The Triple Alpha Process

Quick! List the most important chemical elements for life to exist. You’d certainly say oxygen, probably carbon, maybe hydrogen or nitrogen. But did you think of helium? Why would you, you might ask, when helium is a noble gas which surely plays no role in the chemistry of life? The answer lies in what happens inside stars when their hydrogen fuel starts to run out.

Helium is named after Helios, the Sun god from Greek mythology, because it was first discovered by studying the Sun’s spectrum¹ . In hindsight, this naming may seem misguided, since stars are predominantly made of hydrogen and only contain a relatively small amount of helium. In a way, it seems appropriate, though, because for most of our history humans have recognised the Sun as the ultimate giver of life, and there would be no life without helium.

As you probably know, stars are essentially giant, gravity-powered nuclear fusion reactors. The star’s weight creates enough pressure to allow atoms to fuse in its core. During the star’s main sequence lifetime this happens via a chain reaction called the proton-proton chain, which ultimately takes four hydrogen nuclei (protons) and turns them into a helium nucleus (two protons and two neutrons). That helium nucleus is also called an alpha particle. Once the hydrogen in the core starts to run out, the delicate equilibrium holding up the star against gravity falters, the pressure increases, and a new chain reaction takes over: the triple-alpha process, which I’m sure you’ve now guessed is where this newsletter’s name comes from. We have this process to thank for creating almost all the carbon in the Universe, and therefore making life possible.

It doesn’t all happen at once. First, two helium nuclei fuse to create beryllium. Then that fuses with a third alpha particle to create carbon. Sometimes a fourth helium nucleus joins to create oxygen, but mostly this process stops after three alpha particles. It’s an astonishing stroke of luck for us that these reactions happen the way that they do. The total amount of energy in beryllium plus helium happens to be very close to one of the energy levels allowed for carbon. If that weren’t the case, carbon could not be created this way and the Universe would have a lot more beryllium and almost no carbon. Similarly, if the energy levels were slightly different, the process would go on to create oxygen a lot more often, and again we would end up with no carbon.

Most of the carbon in the universe was created this way, including the carbon in your body and in all the other life on the planet. All the life that we know about is built primarily out of hydrocarbons, which are molecules made from carbon atoms chained together with hydrogen around the outside. In a literal sense, life as we know it would not exist without the triple alpha process.

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Solar System

• Saturn now has a total of 274 known moons, after a further 128 were discovered. [New York Times, National Geographic]

• Geologists in Australia just discovered the oldest known impact crater. The researchers who discovered this think such impacts may have driven the formation of continents, contrary to the usual idea that it was driven by heat from inside the planet. [Phys.org, Space.com, EarthSky]

• The Athena Moon lander toppled over and died after landing on the Moon. [New York Times, Ars Technica, Space.com]

• Unexpected chemistry and magnetism have been found in samples from the asteroid Bennu, which were returned to Earth by OSIRIS-REx in 2023. [New Scientist]

Galaxy

• A new technique uses pulsars to map the Milky Way’s dark matter. [Universe Today]

• A hypervelocity star is helping us learn about the chemistry of the galactic centre. [New Scientist]

• Over 800 new candidate white dwarf-main sequence binaries have been found in Gaia data, by using a machine learning algorithm to discover clues in the stars’ spectra. [Universe Today]

• The closest single star to the Sun, Barnard’s star, has been confirmed to have a planetary system. [Sky & Telescope, NOIRLab]

Universe

• A supermassive black hole has been found moving at over a thousand kilometres per second, evidence of a merger that could help explain how such black holes got so big. [New Scientist]

• After some delays, the SPHEREx observatory successfully launched on its mission to conduct an all-sky galaxy survey. [NASA, The Guardian]

• The Vera Rubin Observatory is complete! The camera has been successfully installed and now we eagerly await the first observations later this year. [Vera C. Rubin Observatory]

• New observations by JWST have found a young galaxy which is brighter and more chemically complex than expected. [Phys.org]

Finally

There’s something captivating about this video of a meteor burning up over Arizona:

If you liked Three Alpha, why not forward it to a friend? You can also find me on BlueSky, @adammc.space. See you next time!

1  In a future edition I’m bound to gush about all the wonderful things we can learn from spectroscopy.