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 Nuclear fusion – University’s Powerhouse

If you wish, catch up on the basics of Atoms and Chemical Elements here

Throughout the Universe and ever since its beginning with the Big Bang, energy is predominantly generated by nucleosynthesis, which is a term used to describe a variety of reactions of nuclear fusion.

Through most of the star's life, energy is produced by hydrogen fusion through a series of steps that ultimately convert hydrogen into helium. Most of the time the inner region of the star, the core, is the only location in the star that is hot enough for this fusion process.

Inside the star, due to the high temperature, atoms are stripped off their electrons and only the bare nucleus can exist. So when we say that hydrogen fuses to helium, only the nuclei of these elements are involved. So what is hydrogen fusion?


Hydrogen fusion – the basics

Heating up
Gas and dust contracts due to self-gravity. This increases temperature and pressure in the centre of the cloud.
Increasing temperature means that the speed of the particles increases. With increasing temperature hydrogen and other atoms are loosing their electrons and a plasma is created mainly consisting of free protons and electrons.
Strong Nuclear Force
Protons have equal charge and therefore repel each other. At very high temperature protons can collide and undergo nuclear fusion: they stick together and form other particles and energy is released. This happens when protons come so close together that the strong nuclear force overcomes the electric repelling of the protons. When this happens a star is born and nuclear fusion will continue.
Fusion creates Energy
When protons collide other nuclei are formed while a little mass (m) is “lost” due to nuclear binding energy; the new nucleus has slightly less mass than the individual constituents together.  This "lost" mass is converted into energy (E) according to E=mc2. Since c2 is a very large number, a very small amount of mass will produce a large amount of energy.
rest mass

p-p Cycle
Hydrogen atoms have only one proton (red circles in the diagram at the right). The next larger element is Helium that has two protons. So the end result of the fusion of hydrogen is indeed Helium. There are different types of fusion processes but in our Sun the main process is the proton-proton cycle (right) converting hydrogen into helium with by-products neutrinos and gamma radiation.



Image credit: Wikipedia

p pcycle

In stars that have more than 1.3 times the mass of the Sun, energy is mainly generated by a different process than the proton-proton cycle described above, the CNO-cycle. It has the same net result of producing helium from Hydrogen, but it involves nuclei of Carbon, Nitrogen and Oxygen as intermediate stages (catalysts).


Image credit: Wikipedia

cno cycle
Dynamic Equilibrium
The explosive force of the fusion counters the contraction due to self-gravity and a dynamic but stable balance is reached.
dynamic equilibrium

Life time
The temperature in the core of the star depends on the mass of the newly formed star and this determines the life time of the star. High mass stars exist for millions of years, moderate stars, like our Sun, live billions of years and low mass stars (red dwarfs) live longer than the present age of the Universe (tens of billions of years).
We discuss this in much more detail below.




Often in astronomy the process of nuclear fusion is described as “burning”, e.g. “hydrogen burning”. This term is very misleading because burning relates to a chemical reaction involving oxygen.
As described, fusion is a reaction between atomic nuclei, which is quite something else.
In this EBook we try to avoid this “burning” issue (pun intended).


Further in this EBook we will discuss the end stages in the life of stars of different mass.
In preparation for that, a good overview of the most important nuclear reactions (nucleosynthesis) that happen in stars is given here.


Further Reading:
How stars are born: video
Going deeper, including history of discovery of fusion






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