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protoplanetary disk
Artist’s impression of a brown dwarf surrounded by a protoplanetary disc in which smaller bodies (planetesimals) are growing and forming into planets. (NASA/JPL-Caltech/T Pyle [SSC])


Nuclear fusion, formation of the elements, and planetary systems

From the 1930s onwards, the growing understanding of how nuclear reactions synthesize elements in the Sun and stars allowed the development of detailed models of stellar evolution. A star like the Sun is presently fusing hydrogen to form the next simplest element, helium. Later in its life it will become a red giant star and fuse helium to make carbon, nitrogen and oxygen. More massive stars continue this sequence on to the formation of elements such as silicon, magnesium and sulphur, through to iron.

In a classic paper of 1957, Margaret Burbidge, her husband Geoffrey Burbidge, William Alfred Fowler and Fred Hoyle showed that almost all elements could be made either in normal stars or during supernova explosions at the end of the life of massive stars. Through winds and explosions stars spread the elements they manufacture through the interstellar gas between the stars. New stars and planetary systems then form by condensation out of dense clouds of interstellar gas.

A crucial first step in planetary formation is believed to be the formation of planetesimals, aggregates of dust and ice, within the disc-shaped cloud surrounding the forming star. In the outer reaches of the solar system, the Oort cloud of comets may be a relic of this early stage in the formation of the solar system.

The elements of the Earth reflect the whole history of star formation, evolution and death in our galaxy.

The only elements that the Burbidge, Burbidge, Fowler and Hoyle theory could not account for were the light elements deuterium, helium and lithium, whose abundances are too high to be accounted for by purely stellar processes. To understand these we have to look to the early stages of a hot Big Bang universe (see chapter 11). 


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