Space · The stars

White dwarfs

The heart a star leaves behind

You need a telescope

White dwarfs — A white dwarf: as small as the Earth, extremely dense, and by now only cooling down.

The numbers

What it is: The bare heart of a dead star
How big it is: As big as the EarthBut it weighs as much as a star
Does it make energy?: No: it just cools down
When they were understood: Seen in the 1700s, understood in 1910

To get an idea

It no longer makes energy: it shines only because it is still extremely hot, like an unplugged iron.

It cools down so slowly that none, in the whole universe, has yet had time to go out.

Fun facts

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

The gravity on its surface is enormous, because of all that mass in so little space.
The first one was spotted around the end of the 1700s, long before anyone understood what it was.
What these strange objects really were was understood only in the year 1910.
Sirius, the brightest star in the sky, has a white dwarf as its companion.
This is exactly the fate that awaits the Sun too, after its red giant stage.

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 is a white dwarf?

Question 2 of 5

How big is it?

Question 3 of 5

Does it still make energy?

Question 4 of 5

How long does it take to go out?

Question 5 of 5

When did people understand what they were?

Riddles

I am the heart a star left behind, as big as your planet but as heavy as a whole star. A teaspoon of me would weigh as much as a car. What am I?

My engine has shut down forever and I make nothing anymore: I shine only because I am still scorching, like an iron pulled out of the socket. What am I?

I am cooling down so slowly that none of my kind, in the whole universe, has yet finished going out completely. And the universe is not young. What am I?

Try it yourself

Activity 1

Take a marble and a soccer ball: imagine squeezing the whole ball into the marble without losing anything. That is the density of a white dwarf.

Activity 2

Unplug a hot iron and touch it every ten minutes: it stays too hot to touch for a long time without any power. That is how a white dwarf shines.

The words

density: How much matter is packed into a certain space.

Sources: Wikidata (Q5871), Wikipedia (Nana bianca).