α α α!

This Universe is full of carbon. Well… Not full really. I mean, technically it’s mostly full of absolutely nothing, closely followed by dark energy and dark matter… and if we’re talking about things we can see, then it’s mostly full of hydrogen… But I’m digressing. The point is that there’s quite a lot of carbon about. Enough for people like me to dedicate a lot of time to studying it. Quite a lot of carbon indeed. And here’s why…


This is the triple alpha process! It’s one of the most important stellar fusion processes. Which is strange, because it’s actually very unlikely to happen. The process overall is called “triple alpha” because essentially you put in three helium nuclei (aka alpha particles) and you get a carbon nucleus out. In reality it is, as always, slightly more complicated.

Shown in the images both above and below, the triple alpha process actually consists of two steps. The first step mashes two helium-4 nuclei into a beryllium-8 nucleus. Be-8, however, isn’t very stable. It has an energy very close* to that of two He-4 nuclei, so it tends to just fall apart again. Interestingly though, a Be-8 nucleus and a He-4 nucleus have almost the exact same energy as an excited carbon-12 nucleus does. This means that Be-8 and He-4 combine readily to make carbon’s most stable isotope** (giving a net energy output of 7.367 MeV). The C-12 then just relaxes and goes off to fuse into other elements.

This curious combination of events makes the chance of a triple alpha process occurring actually quite slim. Statistically, for just 3 alpha particles it would only happen a couple of times in the lifetime of any star. Thankfully, even the smallest stars contain billions of helium nuclei, so it happens reasonably often. Even so, it tends to take a few billion years for any sizeable amount of carbon to form. Two things do increase the rate of helium fusion though — temperature and density. The more massive a star is, the hotter and denser its core is, and so the more helium it burns.

Et voila. How stars create carbon. And eventually us. I love nuclear fusion…

*Actually Be-8 is 93.7 keV higher in energy than two He-4 nuclei. In physics the lowest energy always wins, so if ever you leave Be-8 to its own devices it rapidly decays into two alpha particles.

**Interestingly, this coincidence in energies was predicted by Fred Hoyle before it was observed. Hoyle figured it was the only way to explain how carbon was formed so plentifully.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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8 Responses to α α α!

  1. Pingback: Black hole nucleosynthesis | Supernova Condensate

  2. 6_bleen_7 says:

    Cool! I read a while back that the helium flash isn’t nearly as catastrophic for an observer as it sounds, because although the flash produces a huge amount of energy, it takes a long time for that energy to percolate up to the star’s surface, but with that much power I have to wonder.

  3. invaderxan says:

    Sorry for the delay there. I’ve been rather caught up adventuring in thesis land…
    Kinetic energy makes up that deficit, yes. At the temperatures needed for helium fusion, quite a fearsome amount of kinetic energy at that! I too have been meaning on learning more about nuclear physics, so I took the opportunity to read up a bit about stellar nucleosynthesis while writing my introduction chapter… Fascinating stuff.
    When temperatures in a star’s core become high enough for helium fusion to occur, it does so almost instantaneously, causing a “helium flash”. This burns 60-80% of the helium in the star’s core in a matter of seconds and produces an energy output comparable to the average output of some entire galaxies!

  4. 6_bleen_7 says:

    Oh yeah—forgot to ask: If 8Be is higher in energy than two 4He, doesn’t there need to be a γ on the left side of the equation? Or does kinetic energy supply the deficit?
    I absolutely love nuclear physics, although I know next to nothing about it.

  5. invaderxan says:

    It’s true, it’s ridiculously short. Looking it up, it’s 7×10-17 seconds, which is about 70 attoseconds! Probably the shortest half life I’ve ever seen, actually… Unless I’m mistaken, I don’t believe it’s because of the conditions inside the star changing the half life. While it’s technically in two steps, it’s essentially a three-body reaction. You need that third helium nucleus to be in the right place at the right time, before the beryllium decays…
    And err, that was a bit of an understatement, yeah. :)

  6. invaderxan says:

    Re: Nothing
    Well, okay. That depends how deep you wish to venture down the rabbit hole… :P

  7. 6_bleen_7 says:

    Wow, the half-life of 8Be is so short that the Table of the Nuclides gives it in units of eV, which don’t mean anything to me, but I seem to remember it being shorter than 1 fs (femtosecond). Are the conditions in, say, the Sun’s core extreme enough to change its half-life significantly?
    You’re being quite modest on behalf of stellar cores. Saying that the smallest stars contain “billions” of helium nuclei is kind of like saying that the Pacific Ocean contains “dozens” of water molecules. : )

  8. maxdwolf says:

    Nothing
    I wouldn’t characterize “empty” space as nothing.

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