Physics Is Life
Me

Unit 12, lesson 58. About 15 minutes.

Energy from the Sun

How has the Sun kept shining for almost five billion years?

Play

Getting the experiment ready…

Explain

The Sun gets its energy from nuclear fusion. Deep inside, it is so hot and squeezed that hydrogen nuclei join together.

They make helium, and release a huge amount of energy.

That energy reaches us as sunlight, and it powers nearly all life on Earth.

In fusion, four hydrogen nuclei effectively become one helium nucleus. About 0.7% of the mass is turned into energy.

The energy comes from E = mc². Since c² is so large, a tiny mass gives a huge amount of energy.

Worked example: 1 kg of hydrogen releases 0.007 × 1 × (3 × 10⁸)² ≈ 6.3 × 10¹⁴ J.

  1. Hydrogen nuclei collide
  2. They fuse to make helium
  3. Energy is released
Want the full story?

The real process has several steps, and the sim shows only the idea. The Sun's core is about 15 million °C. Scientists are working on fusion power on Earth, which is very hard because it needs such extreme conditions. Fusion is different from fission, which splits heavy atoms.

The Formula

energy = 0.7% of the mass × the speed of light squared

E=0.007×m×c2E = 0.007 \times m \times c^2
EE
the energy released, measured in joules (J)
mm
the mass of hydrogen used, measured in kilograms (kg)
cc
the speed of light, measured in metres per second (m/s)

Try the numbers

Energy released: 6.29 × 10¹⁴ J

Try it at home

You need some paper, pens.

You need: some paper, pens.

  1. Draw a diagram of four protons joining to make helium.
  2. Label the energy that comes out.
  3. Add arrows to show sunlight reaching a plant, and then you.

What's happening: this energy chain links the Sun's core with the food that you eat.

Physics is life

Your body

Sunlight lets your skin make vitamin D.

Sunlight helps your skin to make vitamin D for strong bones.

Daily life

Solar panels catch sunlight.

Solar panels turn sunlight directly into electricity.

Nature

Plants need the Sun.

Plants use sunlight to make food, and they feed nearly all life.

Space

All stars shine by fusion.

Every star you can see shines by fusion for most of its life.

Quick check

Question 1 of 3

What powers the Sun?

Words to know

Nuclear fusion: Small atoms joining to make a bigger one, and releasing energy.

Nuclear fusion
Small atoms joining to make a bigger one, and releasing energy.
Nucleus
The tiny, dense centre of an atom.
Energy
What makes things able to happen. It is measured in joules.

Meet the scientist

Next up

Einstein's big ideas