How helium is made in the Sun — step-by-step explanation for students

Overview

The Sun makes helium by fusing hydrogen nuclei (protons) in its core. The dominant process in the Sun is the proton-proton (pp) chain. In hotter, more massive stars a related process called the CNO cycle dominates. Fusion turns four protons into one helium-4 nucleus, releasing energy that powers the Sun.

What conditions are needed

• Core temperature: about 1.5 × 10⁷ K (15 million K).
• Core density: roughly 150 g/cm³ (much denser than water).
• Very high pressure so protons are packed close together.
• Quantum tunneling: even with high temperature, protons would classically repel each other. Quantum tunneling lets them get close enough to react.

Why fusion is slow (and why that matters)

Protons are positively charged and repel via the Coulomb force. Even at 15 million K, the average kinetic energy is small compared with the Coulomb barrier. Fusion in the Sun depends on the tiny probability that two protons will quantum-tunnel through the barrier. That makes individual reactions rare, which is good: it gives the Sun a multi-billion-year lifetime.

The proton-proton chain, step by step

1. Step 1: Two protons fuse

   1H + 1H → 2H + e⁺ + ν_e
   Two protons combine to form deuterium (2H), releasing a positron (e⁺) and an electron neutrino (ν_e). This is a weak-interaction process, so it is slow and controls the overall reaction rate.

2. Step 2: Deuterium captures a proton

   2H + 1H → 3He + γ
   Deuterium fuses with another proton to make helium-3 (3He) and a gamma-ray photon (γ). This step is fast compared to step 1.

3. Step 3: Two helium-3 nuclei combine

   3He + 3He → 4He + 1H + 1H
   Two helium-3 nuclei fuse to make a stable helium-4 (4He) nucleus and release two protons back into the plasma. The net effect of the three steps is that four protons become one helium-4 nucleus plus two positrons and two neutrinos.

Net reaction (pp chain): 4 1H → 4He + 2 e⁺ + 2 ν_e + energy

Energy release and mass defect

The mass of four protons is slightly greater than the mass of a helium-4 nucleus. The missing mass (the mass defect) is released as energy according to E = mc². The net energy released per completed conversion of 4 protons to one helium-4 nucleus is about 26.7 MeV. Some of that energy appears as kinetic energy of particles and gamma rays; neutrinos carry away a fraction of it (and escape the Sun almost immediately).

Other branches and the CNO cycle

• The pp chain has sub-branches (ppI, ppII, ppIII) that produce different neutrino energies and involve isotopes like 7Be and 8B. The basic net effect is still conversion of hydrogen to helium.
• The CNO cycle uses carbon, nitrogen and oxygen nuclei as catalysts to convert protons into helium. It becomes the dominant fusion process in stars hotter and more massive than the Sun (roughly >1.3 solar masses). In the Sun, the CNO contribution is minor (<1%).

Where the energy goes

• Gamma rays produced in the core are repeatedly absorbed and re-emitted; it takes photons a very long time (thousands to millions of years) to reach the surface.
• Positrons quickly find electrons and annihilate, producing more gamma rays.
• Neutrinos escape almost unhindered and can be detected on Earth, providing a direct probe of core processes.

Observational evidence

Detection of solar neutrinos (with energies and fluxes matching predictions) and helioseismology (sound waves probing the interior) strongly confirm that hydrogen fusion via the pp chain is what powers the Sun.

Summary

The Sun fuses hydrogen into helium in its hot, dense core primarily through the proton-proton chain. Quantum tunneling allows protons to overcome electrostatic repulsion. The net conversion of four protons into one helium-4 nucleus releases energy that ultimately becomes the sunlight and heat we receive on Earth.

If you want, I can show a diagram of the pp chain, calculate the energy per reaction in joules, or explain in more detail how neutrino measurements confirm the theory.