Nuclear Fusion: Powering the Stars

Nuclear fusion is the opposite of fission: instead of splitting a heavy nucleus, you combine two light nuclei into a heavier one. This is what powers every star in the universe, including our sun. The sun fuses about 600 million tons of hydrogen into helium every second, converting about 4 million tons of mass into energy via $E = mc^{2}$.

The deuterium-deuterium reactions

Two deuterium nuclei can fuse via two equally probable channels:

$^{2}\text{H} + ^{2}\text{H} \to ^{3}\text{He} + ^{1}\text{n} + 3.27\text{ MeV}$

$^{2}\text{H} + ^{2}\text{H} \to ^{3}\text{H} + ^{1}\text{p} + 4.03\text{ MeV}$

The proton-proton chain (how the sun works)

The dominant fusion process in the sun is the proton-proton chain:

1. $^{1}\text{H} + ^{1}\text{H} \to ^{2}\text{H} + e^{+} + \nu_{e}$ (two protons fuse to deuterium + positron + neutrino)
2. $^{2}\text{H} + ^{1}\text{H} \to ^{3}\text{He} + \gamma$ (deuterium + proton → helium-3)
3. $^{3}\text{He} + ^{3}\text{He} \to ^{4}\text{He} + 2\,^{1}\text{H}$ (two helium-3 → helium-4 + two protons)

Net: $4\,^{1}\text{H} \to ^{4}\text{He} + 2e^{+} + 2\nu_{e} + 26.7\text{ MeV}$

Four hydrogen atoms become one helium atom, releasing 26.7 MeV — about 0.7% of the original mass converted to energy.

Why fusion is so hard on Earth

The Coulomb barrier: both nuclei are positively charged and repel each other. To get close enough for the strong nuclear force to take over (at $\sim 10^{-15}$ m), you need temperatures of ~100 million K (10× hotter than the sun's core, because we can't rely on the sun's enormous gravitational pressure). At these temperatures, matter is a plasma.

Current approaches: magnetic confinement (tokamaks like ITER), inertial confinement (lasers like NIF), and newer concepts. In December 2022, NIF achieved "ignition" — more fusion energy out than laser energy in — for the first time.

Fusion vs fission energy comparison

• Fission of 1 kg U-235: ~82 TJ
• Fusion of 1 kg deuterium: ~340 TJ (4× more energy per kg)
• Fusion fuel (deuterium) is abundant: 1 in 6500 hydrogen atoms in seawater is deuterium. The oceans contain enough deuterium for billions of years of energy.

Common mistakes

• Thinking fusion and fission release energy for the same reason. They don't. Fission releases energy because heavy nuclei have less binding energy per nucleon than medium ones. Fusion releases energy because light nuclei have less binding energy per nucleon than medium ones. Both converge toward iron-56 (the most tightly bound nucleus).
• Thinking the sun uses fission. The sun is 100% fusion. No fission occurs naturally in stars (fission requires very heavy elements that are rare in stars).

Try it in the visualization

Watch two deuterium nuclei approach, overcome the Coulomb barrier, and fuse into helium-3 + neutron. The mass defect is shown as the difference between input and output masses, with the deficit appearing as a burst of energy. Toggle between D-D fusion and the proton-proton chain to see the sun's process. The energy comparison bar shows fusion dwarfing chemical reactions.