Fission splits heavy nuclei apart. Fusion does the exact opposite: it joins two light nuclei together to make a bigger one, and releases even more energy per kilogram than fission. This is the reaction that lights up every star in the sky, including our Sun. Here’s the beautiful part — the energy that stops a star collapsing under its own gravity comes from hydrogen nuclei crashing together in its core. Let’s see how, and why it’s so hard to make happen.
📚 What you need to know
Nuclear fusion is the joining of two small nuclei to produce a larger nucleus
Only low-mass nuclei (like hydrogen and helium) release energy when they fuse
In stars, four hydrogen nuclei fuse to form one helium-4 nucleus, releasing huge energy
This energy provides an outward radiation pressure that stops the star collapsing under gravity
Energy is released because the product has a higher binding energy per nucleon
For fusion to occur, nuclei need very high kinetic energy to overcome Coulomb repulsion
This requires extremely high temperature and density, like a star’s core
On Earth, research focuses on the deuterium–tritium (D–T) reaction
What is fusion?
Nuclear fusion is defined as the joining of two small nuclei to produce a larger nucleus. It’s the mirror image of fission — instead of a heavy nucleus splitting, two light ones merge. And just like fission, it can release energy, because the new nucleus is held together more tightly than the pieces that made it.
The catch is that only light nuclei release energy when fused. Looking at the binding-energy-per-nucleon curve, light nuclei sit low on the left-hand side; fusing them moves the product up toward the peak (around iron), so the nucleons end up more tightly bound. That climb in binding energy is released as energy.
Here’s the single idea that ties fission and fusion together: both release energy by moving nuclei towards the peak of the binding-energy-per-nucleon curve. Heavy nuclei are to the right of the peak, so you split them. Light nuclei are to the left, so you join them. Iron sits at the very top — it’s the most stable, so you can’t get energy by fusing or splitting it. One curve, two processes.
Fusion in the Sun
In the core of a star like the Sun, the overall result is that four hydrogen nuclei (protons) fuse into one helium-4 nucleus. Along the way, some protons convert into neutrons, releasing positrons and neutrinos. The reaction gives out a huge amount of energy.
When two protons first fuse, one converts into a neutron through beta-plus decay, producing a deuterium nucleus plus a positron and a neutrino:
First step: two protons fuse11H + 11H → 21H + e+ + νe
This released energy is what creates the outward radiation pressure that balances the inward pull of the star’s own gravity — keeping the star stable and stopping it from collapsing. Without fusion, gravity would win and the star would shrink.
Why fusion is so hard: the Coulomb barrier
Here’s the problem. Both nuclei are positively charged, so they repel each other through the electrostatic (Coulomb) force. To fuse, they must get close enough for the strong nuclear force to take over — but that force has an extremely short range, so the nuclei have to almost touch.
To overcome that repulsion, the nuclei need enormous kinetic energy. That means they must be in an extremely hot and dense environment (like a star’s core), or be accelerated to very high velocities (like in a particle accelerator).
Two nuclei repel as they approach (the rising Coulomb barrier). Only if they have enough kinetic energy to climb over the peak can they fall into the deep strong-force well and fuse.
Fusion on Earth: the deuterium–tritium reaction
Recreating the Sun’s conditions on Earth is incredibly difficult, so fusion research focuses on the easiest reaction to achieve: the deuterium–tritium (D–T) reaction. A deuterium nucleus and a tritium nucleus fuse to make a helium nucleus and a spare neutron:
Deuterium–tritium fusion21H + 31H → 42He + 10n
Deuterium (1p+1n) and tritium (1p+2n) fuse into a helium-4 nucleus and a spare neutron, releasing energy. The helium nucleus weighs slightly less than the parts — that lost mass is the energy.
The total mass of the helium nucleus is less than the total mass of the separate nucleons. That mass difference is released as energy — and because so little fuel is needed, the reaction gives a large amount of energy per unit mass, which is what makes it so attractive for future power stations.
Light nuclei (high KE)
overcome Coulomb barrier
Strong force binds them
mass defect → energy
Larger nucleus + energy
WE 1
Two deuterium nuclei (21H) fuse to form one helium-4 nucleus (42He). The binding energy of a helium-4 nucleus is about 28 MeV, and the total binding energy of the two deuterium nuclei is about 4 MeV. Calculate the energy released.
Step 1 — the idea
Energy released = rise in binding energy from reactants to product.
Step 2 — subtractE = BE(helium) − BE(2 deuterium)E = 28 − 4E = 24 MeV releasedThe product is more tightly bound (higher total binding energy), and that difference comes out as energy. Fusion releases energy whenever the binding energy per nucleon goes up.
WE 2
In the D–T reaction 21H + 31H → 42He + 10n, the masses are: deuterium 2.014102 u, tritium 3.016049 u, helium-4 4.002602 u, neutron 1.008665 u. Calculate the energy released, in MeV. (1 u = 931.5 MeV c−2)
Step 1 — mass beforembefore = 2.014102 + 3.016049 = 5.030151 uStep 2 — mass aftermafter = 4.002602 + 1.008665 = 5.011267 uStep 3 — mass defectΔm = 5.030151 − 5.011267 = 0.018884 uStep 4 — energyE = 0.018884 × 931.5≈ 17.6 MeV releasedThis is the standard D–T value. Notice it’s less per reaction than fission’s ~200 MeV, but per kilogram of fuel it beats fission, because the fuel nuclei are so light.
⚛ Working a fusion question
Check it’s fusion: two light nuclei joining into a bigger one.
Balance the equation: mass numbers and atomic numbers each sum equal.
Energy from masses: Δm = mass before − mass after, then × 931.5 for MeV.
Energy from binding energies: BE(product) − BE(reactants).
Why it needs heat: high KE to beat the Coulomb barrier.
💡 Top tips
Fusion = joining light nuclei; fission = splitting heavy ones.
Only nuclei lighter than iron release energy when fused.
Energy comes from the mass defect / rise in binding energy per nucleon.
Nuclei need high KE to overcome Coulomb repulsion — hence high temperature.
Fusion releases more energy per kg of fuel than fission.
⚠ Common mistakes
Thinking any nuclei release energy when fused — only light ones do
Saying fusion needs a neutron to start it — that’s induced fission
Forgetting the Coulomb barrier is why such high temperatures are needed
Confusing the strong force (very short range) with the electrostatic force
Writing that mass is “destroyed” — it’s converted into energy
Quick recap:Fusion joins two light nuclei into a larger one, releasing energy from the mass defect (a rise in binding energy per nucleon). In stars, four hydrogen nuclei fuse into helium-4, and the energy released creates the radiation pressure that balances gravity. Nuclei need enormous kinetic energy to beat the Coulomb barrier, so fusion needs extreme heat and density. On Earth we chase the D–T reaction (~17.6 MeV).
We’ve seen that fusion in a star’s core is what holds it up against gravity — the outward radiation pressure balancing the inward pull. That balance is the whole life story of a star: born in a collapsing cloud, held steady for billions of years by fusion, and eventually changing dramatically once the fuel runs low. Next page: Star Formation and the equilibrium that keeps a star stable.
Fusion reactions still feeling tricky?
Book a free meeting and we’ll drill the mass-defect energy calculation, the binding-energy route, and the Coulomb-barrier reasoning examiners love to test.
