Linné on line - The solar cycle

Linné on line Physics and the Cosmos Macrocosmos The sun 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),

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,

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,

The net reaction will thus be,

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

followed by

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

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.