Nuclear fusion is a process where two or more nuclei are combined to produce different combinations of protons and neutrons. Despite how technological it might sound, nuclear fusion occurs naturally throughout the universe in stars. This fusion, however, is vastly different from fusion done by fusor enthusiasts in IEC fusion reactors. The fusion processes in stars even vary depending on the type and temperature of the star.
Inertial Electrostatic Confinement
Commonly called a Farnsworth–Hirsch fusor the proper name for our fusion reactor is an Inertial Electrostatic Confinement (IEC) Fusion Reactor. IEC describes the class of fusion we are using. IEC Fusors use an electric field inside of a vacuum to promote the collision of positively charged nuclei. Among amateur fusors, deuterium is the most commonly used isotope. When the deuterium ions collide with other ones they either produce a neutron and helium-3 nuclei or a proton and tritium nuclei. Whichever reaction occurs happens at a rate of 50/50.
The Proton-Proton Chain
Our sun experiences a completely different type of fusion reaction called the Proton-Proton Chain. Unlike an IEC fusion reaction, the Sun does not use an electric field but instead uses intense pressure to fuse particles together. The P-P chain is responsible for creating helium, and the only necessary element for the reaction is hydrogen. The process starts with two hydrogen protons fusing into one deuterium nuclei. This releases a neutrino and positron in the process. Then this deuterium fuses with another hydrogen proton to create helium-3 and a gamma ray. The process must happen twice to this point in order to produce a second helium-3 nucleus to fuse with the first one. The two helium-3’s produce a single, stable helium-4 atom and two protons. This process happens constantly within the sun, it is the foundation of nucleosynthesis, the creation of chemical elements within stars.
In a fusor, we use an electric field to encourage collision at a high enough speed to overcome the natural repulsion force keeping the atoms apart. The sun overcomes this force using gravity. It is so massive that the pressure heats up the core to around 15 million degrees celsius. At this temperature hydrogen becomes plasma and it’s protons move around at extreme speeds that collide, completely overcoming the repulsion force. 
The CNO Cycle and Triple-Alpha Process
There is a second nuclear fusion method in which stars can create helium called the Carbon-Nuclear-Oxygen (CNO) cycle. This process occurs in stars with a higher temperature than our sun which requires nearly 1.3 solar masses. It is possible for the CNO cycle to occur in the sun but it’s estimated to only account for less than 2% of the sun’s helium production. There are 7 hypothesised CNO cycles all of which follow the same principle. Protons are continuously added to a catalyst of either carbon, nitrogen or oxygen as it progresses through several stages of each possible element until it reaches nitrogen-15. At which point, fusion with a single proton will produce one helium-4 nuclei and a carbon-12 nuclei which can restart the process. Depending on which step of the cycle is occurring, each reaction will also release a neutrino or energy in the form of a gamma ray.
The third main type of stellar fusion reaction is called the triple alpha process. This process occurs in stars even hotter than those that experience the CNO cycle. Sometimes called “helium-burning” this process most commonly happens in very heavy stars. Unlike the previous two processes, this process requires helium instead of hydrogen. This means that it can only occur in stars that have already undergone the previous two processes to accumulate enough hydrogen in its core. In the triple alpha process, two alpha particles (helium-4) come together to become one beryllium-8 nuclei. Sometimes this beryllium decays but often a third alpha particle is fused, making a carbon nuclei and releasing energy as a gamma ray.
Like the proton-proton chain, these reactions also use pressure instead of electric fields to encourage fusion. Only these processes both include elements other than helium, with much higher temperature requirements to become plasma. This is why these more advanced processes only happen in hotter, larger stars.
There are more reactions that fuse together heavier elements than hydrogen and helium. Such as lithium-burning, carbon-burning and oxygen-burning. All of these processes have similar reactions to those above, but require most of the lighter elements to be already gone from the star.
The Importance of Fusion
In all of the stellar fusion reactions mentioned, light and energy are released. This energy can take years to travel from the core of the sun to the surface and then to us. Fusion is the source of the sun’s power, and without it, life couldn’t exist. Additionally, nucleosynthesis is necessary for the creation of complex elements needed for our survival. The importance of fusion in the universe cannot be understated, it is important for enthusiasts like the UPS reactor group to continue to do experiments and research.
This image shows energy output (Ɛ) vs temperature (T).
Image credit to The Wikimedia Foundation.
The American Nuclear Society. Nuclear Fusion. Center for Nuclear Science and Technology Information. http://nuclearconnect.org/know-nuclear/science/nuclear-fusion.
EUROfusion. Fusion on the Sun. Eurofusion. https://www.euro-fusion.org/fusion/fusion-on-the-sun/.
Hanania, J. (2020, June 16). Nuclear fusion in the Sun. Energy Education. https://energyeducation.ca/encyclopedia/Nuclear_fusion_in_the_Sun.
Kruszelnicki, K. S. (2012, April 24). Sun makes slow light. ABC (Australian Broadcasting Corporation). https://www.abc.net.au/science/articles/2012/04/24/3483573.htm.
Mihos, C. High Mass Nucleosynthesis. burro.cwru.edu. http://burro.cwru.edu/academics/Astr221/StarPhys/nucleosynth.html.
Rave, C. Nuclear Fusion. http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/fusion.html.
Shapley, P. (2011). Fusion in Stars. http://butane.chem.uiuc.edu/pshapley/GenChem1/L1/3.html.
Swinburne University of Technology. CNO cycle. COSMOS. https://astronomy.swin.edu.au/cosmos/c/cno+cycle.
“Stellar nucleosynthesis.” 2021. In Wikipedia. https://en.wikipedia.org/wiki/Stellar_nucleosynthesis.
“Proton-proton chain” 2021. In Wikipedia. https://en.wikipedia.org/wiki/Proton–proton_chain.
“CNO Cycle” 2021. In Wikipedia. https://en.wikipedia.org/wiki/CNO_cycle.
“Triple-alpha process” 2021. In Wikipedia. https://en.wikipedia.org/wiki/Triple-alpha_process.