Basic Fusion
Basic Fusion
Introduction:
When you hear the word fusion, you might think of a vague source of energy that seems to work only in sci-fi novels. Well, scientists are actually researching this, known as controlled thermonuclear fusion, as a possible source of energy for the future. However, the fusion that people sometimes forget is the kind which goes on every day, and is an important part of our lives. This fusion, in nuclear physics, is the joining of two atomic nuclei. It occurs in stars all over the universe, including our Sun, and is what provides the warmth and light we receive. 
So what is really going on?
For fusion to work, extremely high energies are needed to fuse the nuclei together. This is needed to overcome the electrical repulsion (also known as the coulomb barrier) between two positively charged nuclei, so that they get close enough to have the strong nuclear force bind the nuclei. This nuclear force has an effective range of around 10-15 meters, which is why fusion occurs most easily in stars, where a high density and temperature environment exists. The density and temperature are the primary factors in determining the probablity of the nucleons fusing in the star. Below we discuss two major chain of reactions involving fusion, both which occur generally in main sequence stars (you will learn more about this later). Most of the energy generated within the Sun is created from a sequence of reactions that "burns" hydrogen into helium, known as the proton-proton reaction.
In our Sun, these reactions occur in the central region, where the density is increased to 100 times the density of water on Earth, sending temperatures to about 15 million K (27,000,000 degrees F; consider baking a pizza at 400 degrees F!). At these temperatures, the hydrogen is ionized, or stripped of their electrons, creating a plasma of free electrons and protons, the nuclei of the hydrogen. The heat provides enough energy for the hydrogen ions to collide with enough force to overcome the repulsion between these positively charged nuclei and fuse them. One of the resulting two protons that just fused decays into a neutron through ß+ decay, forming a deuteron. The decay process also releases an anti-electron, or positron, and a neutrino. The positron will later collide with an electron and annihilate each other, releasing two gamma rays, which are high-energy photons. The neutrino interacts very weakly with matter, and will pass right out of the sun. The newly formed deuteron (2H) may collide into another hydrogen nucleus, creating the helium isotope 3He and causing the release of a gamma ray. When two of these 3He isotopes collide, two of its protons are released, creating 4He. This sequence of nuclear reactions, known as the proton-proton reaction, can be written like like this: (1.) 1H + 1H 
2H + positron (ß+) + neutrino (v)
(2.) 2H + 1H 3He + gamma ray (y)
(3.) 3He + 3He 4He + 1H + 1H
Note that steps 1 and 2 must be done twice for each step 3. 3He and 4He are stable isotopes. 4He needs even higher energies to fuse, since the repulsion between the two pairs of positively charged protons in helium is even greater than the repulsion between only two hydrogen nuclei.
In starts with masses similar or less than the Sun, the proton-proton reaction is the primary energy producer. However, another cycle dominates over this reaction in the hotter, more massive stars.
The Carbon-Nitrogen-Oxygen (CNO) Cycle:
The CNO cycle is another sequence of energy producing reactions, which ultimately results in the conversion of hydrogen to helium. It occurs in stars at temperatures greater than 16 million K. Although hydrogen and helium are the main elements in stars, usually some heavier elements are present in much smaller quantities. If Carbon(C), Nitrogen(N), and Oxygen(O) ions are present, they may be involved in the release of energy within stars through the following sequence of reactions: (1.) 12C + 1H 13N + y
(2.) 13N (through ß+ decay) 13C + (ß+) + v
(3.) 13C + 1H 14N + y
(4.) 14N + 1H 15O + y
(5.) 15O (through ß+ decay) 15N + (ß+) + v
(6.) 15N + 1H 12C + 4He
In the above reactions, Carbon(C) acts as the catalyst, that is, it initiated the chain of reactions but was not consumed (notice that 12C reappears in the last step). Here is an animation to help you better understand and visualize the above reactions. Although the above was initiated with carbon, similar reactions are possible starting with nitrogen or oxygen.
Current estimates say the Sun produces 98-99% of its energy through the proton-proton reaction and 1% from the CNO cycle. However, the CNO cycle would be the major energy producer if our Sun was 10-20% more massive.
One Little Problem: the Neutrino
Neutrinos are produced in stars during reactions such as that which occurs in the proton-proton reaction. Whether or not neutrinos have mass is still being debated among physicists. However, it does appear that neutrinos interact weakly with matter, which allows them to pass easily out of the star. Therefore, if the neutrinos can be detected from a star, it would allow us to see a portion of the processes occuring within the core of the star. However, experiments have currently deteected much fewer neutrinos than predicted in the theory. This suggests that our understanding of the Sun is insufficient, or our understanding of neutrinos is insufficient. One proposal for explaining the deficit of neutrinos involves the idea that there are three types of neutrinos, and as they leave the Sun's core, they may change into another type that is undetectable by today's detectors. However, there is no definitive proof of this behavior of neutrinos.
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