Question: When fusion creates elements that are heavier than iron, it requires an excess of neutrons. Therefore, astronomers assume that heavier atoms are minted in supernova explosions, where there is a ready supply of neutrons. But Actually how this happen? How are heavy elements from Iron to Uranium are made?

Jack Miller answered on 22 Jun 2013:

Hi Usman100,

Great question! In short, we know that the heavier atoms are made in supernovae as we can see their presence (just) by spectroscopy, looking that the wavelengths of light emitted from an exploding (large) star. There are other methods of nuclear decay than those you’ve been told about in school, and, in particular, it’s possible for reverse beta decay (often called K-capture) to happen, and the reaction electron+proton -> neutron + electron neutrino to occur (which produces the ‘required’ neutrons you’re talking about). The process works a little bit like this (and please forgive me for copying and pasting a bit from wikipedia — search for Type II supernovae).

This continues until nickel-56 is produced, which decays radioactively into cobalt-56 and then iron-56 over the course of a few months. As iron and nickel have the highest binding energy per nucleon of all the elements, energy cannot be produced at the core by fusion, and a nickel-iron core grows. This core is under huge gravitational pressure. As there is no fusion to further raise the star’s temperature to support it against collapse, it is supported only by degeneracy pressure of electrons. In this state, matter is so dense that further compaction would require electrons to occupy the same energy states. However, this is forbidden for identical fermion particles, such as the electron – a phenomenon called the Pauli exclusion principle.

When the core’s mass exceeds the Chandrasekhar limit of about 1.4 solar masses, degeneracy pressure can no longer support it, and catastrophic collapse ensues. The outer part of the core reaches velocities of up to 70,000 km/s (23% of the speed of light) as it collapses toward the center of the star. The rapidly shrinking core heats up, producing high-energy gamma rays that decompose iron nuclei into helium nuclei and free neutrons via photodisintegration. As the core’s density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and elementary particles called neutrinos. Because neutrinos rarely interact with normal matter, they can escape from the core, carrying away energy and further accelerating the collapse, which proceeds over a timescale of milliseconds. As the core detaches from the outer layers of the star, some of these neutrinos are absorbed by the star’s outer layers, beginning the supernova explosion.

For Type II supernovae, the collapse is eventually halted by short-range repulsive neutron-neutron interactions, mediated by the strong force, as well as by degeneracy pressure of neutrons, at a density comparable to that of an atomic nucleus. Once collapse stops, the infalling matter rebounds, producing a shock wave that propagates outward. The energy from this shock dissociates heavy elements within the core.

Hope that helps!

— Jack