The PP cycle
Nuclear fusion within the stars begins with the different passages of hydrogen transformation into helium. Stars that are going through this cycle are in the most stable and long lasting period of their whole existence because the scale times at which reactions happen, in other words the average survival period of the particle before interacting with others, are very long. Here we are dealing with statistics, which means that some particles will react faster, others slower, while others yet might not react at all. It all depends on the kind of particle and on the temperature of the nucleus. In the case of stars the size of the Sun, for example, the first step of the cycle is the fusion of two hydrogen nuclei to form a deuterium nucleus: this reaction has a scale time of about 10 billion years, but can vary considerably depending on the temperature, which means that in the case of larger and hotter stars it will happen sooner, while in the case of smaller and cooler stars it will take longer.
Why is the temperature so important? Hydrogen nuclei are made up of a single proton, an electro statically charged particle. We know that two particles with the same charge repel one another; yet at very short distances (10-15 meters) they tend to attract one another regardless of the type of charge because of the so called strong interaction. Therefore the trick is to try to draw them close to one another in spite of the electrostatic repulsion and there are better chances of succeeding at this by increasing the particle’s kinetic energy, that is by increasing the temperature. This is when, once the stellar nucleus reaches the10 million degree temperature, nuclear fusion can begin. Following the first step where the deuterium nucleus has formed (one neutron and one proton) from hydrogen nuclei (two protons), the process continues with a very fast reaction: in scale times of just over a second, the deuterium that has just formed fuses with another proton to create a helium-3 nucleus (two protons and one neutron). The helium-3 nucleus survives for an average of 250 thousand years before fusing with another identical nucleus, thus producing a beryllium-6 nucleus (4 protons and 2 neutrons). Since it is very unstable, this configuration breaks down immediately by expelling two protons and leaving a stable nucleus of helium-4 (2 protons and 2 neutrons). All these steps together are called the proton – proton cycle.