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Space · The Sun and its family

Inside the Sun

An explosion held still by its own weight

Only spacecraft can see it

Inside the Sun
The Sun cut in half: the bright core at the center, then the layers that carry the energy out.

How it happens, step by step

  1. The weight presses

    The outer layers of the Sun press toward the center. The deeper you go, the more squeezed the matter is and the hotter it gets.

  2. The nuclei touch

    At the center the push is so strong that hydrogen nuclei, which repel each other, come close enough to stick together.

  3. Helium is born

    The union makes a helium nucleus, which weighs a tiny bit less than the pieces it came from. That bit becomes energy.

  4. The light climbs

    The energy is absorbed and given off again by one layer after another, and takes a very long time to reach the surface.

  5. The balance holds

    Gravity squeezes inward, energy pushes outward. As long as the two forces are even, the star stays as it is.

To get an idea

It is a nonstop explosion held still by its own weight: gravity squeezes inward, energy pushes outward.

The iron in your blood and the calcium in your bones were made in stars that went out long before the Sun was born.

Fun facts

These are not in the book: there was no room on paper.

  1. Fusion happens only in the core: nowhere else in the Sun is the temperature high enough.

  2. Stars make all the chemical elements except hydrogen, which is the starting fuel.

  3. The biggest stars manage to make iron; the heavier elements are born in supernovae.

  4. Before fusion was understood, people thought the Sun was simply cooling down.

  5. Arthur Eddington was the first, in 1920, to suggest that stars live on nuclear fusion.

  6. Inside the Sun, a single proton waits on average billions of years before it manages to fuse.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

Where does fusion happen in the Sun?

Question 2 of 5

What comes out when hydrogen nuclei join?

Question 3 of 5

How much hydrogen does the Sun fuse every second?

Question 4 of 5

Why doesn't the Sun explode or collapse?

Question 5 of 5

Where does the iron in our blood come from?

Riddles

I am an explosion that has lasted billions of years without ever ending, because something holds me still: the weight of what lies on top of me. What am I?

I cooked the iron in your blood and the calcium in your bones long ago, in a scorching place where nobody will ever be able to go. What am I?

Try it yourself

Activity 1

Squeeze a ball of modeling clay in your hand: the more you squeeze it, the warmer it gets. The Sun does the same, with the weight of a whole star.

Activity 2

Look for iron in the list of ingredients of a breakfast cereal. That iron was made inside a star, long before the Sun.

The words

helium
The light gas that is made when hydrogen fuses.
nucleosynthesis
The making of new chemical elements inside stars.

Sources: Wikidata (Q1425536), Wikipedia (Nucleosintesi stellare).