Editing Nuclear physics (section)

=== Production of "heavy" elements ===
{{main|nucleosynthesis}}
According to the theory, as the Universe cooled after the [[Big Bang]] it eventually became possible for common subatomic particles as we know them (neutrons, protons and electrons) to exist. The most common particles created in the Big Bang which are still easily observable to us today were protons and electrons (in equal numbers). The protons would eventually form hydrogen atoms. Almost all the neutrons created in the Big Bang were absorbed into [[helium-4]] in the first three minutes after the Big Bang, and this helium accounts for most of the helium in the universe today (see [[Big Bang nucleosynthesis]]).

Some relatively small quantities of elements beyond helium (lithium, beryllium, and perhaps some boron) were created in the Big Bang, as the protons and neutrons collided with each other, but all of the "heavier elements" (carbon, element number 6, and elements of greater [[atomic number]]) that we see today, were created inside stars during a series of fusion stages, such as the [[proton–proton chain]], the [[CNO cycle]] and the [[triple-alpha process]]. Progressively heavier elements are created during the [[stellar evolution|evolution]] of a star.

Energy is only released in fusion processes involving smaller atoms than iron because the binding energy per [[nucleon]] peaks around iron (56 nucleons). Since the creation of heavier nuclei by fusion requires energy, nature resorts to the process of neutron capture. Neutrons (due to their lack of charge) are readily absorbed by a nucleus. The heavy elements are created by either a ''slow'' neutron capture process (the so-called [[s-process|''s''-process]]) or the ''rapid'', or [[r-process|''r''-process]]. The ''s'' process occurs in thermally pulsing stars (called AGB, or asymptotic giant branch stars) and takes hundreds to thousands of years to reach the heaviest elements of lead and bismuth. The ''r''-process is thought to occur in [[supernova explosions]], which provide the necessary conditions of high temperature, high neutron flux and ejected matter. These stellar conditions make the successive neutron captures very fast, involving very neutron-rich species which then beta-decay to heavier elements, especially at the so-called waiting points that correspond to more stable nuclides with closed neutron shells (magic numbers).