Proton–proton chain

The sun makes light and heat.

It does this in its center. Tiny bits join together. This is called fusion.

Two tiny bits join to make one. This makes a new bit. This new bit joins with others. Soon, they make helium. This makes a lot of energy. The energy keeps the sun hot. It also keeps the sun big. This process can take a long time. It can take billions of years. The sun stays bright because of this.

Stars like our Sun make light and heat through a process called fusion.

This happens in the Sun's hot core. The Sun turns hydrogen into helium using the proton-proton chain.

This chain is a series of steps. In the first step, two protons join together. This makes a new part called a deuteron. This step is very slow. One proton might wait 9 billion years to join another! After that, the deuteron joins a third proton. This makes helium-3.

Next, the helium-3 joins other parts to make helium-4. This is the main goal of the chain. The process releases a lot of power. This power comes from a tiny bit of lost mass. This energy heats the Sun. It also stops the Sun from collapsing under its own weight. Most of the Sun's energy comes from this chain.

Some energy also leaves the Sun as neutrinos. These are tiny particles that pass right through matter.

Stars like our Sun stay bright and warm through a special way it works called nuclear fusion.

This happens in the very center of a star. The proton-proton chain is one way stars turn hydrogen into helium. This chain is the main source of power for stars that are not much bigger than our Sun. In our Sun, this process provides about 99% of its total energy. The other 1% comes from a different way called the CNO cycle.

This process works through a series of steps. First, two protons must fuse together to create a deuteron. This first step is extremely slow because it uses the weak nuclear force. An average proton in the Sun's core might wait 9 billion years to fuse!

Once a deuteron is made, it quickly joins with another proton. This creates helium-3. This second step is much faster because it uses the strong nuclear force. In the Sun, a new helium-3 nucleus only lasts about 400 years before it changes again. Finally, helium-3 nuclei fuse to create the stable helium-4 that we know.

Scientists have worked to understand these star processes for a long time. Arthur Eddington suggested in the 1920s that these reactions power the Sun. At that time, many thought the Sun was too cool for this to happen. Later, the study of quantum mechanics showed that protons can "tunnel" through barriers. This allows fusion to happen even at lower temperatures. In 1938, Hans Bethe and C.L. Critchfield proposed how protons combine to make deuterium. Hans Bethe later won the Nobel Prize in Physics in 1967 for his work on how stars create elements.

There are different paths the chain can take. One model shows that Branch I produces 83.3% of the helium-4 in the Sun.

Branch II produces about 16.68% of the helium-4. A very small amount, just 0.02%, comes from Branch III.

These branches happen at different temperatures. Branch I is common, but Branch III becomes more important if temperatures get much higher. Some energy is always lost as neutrinos, which are tiny particles that fly right through matter.

This chain is why the Sun is a stable star. When protons fuse, a tiny bit of mass is lost. This lost mass turns into a huge amount of energy. This energy creates heat and light. The heat keeps the Sun's core hot so it does not collapse under its own weight. It is like a constant engine pushing outward against gravity. Without this steady flow of energy, the Sun would not be able to support life on Earth.

The proton–proton chain is a fundamental series of nuclear fusion reactions. These reactions allow stars to convert hydrogen into helium. This process is the primary energy source for stars with a mass less than or equal to our Sun. In the Sun, this chain provides about 99% of the total energy output. The remaining 1% comes from a different process called the CNO cycle.

Nuclear fusion occurs when atomic nuclei join together. For this to happen, protons must overcome their mutual electrostatic repulsion. This repulsion is caused by their positive electrical charges. Protons require high kinetic energy, or temperature, to push past this barrier. In the Sun, quantum mechanics allows for a process called tunneling. This allows protons to pass through the repulsive barrier even at lower temperatures.

The chain begins with the fusion of two protons to create a deuteron, which is a nucleus containing one proton and one neutron. This first step is governed by the weak nuclear force. Because the weak force is involved, this reaction is extremely slow. An average proton in the Sun's core waits about 9 billion years to fuse. During this step, one proton undergoes beta plus decay. This converts it into a neutron and releases a positron and an electron neutrino. The positron then annihilates with an electron to produce two gamma rays.

After a deuteron is formed, the process moves much faster. The deuteron fuses with another proton to produce helium-3. This step is mediated by the strong nuclear force. In the Sun's core, a newly created deuteron exists for only about one second. The resulting helium-3 nucleus lasts for about 400 years before it undergoes further reactions. From helium-3, the chain can follow several different paths to create the final product, helium-4.

There are three main branches of the proton–proton chain. Branch I is the most common in the Sun. It produces approximately 83.3% of the helium-4.

Branch II is the next most frequent, producing about 16.68% of the helium-4.

Branch III is much rarer, accounting for only 0.02% of the production.

These branches are influenced by the temperature of the stellar core. For example, Branch III becomes more important if temperatures exceed 14 million Kelvin.

Scientists have a long history of studying these stellar processes. In the 1920s, Arthur Eddington suggested that proton–proton reactions power the Sun. At that time, many believed the Sun was too cool for fusion. In 1938, Hans Bethe and C.L. Critchfield proposed the specific reaction for Branch II. They identified how two protons combine to create a deuterium nucleus. Hans Bethe later won the Nobel Prize in Physics in 1967 for his work on stellar nucleosynthesis.

The energy released by the entire chain is significant. The complete process releases a net energy of 26.73 MeV. This energy comes from a loss of mass during the reaction. About 0.7% of the original mass of the protons is converted into energy. This energy takes the form of gamma rays, kinetic energy, and neutrinos. The gamma rays and kinetic energy heat the Sun's interior. This outward pressure prevents the Sun from collapsing under its own gravity. However, neutrinos do not interact much with matter. They carry away a portion of the energy, such as 2.2% in Branch I and 28.3% in Branch III. This energy loss helps explain why the Sun's luminosity is measured at specific levels.