Nuclear Chemistry - Stellar Nucleosynthesis

Nucleosynthesis in Stars: A Stellar Concept

All elements are synthesized inside stars. The exception being Hydrogen. Through stellar nucleosynthesis, we are able to better understand stellar evolution and space-time distribution of matter in the universe. Due to gravitational contraction, the core of a star will become hot enough for atoms to be stripped off their electrons and collisions between atomic nuclei trigger nuclear reactions. The star's hydrostatic equilibrium is established by radiating away some of the nuclear energy. 

Proton-Proton chain

The fusion of 4 Hydrogen nuclei into Helium through a chain of reactions is called Proton-Proton chain and is involved in the nucleosynthesis in main sequence stars. This process was first discovered in 1939 by Hans Bethe. 

The CNO cycle

The CNO cycle occurs when a star is more massive. resulting in Hydrogen and Helium burning through a chain of reactions involving Carbon, Nitrogen, and Oxygen (taken from previous stars).  These 3 elements are catalysts to synthesize 4 protons into Helium with the same energy outcome as the Proton-Proton chain. 
When Helium dominates the core the temperature is not high enough to trigger reactions involving He4. Gravity then contracts the core even further until alpha reactions are triggered. These reactions produce elements such as C12, O16 and Ne20 by fusing 3, 4 or 5 alpha particles. Though these reactions are not direct, they build up through binary reactions leading to Be8 (which has a very short lifetime: 2.6 10-16 sec) and then the capture of a third alpha particle to form C12. At this stage, the star has entered its red giant phase because Hydrogen occupies the outer layer which expands because of an increase in temperature.
When Helium burning stops, which occurs when Helium is exhausted, the core of the star gives way to gravitational contraction. Stars that are about 1 solar mass or less enter the white dwarf degeneracy state while more massive stars raise the temperature to a level where C12 - C12 collisions start to produce elements that are heavier. An example of this is Mg24, and O16 - O16 collisions producing S32. A massive star will also end up with an Iron core when the star reaches the stage of Iron synthesis and in this stage, more core contraction will not trigger succeeding nuclear reactions. All subsequent reactions will be endothermic because Iron is the most bound element. Therefore no more energy will be provided to support the star against gravitational collapse. This leads to a runaway phase where it then undergoes a supernova explosion in which heavier elements (Uranium, Lead, and Gold) will be synthesized through neutron capture and decay processes.

White Dwarf

Supernova

You may be thinking, "Wow, this is really kind of boring. I wonder when she'll get to the good stuff and how all this pertains to me." Well, wonder no longer! Astrophysicist Lawrence Krauss explains, "Every atom in your body came from a star that exploded. [In other words,] you are all stardust. You couldn’t be here if stars hadn’t exploded, because the elements - the carbon, nitrogen, oxygen, iron, all the things that matter for evolution and for life - weren’t created at the beginning of time. They were created in the nuclear furnaces of stars, and the only way for them to get into your body is if those stars were kind enough to explode.... The stars died so that you could be here today." The entire process of stellar nucleosynthesis is working up to the point where the core is iron therefore there is nowhere left to go and the star collapses. Lawrence Krauss describes, "That whole collapse happens in one second.... There's a shock wave and that shock wave ... spews out all of the atoms that were created during the life history of a star. The carbon, the nitrogen, the helium, the iron. And that's vitally important because every atom in your body was once inside a star that exploded.... The atoms in your left hand probably came from a different star than in your right hand, because 200 million stars have exploded to make up the atoms in your body." 

Work Cited:
http://philosophy-of-cosmology.ox.ac.uk/stellar-nucleosynthesis.html
https://www.thoughtco.com/stellar-nucleosynthesis-2699311