The Sun: an almost inexhaustible source of energy
Editorial review 2026
Why does the Sun shine? And for so long?
The Sun formed from a cloud of stellar matter composed mainly of hydrogen, the most widespread atomic element in the universe and whose nucleus is reduced to a proton. As it contracted under the effect of gravitational attraction, this cloud heated up until nuclear reactions ignited, from which the Sun derives its energy, part of which radiates in the form of light and heat.
The nuclear reaction from which the Sun derives its energy is the fusion reaction of two protons into a deuterium nucleus. It uses hydrogen as fuel. This reaction releases about 2 million electronvolts (MeV), compared with the few electronvolts released by the combustion of a carbon atom from a speck of coal dust or a drop of gasoline (4 eV). For the reaction to ignite, protons (hydrogen nuclei) must come into contact, which is only possible at temperatures above one million degrees.
The Sun therefore derives its energy from a phenomenon very close to radioactivity. The deuterium nucleus resulting from the fusion of two protons is composed of one proton and one neutron. One of the two protons has transformed into a neutron. The transformation of a proton into a neutron involves the nuclear forces responsible for beta radioactivity, which physicists call weak forces.
Without the action of these weak forces, this transformation would be impossible. Two protons, even in contact, are incapable of fusing because they repel each other. However, each of the two protons has the ability to transform temporarily into a neutron by emitting a particle called the W boson. This W boson is generally immediately reabsorbed, the neutron becoming a proton again. Exceptionally, the fleeting W may have time to decay into a positron and a neutrino. The neutron no longer becomes a proton again. It can then fuse with the other proton to form a deuterium nucleus.
Weak forces active inside the Sun!
The 3 stages of the fusion reaction of two protons coming into contact to form a deuterium nucleus accompanied by a positron and a neutrino: 1) One of the two protons transforms for a very brief time into a neutron and a W boson; 2) Instead of being reabsorbed, the W decays into a positron and a neutrino; 3) Alongside these two particles, the proton and neutron bind together into a deuterium nucleus.
© IN2P3
This fusion reaction (called primordial) does not only release 2 MeV of energy. It is not only the source of the Sun’s heat. Weak forces play another very important role in stars: they generate neutrons. Neutrons, which are unstable outside nuclei, are in fact practically absent from stellar gas clouds, unlike protons. Without them, the atoms of all the elements around us could not form. The neutrons in deuterium are used to produce helium nuclei through other fusion reactions, and later nuclei essential to life such as carbon and oxygen.
The Sun burns slowly …
The Sun has been shining for 4.5 billion years and should continue shining for another six billion years before exhausting its hydrogen. It would have collapsed long ago if the pressure of the radiation it emits had not counteracted the effect of gravitational forces.
Why has the Sun been shining for so long? The combination of circumstances leading to hydrogen fusion is very difficult to achieve. Only a tiny fraction of collisions between protons ends in fusion because protons electrically repel each other and the forces that transform a proton into a neutron are weak. The Sun lives at a very slow pace. For our greatest benefit, the solar star consumes its reserve of protons sparingly.