Uppsala universitet
Skip links
På svenska

Linné on line arrow Physics and the Cosmos arrow Macrocosmos arrow The sun arrow The solar cycle

The solar cycle

Energy is released in the sun when hydrogen nuclei are fused through nuclear reactions (fusion) into helium nuclei. When the supply of hydrogen in the sun has decreased substantially in the centre it will also be possible to burn helium (through fusion). Subsequently, heavier elements can also be created through further fusion processes. However, no elements heavier than iron can be formed this way, since iron is the most tightly bound nucleus, i.e., has the largest binding energy per nucleon. In order to produce heavier elements energy must by added and this cannot happen through spontaneous fusion.

Since the sun is a light star, relatively speaking, the fusion processes will stop before iron is formed due to the fact that it will explode before that.

Coming back to the processes that occur in the sun today: The first step in the solar cycle is when two hydrogen nuclei (protons) are fused into heavy hydrogen (deuterium),

1H + 1H -> 2H + e+ + neutrino

One could also envision that 2He is formed, but this state is highly unstable, due to the repulsive Coulomb force between the two positive charges, and quickly decays. The next step in the cycle is that the deuteron (2H) is fused with another proton,

2H + 1H -> 3He + photon

The 3He-nucleus that is formed could in principle react with another proton, but that would form 4Li which is highly unstable and quickly decays back into its original parts. It could also react with deuterons, but these fuse very easily with protons, so they will disappear without reacting with 3He. Instead 3He will react with other 3He-nuclei,

3He + 3He -> 4He + 2 1H + photon

The net reaction will thus be,

4 1H -> 4He + 2 e+ + 2 neutrinos

When enough amounts of 4He have been formed, these can also serve as catalysts for other reactions. For example

3He + 4He -> 7Be + photon

followed by

7Be + e- -> 7Li + neutrino, 7Li + 1H -> 2 4He


7Be + 1H -> 8B + photon, 8B -> 8Be + e+ + neutrino, 8Be -> 2 4He

As is evident from these reactions, 8Be is unstable and heavier elements can therefore not be formed by fusion of two 4He. Instead they must go through some other channel. This can only happen when the concentration of helium is so high that the probability for three 4He to collide at the same time is substantially increased. At this point the reaction

3 4He -> 12C

can take place. From the carbon, other heavier elements can also be formed by absorption of 1H and 4He.

In some of these reactions neutrinos are created. For a long time one did not understand the amount of neutrinos observed from the sun, the solar neutrino problem. However, in the beginning of the new millennium it became clear that the amount of neutrinos reaching the earth from the sun is in good agreement with what is expected from the solar model, once one takes into account the oscillations into neutrinos of different flavour from the one produced in the sun.