conditions the chemical properties of components are irrelevant, as no compounds may exist, atoms being in highly ionized states stripped of their electrons.

Gases thrown out by a supernova cool, and can subsequently be incorporated into new stars. The formation of planetary systems can be common in the Universe. Studies suggest that other planets and the Earth formed about the same time as the Sun. While the Sun formed at the center, chemical reactions in the cooler outer parts of the gas concentration produced solid particles, which collected under gravitational forces, first into small bodies known as planetesimals, and subsequently into the planets. In the outer parts of the Solar System temperatures were low enough to form 'ices' of water, carbon dioxide, solid methane and ammonia, which are constituents of the giant planets Saturn and Jupiter.