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Space · The stars

Red giants

When a star swells up

You can see it with your eyes

Red giants
An enormous, thin red giant next to the star it used to be, much smaller.

The numbers

Which stars
Those of small or medium massFrom a third to eight times the Sun
Surface temperature
8,500°F(4,700 °C)Less than that
What color they are
From yellow-orange to red

How it happens, step by step

  1. The fuel runs out

    In the star's heart the hydrogen runs out, and with it the push that held its own against gravity.

  2. The center shrinks

    Without that push the heart collapses in on itself and, as it collapses, gets even hotter than before.

  3. A shell lights up

    That heat lights up the layers just outside the heart, where there is still hydrogen. A new fire is born.

  4. The star swells

    The new fire pushes the outer layers away, and they move very far from the center of the star.

  5. It turns red

    Far from the center those layers cool down, and a cooler surface glows a redder color.

To get an idea

The surface of a red giant is much cooler than it was before, and that is why it turns red.

The Sun will become a red giant too, and it will swell so much that it swallows the closest planets.

Fun facts

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

  1. The color ranges from yellow-orange to red, and it matches the spectral classes called K and M.

  2. A red giant is brighter than before, because the surface giving off light is immense.

  3. The Sun will become a red giant too, but only billions of years from now.

  4. When that happens it will swell so much that it swallows the planets closest to it.

  5. Many orange stars you can see with the naked eye are already in this stage.

  6. Giant stars can also be told apart by where they fall on the Hertzsprung-Russell diagram.

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

What does a star do when the hydrogen in its heart runs out?

Question 2 of 5

Why does the surface get cooler?

Question 3 of 5

Then why does it look brighter?

Question 4 of 5

Which stars become red giants?

Question 5 of 5

What will become of the Sun?

Riddles

When I run out of firewood I do not go out: I swell up. I become enormous, my skin cools down and I change color, and whoever looks at me from afar sees me brighter than before. What am I?

I am cooler on the surface and yet I shine more, and it is no trick: it is that I have become immense. What am I?

Billions of years from now the star that warms you will become like me, and the worlds closest to it will not come out of it well. What am I?

Try it yourself

Activity 1

Compare it with Rigel, Orion's foot, which is blue: you have in front of you two stars at opposite ends of the temperature scale.

Activity 2

Blow up a balloon and hold it against your cheek: the surface is wider but cooler than when it was flat. It is the same idea.

The words

spectral class
The group a star is put in according to its color.

Sources: Wikidata (Q744622), Wikipedia (Gigante rossa).