When I was in the Navy, I had innumerable firefighting trainings (including one with a simulated fire that we had to fight, full gear, hose and all—exhausting and sweaty work!). One thing we went over every time was the fire triangle, the three components that are necessary for a fire to burn: fuel, oxygen, and heat. Remove one side of the triangle, and the fire will go out. So it isn’t surprising to me that many people carry this piece of knowledge and try to apply it to the Sun. How can the Sun burn, they ask, without oxygen1? It’s a very common and understandable misconception, especially because the Sun does have plenty of fuel and heat, and it does burn. But of course it can’t be the case that the Sun needs oxygen to burn, because it continues to shine in space.
This is because the Sun burns, but it isn’t burning. Or at least it isn’t “on fire.” The way words can take on different shades of meaning is one of the beauties of language, but it can also be a source of confusion as it is here. The Sun burns in the sense that it releases energy by consuming fuel, but the process by which that happens is not combustion; it is nuclear fusion.
Combustion—that is, a typical fire supported by the fire triangle—is a chemical reaction between the fuel and oxygen (or another oxidizing agent). The energy produced from combustion comes from the breaking of atomic bonds. In a combustion reaction, the bonds in the fuel (the reactant) contain more energy than the bonds in the products (which might be gases, water, etc.). Given the conservation of energy, there must be the same amount of energy on either side, so on the product side, that energy becomes heat.
In nuclear fusion, the same concept applies. There is fuel (hydrogen, for the Sun) and there are products (helium, primarily). However, the energy difference here is not from changing atomic bonds. It’s because the mass of the helium is less than the mass of the hydrogen that it is produced from (via a series of interactions). Since energy and mass are interchangeable, the conservation of energy, which is more accurately the conservation of mass-energy, dictates that there must have been energy released during the reaction.
While a fire needs the triangle of fuel, oxygen, and heat, the triangle for nuclear fusion is fuel, heat (that is, high temperatures), and density. The temperatures and densities required are extreme, but they are found in the core of the Sun, where hydrogen starts fusing into helium at about four million kelvin. The density of the Sun’s core is about 1250 pounds per gallon, which is 150 times denser than water and 13 times denser than lead. Given the tiny mass of hydrogen, that means an ice cream scoop of the Sun’s core would contain roughly ten octillion hydrogen atoms. And it’s a good thing, too, because it turns out that hydrogen atoms really don’t like to combine together. They are both positively charged, and in fact they basically wouldn’t ever come together if not for quantum tunneling. The Sun is basically shining through sheer force of quantity.
You can see why achieving these conditions on Earth is so difficult—fusion ignition was achieved in a laboratory for the first time ever just in 2022, and we’re still a long way from being able to use it in place of combustion. But we can still take advantage of the amazing way the Sun burns, whether that’s enjoying the sun on your face, growing a garden, or harnessing that power via solar panels. It’s thanks to this fusion that there is light and life in the universe!
Cheers to our not-burning burning Sun!
1. Meaning “oxygen” as part of the atmosphere, O2 gas. The Sun does contain trace amounts of oxygen, less than one atom per thousand, but that isn’t the type of oxidizing agent meant in the fire triangle. [back]