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Nuclear Fusion

In nuclear fusion, light nuclei join to make a heavier nucleus and release energy. In the Sun's core, at about 15 million °C, four hydrogen nuclei join in steps (the proton–proton chain) to make one helium nucleus. The helium weighs about 0.7% less than the four hydrogens; that missing mass becomes energy by E = mc². Gravity squeezing in and fusion heat pushing out keep a star steady for billions of years.

🎬 Step-by-step story

  1. The Sun is a giant ball of hydrogen. Deep in its core it is about 15 million °C. Sunlight starts there.
  2. Two hydrogen nuclei both have a plus charge, so they push each other away. At 6 million °C they are too slow: they turn back. Raise the temperature to 15 million °C and press Fire: they get close enough to join.
  3. When hot, fast nuclei meet, four hydrogen nuclei join into one helium nucleus. Light and heat come out. This is nuclear fusion.
  4. Weigh it: the helium plus the light weigh a little less than the four hydrogens. About 0.7% of the mass has gone, and it became energy: E = mc².
  5. Why does the Sun not blow up or shrink? Gravity squeezes in, fusion heat pushes out. Move the fusion-rate slider and see the star correct itself.
  6. Free play: change the temperature and fire two nuclei. Find the temperature where they just fuse.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Is the Sun burning like a fire?

No. A fire needs oxygen and is a chemical change. The Sun makes its energy by fusion in the core, which is a nuclear change. Watch hydrogen nuclei turn into helium.

Why must it be so hot for fusion?

Hydrogen nuclei are positive, so they repel. Slow ones turn back at 6 million °C in the 3D. Only fast, hot ones get close enough to join.

Where does the energy come from? Is mass destroyed?

A little mass (about 0.7%) is changed into energy, E = mc². Mass and energy together stay the same; the bar shows what turned into light.

Why does the Sun not explode or shrink?

Gravity pushes in and fusion heat pushes out, and they balance. If fusion rises, the star expands and cools; if it falls, the star shrinks and heats up.

Will the Sun ever run out?

Yes, in about 5 billion years. It has used about half of the hydrogen in its core. A bigger star would run out sooner because it burns fuel faster.

Can we make fusion energy on Earth?

Scientists try it in magnetic machines at over 100 million °C. The idea is the same as your Fire button: make nuclei fast enough to join. Try finding the temperature that just works.

What is nuclear fusion?

The centre of an atom is its nucleus. In nuclear fusion two light nuclei join to make one heavier nucleus. Energy comes out. It is the opposite of fission, where one heavy nucleus splits. Fusion is not burning: nothing is set on fire and no oxygen is used.

Fusion powers the Sun and all the stars you see at night. The energy that reaches us as sunlight is the energy of fusion in the Sun's core, and it drives almost all energy on Earth.

Why it needs a very high temperature

Nuclei have a plus charge, and plus pushes plus away. To join, two nuclei must get so close that a very short-range pull (the strong nuclear force) takes over. That needs high speed. A high temperature means fast particles, and a high pressure packs them close so they meet often.

In the Sun's core it is about 15 million °C and the matter is squeezed to over 100 times the density of water. Even then, most meetings fail; once in a very long while the nuclei sneak through the push-away barrier (quantum tunnelling). The Sun is so big that the rare successes still add up to a huge output.

The proton–proton chain

The Sun joins four hydrogen nuclei (protons) into helium in small steps, called the proton–proton chain:

  1. Two protons join. One turns into a neutron and makes deuterium (heavy hydrogen), with a tiny particle called a neutrino and a positron leaving.
  2. Deuterium meets another proton and makes helium-3, with a flash of gamma light.
  3. Two helium-3 nuclei meet and make helium-4, giving two protons back.

Net result: 4 H → 1 He + energy. Stars heavier than about 1.3 Suns use a different chain, the CNO cycle, which uses carbon, nitrogen and oxygen as helpers but ends the same way.

Mass becomes energy

Einstein showed that mass and energy are two forms of the same thing: E = m c², where c is the speed of light (3 × 10⁸ m/s). The helium nucleus is a little lighter than the four hydrogens that made it; this lost mass (the mass defect) is carried away as light, heat and neutrinos.

About 0.7% of the hydrogen's mass is lost. Because c² is huge, a tiny mass gives a huge energy: 1 kg of hydrogen fused gives about 6 × 10¹⁴ J. The Sun turns about 4 million tonnes of mass into energy every second.

A steady star, and fusion on Earth

A star on the main sequence is one that is fusing hydrogen in its core. It stays steady because gravity squeezes in and fusion heat pushes out. If fusion grows too strong, the core puffs up and cools, and fusion slows. If it grows too weak, gravity squeezes the core, it heats up, and fusion speeds up. This self-correcting balance lets the Sun shine for about 10 billion years.

Big stars have more fuel but burn it far faster, so they live only millions of years; small stars live for hundreds of billions. On Earth, scientists try to fuse two heavy forms of hydrogen (deuterium and tritium) at over 100 million °C in magnetic "bottles". It gives no carbon dioxide, but is very hard to keep going.

Try it

In the 3D, use the temperature slider and press Fire. Predict first: will they fuse at 10 million °C? Then check. At home, hold two magnets with the same poles facing and try to push them together: the push-back you feel is like the push between two nuclei.

Key formulas and definitions

Worked examples

1. 1 kg of hydrogen is fused to helium and 0.7% of the mass is lost. How much energy is released?

Mass lost = 0.007 kg. E = m c² = 0.007 × (3 × 10⁸)² = 0.007 × 9 × 10¹⁶ = 6.3 × 10¹⁴ J.

2. The Sun gives out 3.8 × 10²⁶ J every second. How much mass does it turn into energy each second?

m = E / c² = 3.8 × 10²⁶ / 9 × 10¹⁶ ≈ 4.2 × 10⁹ kg, about 4 million tonnes per second.

3. Four hydrogen atoms weigh 4.0313 u and the helium atom made from them weighs 4.0026 u. Find the energy released (1 u = 931.5 MeV).

Δm = 4.0313 − 4.0026 = 0.0287 u. E = 0.0287 × 931.5 ≈ 26.7 MeV.

4. 1 kg of coal gives about 3 × 10⁷ J when burnt. How many times more energy does 1 kg of hydrogen give when fused (6.3 × 10¹⁴ J)?

6.3 × 10¹⁴ / 3 × 10⁷ = 2.1 × 10⁷, so about 21 million times more.

Common mistakes

Practice quiz

1. Nuclear fusion is the joining of:
2. The Sun's core is at about:
3. In the Sun, four hydrogen nuclei finally make one nucleus of:
4. The energy released in fusion comes from:
5. Fusion needs very high temperature because nuclei:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is nuclear fusion in simple words?

It is when two small nuclei join to form a bigger one and give out energy. It is how the Sun and stars shine.

What is the difference between fusion and fission?

Fusion joins light nuclei (for example hydrogen into helium). Fission splits heavy nuclei (for example uranium). Both release energy because some mass turns into energy.

Can we use fusion for electricity on Earth?

Scientists are working on it, with machines that hold gas at over 100 million °C in magnetic fields. It does not yet give more power than it uses over long periods.

Where this is taught

South Korea고등학교 1학년Environment and energy
South Korea고등학교 2학년Quantum and the micro world
South Korea고등학교 3학년Stars and exoplanet systems

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