What Powers Main-Sequence Stars?

Hint: It’s not just fusion!

First Thing’s First…

My post on protostars and star birth was released last week, check that out if you want things chronologically. If you’re just here for MS stars, in this post we’ll be covering three different ways stars can use fusion to release energy.

The proton-proton chain reaction is the power source for small stars

The proton-proton chain reaction is a simple process that occurs mostly in small sun-sized stars. As the name implies, we start off with 2 protons (2 hydrogen nuclei) and end up with a helium nuclei and the right materials to begin the process again (2 more hydrogen nuclei).

There are multiple steps to get from a hydrogen nucleus (a proton) to a helium nucleus (2 protons + 2 neutrons). We do this by taking 1 proton and adding a few particles bit by bit.


We start by creating deuterium using 2 protons. One of the protons undergoes a quark flavour change and becomes a neutron and in the process we also get a positron and neutrino pair. This is classic weak interaction.

The next step is where we experience an elemental change. By adding a proton, we can turn the deuterium into almost helium, and in the process we release a photon. Since both nuclei are and proton are charged AND we emit a photon in the collision, we can safely say that this interaction is governed by the electromagnetic force.

Lastly, let’s take 2 of those nearly helium nuclei and smash them together to get proper helium! Judging by the fact that there’s no quark flavour change and we’re only using hadrons here (protons + neutrons), it seems that this collision is governed by the strong force, but it still could be a weak interaction.

The CNO cycle powers bigger stars

Vega by Francisco José Sevilla Lobato

The Carbon-Nitrogen-Oxygen cycle is a far more complex cycle that dominates in larger stars. It outputs more energy, but to do that it requires a much higher temperature within the star.

There are 7 different types of CNO cycles, but the main process is the same. We start with 4 protons (hydrogen nuclei) and 2 electrons, which react to make a helium nucleus, 2 electrons and 2 positron-neutrino pairs. The positron and electrons will annihilate to leave us with a helium nucleus and 2 neutrinos. Plus some energy! We can’t forget that!

You may have noticed that carbon, nitrogen, and oxygen aren’t seen here at all! This is because they are catalysts in this cycle, NOT reactants NOR products.

Triple Alpha is needed for the mightiest stars!

We have our hydrogen fusing into helium. What next?

When the star runs out of hydrogen, it turns to helium for its main source of energy and begins the triple-alpha process. This can only be done if the star is hot enough, so it may need to contract in order to heat up.


The first step is to create Beryllium-8, an unstable isotope of Beryllium. It doesn’t last long, so we need another helium nucleus to super speedily collide with the Beryllium to make carbon. At low temperatures the carbon just doesn’t stabilise easily and decays back into some helium nuclei, however, this process is highly effective at high temperatures.

From here on carbon can be used to create heavier elements up to iron. But that’s for another time!

Next week we’ll be taking a break from stars, and we’ll have a look at the biggest disc-destroyer of them all: Lunar eclipses!


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