High Mass Nucleosynthesis

The term nucleosynthesis means synthesizing atomic nuclei through nuclear reactions. That's what stars do.

So we've made helium. Now what? For main sequence stars, now nothing. The main sequence is solely populated by stars turning hydrogen into helium.

But let's say we wanted to continue the fusion process -- can we turn helium into something else?

What would we have to do to the centers of main sequence stars to ignite helium burning?
How could stars do this?

The Triple Alpha Process

At sufficiently high temperatures and densities, a 3-body reaction called the triple alpha process can occur:

Two helium nuclei ("alpha particles") fuse to form unstable beryllium. If another helium nucleus can fuse with the beryllium nucleus before it decays, stable carbon is formed along with a gamma ray.

How does this depend on temperature? Around 10⁸ K, the relationship is E_3a ~ T⁴¹. A 10% increase in temperature results in a 50x increase in energy production!

At these temperatures, other reactions can also occur by the capture of more helium nuclei:

Past this, we need yet higher temperatures and densities to do anything more. How do we get those higher temperatures and densities?

Late stage nucleosynthesis

If a star has sufficient mass (and the Sun doesn't!), the central temperatures and densities can climb enough to overcome the Coulomb barrier for combining heavy elements.

For example, carbon burning (at 6x10⁸ K):

and oxygen burning (at 10⁹ K):

(Most of) these reactions create energy. If the star is massive enough, it can shine by making progressively more massive nuclei in its core. Until, that is.....

Iron

Once Iron (Fe) is made, we're stuck. Iron is the most stable of elements -- it doesn't won't undergo nuclear fusion. If a star is massive enough to create Iron, it's out of fuel. What now? All hell breaks loose. We'll see this later....

The Beauty of Nucleosynthesis

As we will see later, when the Universe formed it was full of almost entirely hydrogen and helium -- no heavy elements. Yet here we are. Where did our atoms come from?