Galaxy clusters

Galaxies live in groups

You need a telescope

Galaxy clusters — A cluster: every little smudge is a whole galaxy, and between them lies invisible scorching gas.

The numbers

What it is: Hundreds or thousands of galaxiesHeld by gravity
What is in between: Thin, scorching hot gas
When there are few: They are called groups
They were the biggest: Until superclusters were found

To get an idea

They must not be mixed up with star clusters, which all sit inside a single galaxy.

Between the galaxies there is scorching gas, which holds more matter than all their stars together.

Fun facts

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

The space between the galaxies of a cluster is filled with thin, scorching hot gas.
That gas holds more matter than all the stars in the cluster put together.
To see it you need instruments that collect X-rays, not ordinary telescopes.
Until the 1980s clusters were thought to be the biggest structures in the universe.
Then superclusters were discovered, gatherings of galaxies even bigger than the clusters themselves.

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 cluster of galaxies?

Question 2 of 5

How is it different from a star cluster?

Question 3 of 5

What is a small gathering called?

Question 4 of 5

What is between one galaxy and the next?

Question 5 of 5

What is the biggest structure known today?

Riddles

I am not a group of stars but a group of star cities: each of my members holds billions of them. We are held together by the same force that keeps you on the ground. What am I?

Among my residents there is more stuff in the seemingly empty space than inside all their stars put together, but you need special eyes to see it. What am I?

For decades I was the biggest in the kingdom, then in the 1980s someone bigger than me was found and I lost the title. What am I?

Try it yourself

Activity 1

In those pictures, look for bent arcs of light: they are even more distant galaxies, whose light has been bent by the mass of the cluster.

Activity 2

Try counting how many smudges you can make out in a single photograph, and then multiply by the billions of stars in each one.

The words

supercluster: An even bigger gathering, which holds several clusters.

Sources: Wikidata (Q204107), Wikipedia (Ammasso di galassie).