Stars are powered by a process called nuclear fusion. This is where atomic nuclei combine under enormous pressure and temperature. In the Sun, the dominant nuclear fusion process is called the proton-proton chain. This is the pathway for hydrogen nuclei to combine to make helium. Getting from hydrogen to helium involves a few steps.
Step One – deuterium
Two hydrogen nuclei (protons) crash into each other at just the right angle and combine forming an isotope of hydrogen called deuterium. In the process, one of the protons converts to a neutron and a positron and a neutrino are released.
Step Two – helium-3
Another hydrogen nucleus crashes into the deuterium. If it overcomes the repulsive electromagnetic force of the deuterium, it’ll join the nucleus and make an isotope of helium called helium-3. It releases a neutrino as well.
Step Three – helium-4
After a while, a few helium-3 nuclei are flying around, and eventually two will collide; they have to overcome the repulsion between the positive charges of both nuclei for this to happen. If this happens, both nuclei will combine with two protons flying off, and the remaining two protons and two neutrons will combine to make a helium-4 nucleus.
Energy. At every step, energy is released, and that energy is either carried away by particles or converted into kinetic energy. The energy itself comes from the loss of mass that occurs in the nucleus. The masses of protons and neutrons that are bound together in a nucleus are a tiny bit smaller in mass than free protons and neutrons flying around on their own. Ultimately, it’s the difference between the mass of four free protons and the mass of the helium-4 nucleus that accounts for the energy released in the process.
The diagram below shows each step of the proton-proton chain and how energy is released during the process.
Proton-proton chain diagram by Sam Leske. Click the reset button to start again.
References
Featured photo is from NASA https://science.nasa.gov/blogs/the-sun-spot/2023/09/26/layers-of-the-sun/
