When the first Hydrogen Helium stars formed the heavier elements began to form in the center of the stars by stellar nucleosynthesis.
Stellar nucleosynthesis
In
astrophysics,
stellar nucleosynthesis is the
creation of
chemical elements by
nuclear fusion reactions within
stars. Stellar nucleosynthesis has occurred since the
original creation of
hydrogen,
helium and
lithium during the
Big Bang. As a
predictive theory, it yields accurate estimates of the
observed abundances of the elements. It explains why the observed abundances of elements change over time and why some elements and their
isotopes are much more abundant than others. The theory was initially proposed by
Fred Hoyle in 1946,
[1] who later refined it in 1954.
[2] Further advances were made, especially to nucleosynthesis by
neutron capture of the elements heavier than
iron, by
Margaret and
Geoffrey Burbidge,
William Alfred Fowler and
Fred Hoyle in their famous 1957
B2FH paper,
[3] which became one of the most heavily cited papers in astrophysics history.
Stars evolve because of changes in their composition (the abundance of their constituent elements) over their lifespans, first by
burning hydrogen (
main sequence star), then
helium (
horizontal branch star), and progressively burning
higher elements. However, this does not by itself significantly alter the abundances of elements in the universe as the elements are contained within the star. Later in its life, a low-mass star will slowly eject its atmosphere via
stellar wind, forming a
planetary nebula, while a higher–mass star will eject mass via a sudden catastrophic event called a
supernova. The term
supernova nucleosynthesis is used to describe the creation of elements during the explosion of a massive star or
white dwarf.
The advanced sequence of burning fuels is driven by
gravitational collapse and its associated heating, resulting in the subsequent burning of
carbon,
oxygen and
silicon. However, most of the nucleosynthesis in the mass range
A = 28–56 (from silicon to nickel) is actually caused by the upper layers of the star
collapsing onto the core, creating a compressional
shock wave rebounding outward. The shock front briefly raises temperatures by roughly 50%, thereby causing furious burning for about a second. This final burning in massive stars, called
explosive nucleosynthesis or
supernova nucleosynthesis, is the final epoch of stellar nucleosynthesis.
A stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the
abundances of elements found in the universe. The need for a physical description was already inspired by the relative abundances of the chemical elements in the solar system. Those abundances, when plotted on a graph as a function of the atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions (see
history of nucleosynthesis theory).
[4] This suggested a natural process that is not random. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the
energy released from nuclear fusion reactions accounted for the longevity of the
Sun as a source of heat and light.
More to follow . . .
Science has yet to create a star, but we are working on it.
