Atomic nucleus
Since the binding energy per nucleon peaks around iron, energy is only released in fusion processes occurring below this point. Since the creation of heavier nuclei by fusion costs 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) or by the rapid, or 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 because the conditions of high temperature, high neutron flux and ejected matter are present.  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).  The r process duration is typically in the range of a few seconds.