In December, the United States Department of Energy made headlines around the world with a breakthrough in fusion energy: for the first time, they created a reaction that produced more energy than the power of the laser that triggered it. Although this is a big step forward, the energy to operate the lasers was still a factor of 100 greater than the energy produced by the reaction, so there is still a lot of work to be done. But it raises some interesting questions: how does fusion energy work, and what does it all have to do with the life and death of stars?
If you know the basics of nuclear energy and nuclear weapons, you might notice an apparent contradiction. Nuclear power plants and atomic bombs are both based on splitting atomic nuclei to produce energy (fission), while fusion and hydrogen bombs work on the power you get by sticking the nuclei together.
How can both be possible? It has to do with the strange adhesion of atomic nuclei, and how that adhesion depends on the number of protons and neutrons in the atom.
Let’s start with the nuclear reaction that powers the Sun: the fusion of hydrogen into helium. A neutral hydrogen atom is a proton to which an electron is attached. Newborn stars are mostly hydrogen nuclei (i.e. only protons), with a few helium nuclei, electrons, and a trace of other bouncing elements.
Because protons are all positively charged, they repel each other electrically, but given enough heat and pressure, they sometimes collide. When they do, they begin to interact with the powerful nuclear force, and that’s when everything changes. At these close distances, the strong force is attractive and stronger than the electrical repulsion, so two protons crushed in extremely close quarters attract each other.
Protons squashed together in the core of a star go through a few stages of transmutation before becoming helium, but the key is that larger nuclei are more tightly bound than smaller ones. You can think of this as adhesion.
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Generally speaking, elements lighter than iron get stickier as they get heavier, and when you fuse less sticky nuclei into stickier nuclei, you release energy. Imagine a slinky on a staircase. You have to push the slinky for it to start, but once you do it gains power on the way down and can continue as long as the stairs keep going down.
This is why fusion energy is possible in principle: if you can start a reaction and maintain it, you can create a system in which hydrogen is transformed into helium and energy is released. Hydrogen bombs work on the same principle, but 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 the fusion of helium, carbon, neon, oxygen, and silicon. For all of these, adding more protons increases the stickiness 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 stickiest of all the nuclei abundant in stars – technically there is a slightly more tightly bound form of nickel, but it is rarely produced in stars. This means you can get energy by fusing smaller nuclei together to create iron, but if you try to add more protons you’ll end up with something less tightly bound, so the process take rather than giving energy.
Iron is the atom at the bottom of the staircase, with stairs leading to hydrogen on one side and the heavier 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 is no more energy produced to keep the star from collapsing on it. -same.
At this point, the star explodes into a supernova, creating either an incredibly dense neutron star or a black hole. The explosion itself pumps energy into the stellar debris, which can create heavier items, like throwing the slinky down the stairs.
On the heavy side of the “iron peak” of adhesion, the heavier elements are less tightly bound, so the nuclear reactions that pull the nuclei apart produce energy. This is how fission works: very heavy elements such as uranium and plutonium are split in a controlled way in nuclear power plants, or explosively in atomic bombs. It still takes some effort to get the process started, like that initial slinky push, but the energy release can be immense.
Whether fusion energy will ever power our cities remains to be seen. But until then, we can still appreciate the giant fusion reactor in the sky, and the fact that it’s a safe distance away and there are billions of years of hydrogen left to burn.
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