Протон-протонний цикл

The proton-proton chain is the starting process by which all elements are eventually formed in a star's nucleosynthesis. The proton-proton chain forms helium from hydrogen, the crucial first step in order for anything to be created, although the other processes are detailed on other pages.

This theory was first proposed by Arthur Stanley Eddington, who was the foremost experimentalist at the time. This didn't prevent his theories from being controversial, as this one was. However, the proton-proton chain or PP Chain is now mainstream.

A star is usually formed with vast reserves of hydrogen. For stars bigger than our Sun, this process is not actually the most important; that would be the CNO cycle. However, in our Sun and smaller stars, this process dominates helium formation.

The process starts out with the formation of deuterium, a one-neutron isotope of hydrogen. This is accomplished through high pressures and temperature that squash two protons so close together, the reclusive electromagnetic force is broken. The result is diproton, an isotope of helium with two protons and two electrons. This is so unstable that it usually instantly decays back to two hydrogen atoms. However, there is a one-billionth chance a proton will undergo beta-plus decay (basically emission of a positron, a positively-charged version of the electron, also technically anitmatter) and become deuterium. Energy and a type of neutrino called an electron neutrino (no relation to electrons) are released during the process

The fact that a positron is emitted is not set in stone. The reaction can actually absorb an electron instead, leading to a higher energy reaction known as the proton-electron-proton chain or PEP Chain.

In fact, deuterium is consumed after than it can be replaced. At the beginning of the universe, this was called the deuterium bottleneck and prevented the formation of helium. In larger stars, this makes the PP Chain unwieldy.

This first step is in fact that slowest step of the cycle, because diproton is so ridiculously unstable. It would take billions of years for all of the hydrogen in the Sun to be used up like this.

Here is the relevant equation for the first step:

$$^1_1\text{H}+^1_1\text{H}\rightarrow {^2_2}\text{He}+\text{gamma rays} \rightarrow ^2_1\text{H}+\text{positron}+\text{electron neutrino}+\text{gamma rays}$$

Note that $^2_1\text{H}$ is interchangeable with $^2_1\text{D}$, so you might see $^2_1\text{D}$. Both are deuterium.

The next step is extremely fast; one deuterium combines with another hydrogen to form helium-3 almost every millisecond. For more information see Deuterium Burning.

Here is the relevant equation for the second step:

$$^2_1\text{H}+ ^1_1\text{H}\rightarrow ^3_2\text{He}+\text{gamma rays}$$

There are actually three different processes to form the heavier helium-4 from helium-3, as well as one theoretical one. If you want, you can knock yourself out over the next few Continues about each of these steps. The basic information is that helium-three combines with more helium, which creates beryllium-7 or -8. These berylliums are extremely unstable (although, beryllium-8 will play a much more important role later in other reactions, where it is much more likely to react into carbon than 3 random helium nuclei in the core), so they usually decay into helium-4 and more energy. The total reaction generates 1 helium-4 and 26.73 megaelectron-volts (MeVs) worth of energy from 4 hydrogen-1s. The energy comes from a 7% decrease in mass which is converted into energy through Einstein's famous equation. The most important reaction fueling all of this is the first one, most commonly called the pp1 branch. Helium-4 will results, which is sometimes known as an alpha particle because they both have 2 protons and 2 neutrons. Helium-4 is the building block of all later elements, and also happens to be a byproduct of decay of heavier elements.

The proton-proton chain is very important for our everyday lives, because it literally provides the energy that shines down on our heads. In step on of the proton-proton chain, you may remember a positron is released. That positron will go on to react with an electron, resulting in the photons that are streaming through your windows. The beryllium and hydrogen produced in the PP chain will provide the fuel that allow for the creation of heavier elements--carbon, iron, uranium--that will seed the galaxy and clump together be contained in whole solar systems. This is what you are made of--the byproducts of an ongoing reaction that created the elements allowing you to live.

Continue to see some details of the four branches of the proton-proton chain.

So you decided to continue.

The main branch of the proton-proton chain is, as I said up above, the pp1 branch.