Origin of Heavy Elements & Supernova

The Origin of Heavy Elements
During the evolution of both high-mass and low-mass stars, fusion reactions creates different elements from the Proton-proton chain reaction in low-mass stars and the CNO cycle in high mass stars.  Essentially, these two reaction processes give a net result of the fusion of H to He inside the core of stars. As the evolution of stars moves forward, more elements are created. However, on a practical level, the specific processes of the formation of heavy elements in low-mass and high-mass stars vary from each other, as the evolution of these stars differs.  Generally speaking, fusion in the cores of stars created heavy elements as products of their different fusion processes. However, the specific processes vary in the following two types of stars:

Low-Mass Stars:

In low-mass stars, after the main sequence, during which hydrogen is turned into helium in the core, the core itself starts contracting once the hydrogen becomes exhausted. However, while hydrogen is exhausted, the temperature in the core has not yet reached high enough to fuse helium into carbon. Gradually as the core contraction heats up hydrogen in a shell directly above the core, the shell starts to ignite hydrogen fusion, during which helium gets deposited on core. The shell burning accelerates due to the increased gravitational contraction from helium deposition onto the core.  This causes the core temperature to continue to increase, and very suddenly the entire core becomes capable of igniting helium fusion.  This process produces Carbon, a heavy element. As Carbon is produced through the fusion processes in low-mass stars, element of carbon itself served basically as the end point for further fusion processes possible in low-mass stars, as fusion processes that produces heavier elements require more energy that is offered in high-mass stars. So let’s see how high-mass stars make things happen.

source: https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect17/lecture17.html

High-mass Stars:

High-mass stars also ignite fusion of hydrogen to helium in their cores, however, their core temperature are higher, so the chain of reactions is different  when compared with low-mass stars (p-p chain in low-mass stars, CNO cycle in high-mass stars).  Specifically, the CNO cycle is capable of changing carbon into nitrogen and oxygen. Similar to low-mass stars, high-mass stars also go through processes such as helium shell burning and fusion of helium. What makes high-mass stars unique is that the fusion reactions of carbon and other heavy elements continue in their cores and in multiple shells in high-mass stars. High core temperatures in high-mass stars allow the fusion of elements as heavy as iron in their cores.

source: http://www2.astro.psu.edu/users/cpalma/astro10/class12.html

What's after "iron": Supernova!

As you might ask, will there be heavier elements than iron formed through the fusion reactions happening in the cores. The answer is “NO” since iron is a dead end for fusion because nuclear reactions involving iron do not release energy.  Accordingly, as the iron formed in high-mass stars builds up in their cores, the degeneracy pressure gradually becomes incapable of resisting gravity. The core then suddenly collapses, creating a supernova explosion. 

source: http://www.huffingtonpost.com/2013/04/16/supernova-bacteria-deep-sea-fossils_n_3091277.html

More Elements Emerged

Specifically, the core degeneracy pressure lowers down because electrons combine with protons. In this process, neutrons and neutrinos are created, and later the neutrons collapse to the center, forming a neutron star, which is about the same size as an average city like Boston. Through energy and neutrons released in a supernova explosion, elements heavier than iron can finally be produced, such as gold and uranium.