Ever wondered what makes the Sun (and all stars ever) shine? That would be nuclear fusion, the process responsible for lighting up our sky, day and night. It is absolutely key to the lives of stars, from their birth, to their explosive demise billions of years later. Ultimately it generates the heat that keeps our planet warm enough to sustain life. This is how it works.
At its simplest, nuclear fusion is just two atoms smashing together and creating a new, different atom, releasing energy in the process. Feel free to stop reading there, unless you fancy delving into some nuclear theory, with just a dash of quantum mechanics added for good measure.
All atoms consist of a nucleus, made up of protons and neutrons, surrounded by electrons. The nucleus is the important bit for nuclear fusion, so we can forget about the electrons here. The only thing that determines which element an atom is, is the number of protons in the nucleus. Hydrogen, the simplest element, has just one proton, Helium has two, Lithium three, Gold has 79, and so on. Protons have a positive charge, while neutrons are neutral, so atomic nuclei repel one another.
Another thing almost all nuclei have is some missing mass; adding up the mass of all the individual protons and neutrons gives a bigger answer than the actual observed mass of a nucleus. Thanks to Einstein’s famous equation, E = mc^2 (E being energy, m mass and c the speed of light), we can convert mass to energy. The missing mass is accounted for as energy. In fact, it is this energy that holds the nucleus together, which is why it’s known as the binding energy. It takes a different amount of energy to hold different nuclei together.
The Sun, like all other stars, is made up mainly of Hydrogen, the simplest of the elements. Thanks to the Sun being immensely massive (it weighs in at some 2 billion billion billion tonnes), there’s a huge gravitational force acting, compressing the Hydrogen gas, until it gets up to at around 15 million degrees Celsius, where nuclear fusion can kick in.
When two nuclei get close enough to one another, the strong nuclear force can take hold and grip them together. “Close enough” in this case turns out to be about 1 femtometre, or about 100 billion times shorter than the width of a hair. This is a bit of problem, as nuclei repel one another due to their positive electric charge. Even at the ridiculous temperatures at the core of stars, nuclei’s energies are insufficient to overcome the electric repulsion and allow nuclei to come close enough for the strong force to kick in.
Now this is where the quantum mechanics comes in. Quantum mechanics allows particles to occasionally be found where, according to classical theory, they shouldn’t be. Thus the nuclei can get close enough for the strong force. However, before quantum mechanics was discovered, nuclear fusion was still thought to be the source of the Suns power, with the temperature problem dealt with by legendary astrophysicist Arthur Eddington; “we do not argue with the critic who urges that the stars are not hot enough for this process; we tell him to go and find a hotter place.”
So now we’ve got two Hydrogen nuclei, i.e. two protons, at a distance small enough that the strong force can do its business. Lots of nuclear goings on follow, with a hopefully helpful diagram:
The two protons are bound together creating a Helium nucleus. One of these then switches to a neutron, which releases a neutrino, a wraith-like particle that very rarely interacts with anything, and a high energy photon, also known as a gamma ray. A positron, or anti-electron, is also emitted. We now have a Hydrogen nucleus again, a neutron bound to a single proton. The change in the mass and binding energy in two free protons becoming a bound neutron and proton releases the energy that the photon and neutrino carry away.
Another proton then joins the party, giving a Helium nucleus, with two protons and one neutron. Again the binding energy changes, releasing another high energy photon.
Two of these Helium nuclei then collide and fuse, but two protons are also ejected, so the final product is still a Helium nucleus as it still has two protons, along with the two neutrons. This nucleus is stable and will hang around in the star until all the Hydrogen fuel is all used up and gravity resumes its compression of the star, heating it and Helium fusion begins. But that’s more complicated and something to put aside for later.
All in all, this process of fusing Hydrogen into Helium releases 26.7 MeV of energy (or 4 trillionths of a single Joule). The Sun has an energy output of 380 trillion trillion joules per second, which means that every single second, for 10 billion years, the process of Hydrogen fusion above happens 1038 times. That’s 10 followed by thirty-eight zeroes. Every single second. For 10 billion years. The scale of these numbers is simply beyond comprehension.
Nuclear fusion doesn’t just keep the lights on in the sky, it also forges all of the elements up to Iron, as stars repeatedly exhaust their fuel, collapse and heat and begin fusion of the products of the previous stage. For instance, in about 5 billion years the Sun will run out of Hydrogen and then begin to fuse Helium into Carbon. It will also expand greatly and probably engulf the Earth and whoever’s left by then, but that’s definitely a problem for future generations.
As for the current crop of humanity, we first brought fusion to Earth in 1952 with the Hydrogen bomb Ivy Mike, which was about 500 times more powerful the fission powered nuclear bomb dropped on Hiroshima. Thankfully no fusion powered bomb has ever been used in warfare. Indeed, most fusion research is now carried out with the goal of harnessing nuclear fusion as an energy source in grand devices known as stellarators and tokamaks. Unfortunately, they remain inefficient, requiring more energy to be put in to initiate fusion than is released by the fusion, but work is very much ongoing to solve this. Nevertheless, humanity still has a great deal to learn before we can harness the vast power of the stars.