The nuclear fusion reaction takes place at the heart of stars. At extreme temperatures, of the order of millions of degrees Celsius, matter takes the form of plasma. In this fourth state of matter, where it is neither solid, liquid nor gaseous, matter is comparable to a ‘soup’ where nuclei and electrons are no longer bound and circulate freely. Although rare on Earth, this is the most common state of matter in the Universe. In a plasma, two ‘light’ nuclei colliding at high speed can fuse, creating a heavier nucleus. During this reaction, the mass of the fusion products is less than the sum of the masses of the initial elements: this difference in mass is released in the form of energy, according to Einstein’s formula E=mc2 … energy that is the source of the heat emitted by the Sun.
In the cores of stars, reactions involving four hydrogen nuclei are needed to form heavier nuclei and release the energy of fusion of hydrogen into helium.
