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.
Watch an introduction to the Hot CNO Cycle in the video below with Dr James Keegans, a teaching fellow at the University of Hull.
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:
A proton is captured, and then a photon is released to further stabilise the new isotope.
A proton is captured, and then an alpha particle is immediately emitted.
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.
Students can explore the Hot CNO cycle on an A3 sheet as a board game. Start on any carbon or nitrogen isotope and follow the instructions under each tile. You can find details about the game in the instructions document, which also includes a blank grid for the students to draw their cycles onto!
The game can also be played on a larger scale. In the loan kit, we provide laminated A4 and A3 sheets for you to set up in the same grid as the small board game version. You can find a written version of these instructions provided by the National STEM Learning centre.
For information about how to play the Hot CNO Cycle game, in particular the floor tiles version, please watch the video below by Bethany and Jenn from the University of York.
Module 3: Nuclear Astrophysics
Understanding the Hot CNO cycle allows us to determine how stars generate their light, how they evolve throughout their life, and how they die - whether in violent explosions or otherwise. It also allows us to predict which chemical elements are produced during the life and death of stars through the so-called nucleosynthesis processes.
After your class has explored the Hot CNO cycles using the interactive activities, there is a student handout for them to consolidate what they've learned. There are also some teacher notes to accompany these activities.
You can find some Isaac Physics questions you can set to your students related to particle physics below. Please feel free to look at the answers for these questions, as well as our page for help setting up an account on Isaac Physics!
Post-16 Level
To create your own set for the Hot CNO cycle, you'll need the following for each pair/small group:
A single A3 sheet; or printed A4 or A3 laminated tiles/sheets;
Plastic coins, or anything with to be flipped/rolled for two equal outcomes (e.g. dice, or online coin flip);
Stopwatches, or anything with a timer, (e.g. a mobile phone);
Counters, or any small object to track progress around the board.
Cycling the hot CNO: a teaching methodology
An interactive activity to teach the hot Carbon, Nitrogen and Oxygen (HCNO) cycle is proposed. Justification for why the HCNO cycle is important is included via an example of x-ray bursts. The activity allows teaching and demonstration of half-life, nuclear isotopes, nuclear reactions, protons and α-particles, and catalytic processes. Whilst the process example is specific to astrophysics it may be used to teach more broadly about catalytic processes. This practical is designed for use with 10–20 participants, with the intention that the exercise will convey nuclear physics principles in a fun and interactive manner.
Frost-Schenk JW, Diget CA, Bentley MA, Tuff A. Cycling the hot CNO: a teaching methodology. Physics Education. 2017 Dec 22;53(2):024001.