The origin of elements began with the Big Bang, which formed the lightest elements like hydrogen, helium, and lithium through nuclear fusion in the early hot dense universe. As the universe expanded and cooled, the first stars and galaxies formed, allowing nuclear fusion to produce heavier elements up to iron inside stars. When massive stars collapsed in supernova explosions, even heavier elements were formed through further fusion and neutron capture processes. This cycling of matter between generations of stars through nuclear reactions is how all the chemical elements from carbon to uranium came to exist in the present universe.

1. THE ORIGIN OF ELEMENTS
The chapter demonstrates the hitherto known data about the origin of the elements. In this
summary will be exposed the most important points of this journey through time. The
origin of the universe gives us an answer to the question of how the elements were formed
and about the abundance of them in it. According to the big bang theory, all matter in the
universe was contained in a primitive nucleus, it is assumed that this nucleus exploded and
distributed matter and radiation evenly through space.
The universe in initial conditions had matter formed by elementary particles. A second later
the universe had a temperature of approximately 10.10 K and was a sea of neutrons,
protons, electrons, positrons, photons, and the electron-, muon-, and tau-neutrinos and their
antineutrinos, ie hot plasma. In a second, after a period of extensive particle - antiparticle
annihilations, the electromagnetic photons are formed. Then the strong nuclear forces
caused a large amount of neutrons and protons that combine to give nuclei of Deuterium (n
+ p) and Helium (2n + 2p). Also the isotopes of helium an lithium were formed, This
process is known as big bang nucleosynthesis. The temperature was so high that there were
no atoms. As a consequence of the expansion the temperature was decreasing and when
109 ºK was reached nuclear reactions took place in the plasma. Started the weak decoupling
where positive particles can capture electrons and form atoms and the novelty is that these
reactions are not affected by the electromagnetic radiation that is the atoms can interact
with each other independently of the radiation. This interaction leads to the formation of
different atoms, which begin to condense and form the nucleus of stars and the radiation
expands with the universe.
With the presence of gravity, the synthesis of nuclei of chemical elements did not stop. The
existence of gravitational attraction led to the formation of zones in space with a large
concentration of matter. This process initiated the formation of galaxies and stars which
made possible the formation of new chemical elements. The process of formation of matter
had a new heating and increase of density which was enough to produce new nuclear
reactions. There are some Reaction Cycles during the Synthesis of Heavy Nuclei. First,
hydrogen burning where hydrogen is converted to helium, this cycle is called proton-
proton. Second, Helium Burning: most of the hydrogen is burned. Helium burning is done,
triple-alpha process, fuse to form a carbon nucleus. As soon as carbon is present, the alpha
process begins where the formation of neon, oxygen and silicon takes place. Third, Burning
of Heavier Elements: If the star is massive (> 8 Solar masses), another set of nuclear fusion
reactions begins. These include the burning of carbon, neon, oxygen and silicon that lead to
the formation of heavier elements and finally iron. And finally, Production of Heavier
Elements Than Iron: After the production of iron, the star collapses under its own gravity to
a neutron star or a black hole.