Frontier Adventure

Carbon-12 is an Essential Building Block for Life and Scientists Have Finally Figured Out How it Forms in Stars


Artist’s impression of a red giant star. Their cores are cauldrons where carbon-12 is produced. Credit:NASA/ Walt Feimer

Each of us is, as it says in Max Ehrmann’s famous poem “Desiderata”, a child of the universe. It speaks metaphorically about our place in the cosmos, but it turns out to be a very literal truth. Our bodies contain the stuff of stars and galaxies, and that makes us children of the cosmos. To be more precise, we are carbon-based life forms. All life on Earth is based on the element carbon-12. It turns out this stuff is a critical gateway to life. So, how did the universe come up with enough of it to make you and me and all the life on our planet? Astrophysicists and nuclear physicists think they have an answer by using a supercomputer simulation of what happens to create carbon. As it turns out, it’s not very easy.

The recipe for carbon-12 requires a pressure cooker and a lot of source material. The environment inside a star or during a stellar collision or an explosion provides the pressure cooker. The ingredients inside are helium-4 atoms and a theoretically forbidden nucleus of something called beryllium-8 (8Be). Put them all together and eventually, you get carbon-12. Sounds simple, right?

Well, not exactly. There’s no way to replicate this recipe in the lab to test it and prove the process. That’s because you need temperatures and pressures that exist only inside stars. To understand why we can’t reproduce the birth of carbon, here’s a simple outline of a complex process that astrophysicists think is happening.

Diving into Stars

The cores of stars engage in a process called nucleosynthesis. That’s a fancy term for “cooking up new elements.” So, for most of its life, a star is a giant sphere of hydrogen. Inside, the conditions are just right for it to fuse hydrogen atoms to make helium atoms. Take it a step further, and fuse two helium nuclei together, and you get that 8Be. There’s a problem though: the beryllium isotope should not exist. So, we’re at an impasse. If these things can’t be made, you don’t get carbon, and life maybe doesn’t get to exist. What’s the solution? Not silicon-based life forms (as we often hear about in science fiction), or, we still wouldn’t exist. (At least not in our current form.)

Getting to Carbon-12

So, carbon is important. It’s everywhere. Carbon is the fourth most abundant element after hydrogen, helium, and oxygen. The Big Bang created hydrogen and helium. Stars make oxygen and carbon. Older stars such as red giants are prodigious producers of carbon and they are everywhere. So, obviously, it happens. How? It turns out that things are fast and furious inside a star. The “forbidden” 8Be actually can exist, even if only for brief moments. However, it decays in a few nanoseconds.

A graphical look at the triple-alpha process that produces carbon-12. Image credit: Borb, CC BY-SA 3.0.

So, there isn’t much time to create carbon-12. But, it happens, or we wouldn’t be here to talk about it. Here’s how. There’s a chemical reaction involving three helium nuclei called “alpha particles” and the very short-lived 8Be. If conditions are just right inside the star, the alpha particles and the 8Be can fuse together. This forms the basis for carbon-12 production. This is the “triple alpha” process and occurs inside these older stars with extremely hot interiors (like above 108 K).

Modeling the Formation of Carbon-12

We can’t look inside an old red giant star
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