How does the Sun produce energy?

1. Every morning the Sun lights and warms our world. Plants grow toward it and we feel it on our skin. But where does all that light come from?
2. The Sun is a star: a huge ball of glowing gas, mostly hydrogen. More than a million Earths could fit inside it. It is not burning like a fire. It shines in a different way.
3. The Sun's own gravity pulls all that gas toward the middle. The center, called the core, is squeezed so hard that it reaches about 15 million degrees.
4. In the core, hydrogen bits move so fast that when they crash, they stick together. Step by step, four hydrogens become one helium, and a flash of energy comes out. This is called fusion.
5. The energy cannot fly straight out. The Sun is so thick that light gets bumped around in a zigzag for many thousands of years. Near the top, hot gas carries it up like bubbles in boiling soup.
6. Once it leaves the surface, light races across 150 million kilometers of space. About eight minutes later it reaches Earth, where it warms the land, drives the weather and feeds every green plant.

A slice has been cut out of this model Sun. Tap each layer to see what happens there.

• Core. The core is the Sun's power station. It is about 15 million degrees, and here hydrogen fuses into helium and gives out energy.
• Radiative zone. In this thick layer, light cannot travel straight. It is bumped from one bit of gas to the next, zigzagging outward for many thousands of years.
• Convection zone. Near the top, the gas churns like soup in a pot. Hot gas rises, lets its heat out at the surface, cools and sinks again.
• Surface (photosphere). This is the bright layer we see from Earth. It is about 5,500 degrees, and from here light finally escapes into space.
• Sunspots. Sunspots are cooler patches on the surface. They look dark only because everything around them is even brighter.
• Corona. The corona is the Sun's thin outer atmosphere. It streams far out into space, and it is the pale crown you can see around the Sun during a total solar eclipse.

This box is a tiny piece of the Sun's core. Drag the plate to squeeze and heat the hydrogen. Watch what the bits do, and watch the little Sun.

Experiment: What happens to the hydrogen when you squeeze and heat it more and more?

1. Start with a gentle squeeze. Watch the hydrogen bits bounce.
2. Drag the plate, or use the slider, to squeeze harder.
3. Watch for flashes, and look at the little Sun on the right.

What we found: With a gentle squeeze the hydrogen bits are too slow: they bounce apart and nothing happens. Squeezed hard, the core gets hotter and more crowded, so the bits crash hard enough to stick together into helium. Every time that happens, a little energy comes out as light. That is how the Sun shines.

Turn on JavaScript to try it yourself.

Set the squeeze so the planet gets just the right amount of light: not icy, not baking. Press Send the light to test it.

The Sun is a star, and it is our nearest one. It is a giant ball of very hot gas, mostly hydrogen, the lightest stuff there is. The Sun is so heavy that its own gravity squeezes it toward the middle. In the center, called the core, the gas is crushed and heated to about 15 million degrees.

At that heat, the tiny centers of hydrogen atoms fly around at enormous speeds. Most of the time they just bounce off each other. But some crash so hard that they stick. Step by step, four hydrogens join to make one helium. This joining is called fusion, and each time it happens a little energy is set free. Clapping your hands is only a small picture of this: your hands do not change, but the hydrogen really does turn into something new.

That energy has a long trip ahead. The inside of the Sun is so thick that light cannot go straight. It is bumped this way and that for many thousands of years. Near the top, the gas churns like soup in a pot: hot gas rises, cools and sinks again, carrying the heat to the surface.

From the surface, the light finally flies free. It crosses 150 million kilometers of space in about eight minutes and lands on Earth as sunshine. The Sun has been shining like this for about four and a half billion years, and it has enough hydrogen to keep going for billions more. One warning: never look straight at the Sun. It is bright enough to hurt your eyes.

Key words

Fusion

When small bits of atoms are pushed together so hard that they join into a bigger one and give out energy.

Core

The hot, squeezed center of the Sun, where fusion happens.

Plasma

Gas so hot that it glows and its atoms are torn into smaller charged bits.

Nucleus

The tiny, heavy center of an atom. A hydrogen nucleus is a single proton.

Photosphere

The bright visible surface of the Sun, where light escapes into space.

Photon

A tiny packet of light.

Check your thinking

1. What happens in the Sun's core to make its energy?

   Show the answer

   Hydrogen bits fuse into helium. In the hot, squeezed core, hydrogen joins to make helium, and energy comes out each time.

2. About how long does sunlight take to travel from the Sun to Earth?

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   About 8 minutes. Light is the fastest thing there is, but the Sun is 150 million kilometers away, so the trip takes about 8 minutes.

3. Why does fusion happen in the core and not at the surface?

   Show the answer

   Only the core is hot and squeezed enough. Hydrogen bits must crash very hard to stick. Only the core, at about 15 million degrees, is hot and crowded enough.

4. If the Sun's core were squeezed less and became cooler, what would happen?

   Show the answer

   Less hydrogen would fuse, so the Sun would shine less. Cooler, less crowded hydrogen bits bounce apart instead of sticking, so less energy is made.

5. A planet orbits three times as far from the Sun as Earth does. How much sunlight does each square meter of it receive, compared with Earth?

   Show the answer

   One ninth as much. Light spreads over a sphere, so its intensity falls with the square of the distance: 1 divided by 3 squared is one ninth.

6. Why does energy made in the core take so long to reach the surface?

   Show the answer

   Photons are absorbed and re-emitted in random directions again and again. The Sun's interior is dense and opaque, so energy moves outward in a slow random walk before convection carries it the last part of the way.