Hot CNO Cycle

The Hot CNO cycle is a sequence of reactions that provides most of the energy radiated by stars hotter than our Sun. It is one of the two known sets of fusion reactions within stars that converts hydrogen to helium, the other being the proton-proton chain, which is the main source of energy for our Sun.

As opposed to the proton-proton chain, which consumes all its constituents, the CNO cycle fuses four protons together, using carbon, nitrogen, and oxygen as catalysts. In the hot CNO cycles, (as opposed to the cold CNO cycles) the radioactive species will capture a proton before it has a chance to beta decay.

One complete cycle produces one alpha particle. At very high temperatures, alpha particles can be captured to create heavier elements. When this happens, the cycle is broken, and it is known as the Hot CNO cycle.

In the Hot CNO cycle, four protons are converted into a helium nucleus through alpha decay. This is an example of hydrogen fusion, but is a catalytic process where carbon, (C) nitrogen, (N) and oxygen (O) are used to speed up the fusion processes. 

Each of the steps in the loops are one of the following three nuclear equations:

1. A proton is captured, and then a photon is released to further stabilise the new isotope.

2. A proton is captured, and then an alpha particle is immediately emitted.

3. These isotopes will decay via β+ decay.

Each of these three loops lead to the net stellar fusion equation: 4H1 → 2He4 + 2β+ + 2νe 

The resources included in the loan kit for these activities are:

• A single A3 laminated sheet with all isotopes;

• A4 laminated sheets of each isotope, to be laid out across classroom tables;

• A3 laminated sheets of each isotope, to be laid out across a classroom floor;

• 50p coins, to flip to simulate the random process nature;

• Stopwatches, for you to time the half-life of the nuclear processes;

• Counters, to track your progress moving around the board.