In December, the US Department of Energy made headlines worldwide with a fusion power breakthrough: for the first time, they created a reaction that produced more energy than the laser power that ignited it. While this was a major step forward, the energy to run the lasers was still a factor of 100 more than the energy produced by the reaction, so there’s a lot of work yet to be done. But it brings up some interesting questions: how does fusion power even work, and what does all of this have to do with the life and death of stars?


If you’re familiar with the basics of nuclear power and nuclear weapons, you might notice an apparent contradiction. Nuclear power plants and atomic bombs are both based on splitting atomic nuclei to produce power (fission), whereas fusion and hydrogen bombs work on the power you get by sticking nuclei together.

How can both be possible? It has to do with the weird clinginess of atomic nuclei, and how that clinginess depends on how many protons and neutrons the atom has.

Let’s start with the nuclear reaction that powers the Sun: hydrogen fusing into helium. A neutral hydrogen atom is a proton with an electron bound to it. Newborn stars are mostly hydrogen nuclei (i.e. just protons), with some helium nuclei, electrons, and a trace of other elements bouncing around.

Because protons are all positively charged, they electrically repel each other, but with enough heat and pressure they will sometimes smack together. When they do, they start to interact with the strong nuclear force, and that’s when everything changes. At those close distances, the strong force is attractive and stronger than electric repulsion, so two protons smashed into extremely close quarters attract.

The smashed-together protons in the core of a star go through a few stages of transmutation before becoming helium, but the key is that the larger nuclei are more tightly bound than the smaller ones. You can think of it like clinginess.

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Generally speaking, elements lighter than iron get clingier as they get heavier, and when you fuse less clingy nuclei into more clingy nuclei, you get energy out. Imagine a slinky on a flight of stairs. You have to give the slinky a push to get it started, but once you do, it gains energy as it descends and can keep going as long as the stairs keep going down.

This is why fusion power is possible in principle: if you can get a reaction started and keep it going, you can create a system in which hydrogen is transformed into helium and energy is released. Hydrogen bombs work on the same principle, just more explosively.

In stars, fusion is responsible for creating some of the most common elements on Earth. When a massive star converts all the hydrogen it can, it moves up the periodic table, creating concentric shells for helium, carbon, neon, oxygen, and silicon fusion. For all these elements, adding more protons increases the clinginess of the nucleus, and so energy is produced in the process. But something changes when you get to iron, and it’s catastrophic.

Iron is the clingiest of all the nuclei that are abundant in stars - technically there’s a form of nickel that is slightly more tightly bound, but it’s rarely produced in stars. What that means is that you can get energy by fusing smaller nuclei to create iron, but if you try to add more protons, you’ll end up with something less tightly bound, so the process will take rather than give energy.

Iron is the atom at the bottom of the staircase, with stairs leading up to hydrogen on one side and to the heaviest elements on the other. The consequence for a star is that once it has a core full of iron, fusion no longer works there, and there’s no more energy being produced to keep the star from collapsing on itself.

At that point, the star explodes in a supernova, creating either a fantastically dense neutron star or a black hole. The explosion itself pumps energy into the stellar debris, which can create heavier elements, like throwing the slinky up the stairs.

On the heavy side of the 'iron peak' of clinginess, heavier elements are less tightly bound, so nuclear reactions that break nuclei apart produce energy. That’s how fission works: very heavy elements like uranium and plutonium are split in a controlled way in nuclear power plants, or in an explosive way in atomic bombs. It still takes some effort to get the process started, like that initial push of the slinky, but the energy release can be immense.

Whether or not fusion energy will someday power our cities is yet to be seen. But in the meantime, we can always appreciate the giant fusion reactor in the sky, and the fact that it is at a good safe distance and has billions of years’ worth of hydrogen left to burn.

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Dr Katie Mack is a theoretical astrophysicist exploring a range of questions in cosmology, the study of the universe from beginning to end. She currently holds the position of Hawking Chair in Cosmology and Science Communication at the Perimeter Institute for Theoretical Physics, where she carries out research on dark matter and the early Universe and works to make physics more accessible to the general public. She is the author of the book The End of Everything (Astrophysically Speaking) and has written for a number of popular publications, such as Scientific American, Slate, Sky & Telescope, Time, and Cosmos magazine.