What happens in nuclear fusion

1. In fusion, two small nuclei come together and merge into a single larger nucleus, so the number of particles bound into one nucleus goes up rather than down.
2. The reaction most often used as an example joins two isotopes of hydrogen: deuterium, ${}_1^{2}H$, which has one proton and one neutron, and tritium, ${}_1^{3}H$, which has one proton and two neutrons.
3. Written as a nuclear equation this is ${}_1^{2}H{+}_1^{3}H\to {}_2^{4}He{+}_0^{1}n$.
4. Both sides balance, because the mass numbers give $2+3=4+1$ and the atomic numbers give $1+1=2+0$.
5. The larger nucleus created here is helium-4, and a spare neutron is left over that carries away most of the energy released.
6. Fusion of small nuclei releases energy, and this holds for nuclei up to about the size of iron; joining nuclei larger than that would take energy in rather than give it out.

The loss of mass

1. Weigh everything before the reaction and everything after it, and the total mass afterwards is very slightly smaller.
2. No particles have gone missing, since the number of protons and the number of neutrons are both unchanged; it is the mass itself that has decreased.
3. That loss of mass is where the released energy comes from, so mass and energy are two forms of the same thing rather than two separate quantities.
4. The fraction of mass lost is extremely small, but the energy that appears is very large, which is why fusion releases more energy per kilogram of fuel than any chemical reaction and more even than fission.
5. A full description of fusion therefore needs three linked statements: small nuclei join, a larger nucleus is created, and a loss of mass is accompanied by a release of energy.

Fusion in stars

1. A star is a vast ball of gas that is mostly hydrogen, and in its core hydrogen nuclei fuse to form helium.
2. Every one of those reactions involves a small loss of mass, so every one releases energy, and an enormous number of them happen every second.
3. The energy released keeps the core extremely hot and eventually works its way out to the surface, from where the star radiates it away as light, infrared and other electromagnetic radiation.
4. Fusion is therefore the energy source for stars, including the Sun, and all the energy arriving at the Earth as sunlight was released by fusion in the Sun's core.
5. Because mass is lost in each reaction, the Sun is very slowly getting lighter as it shines, even though it is losing only a tiny fraction of its mass over its whole lifetime.
6. Fusion in stars also builds the heavier elements, so the carbon, oxygen and nitrogen in your body were made inside stars that existed before the Sun.

Fusion compared with fission

1. Fusion and fission are both nuclear reactions, so both change the nuclei of atoms rather than rearranging their electrons.
2. Both release energy, and in both cases the energy comes from a small loss of mass between the particles going in and the particles coming out.
3. The difference that defines them is the direction of the change: fusion joins small nuclei into a larger one, whereas fission splits one large nucleus into smaller ones.
4. The fuels are at opposite ends of the periodic table: fusion uses the lightest nuclei, such as isotopes of hydrogen, while fission uses the heaviest, such as uranium.
5. Fission has to be started by a neutron being absorbed, whereas fusion happens only where nuclei are already being driven together under extreme conditions.
6. The products differ too: the daughter nuclei from fission are radioactive, while the helium produced by fusion is a stable, harmless gas.
7. Where each one is found also differs: fission is what runs nuclear power stations on the Earth today, and fusion is what powers every star.