Say it like this

An enormous group of stars travelling together. Ours has a couple of hundred billion in it.

For the grown-up

A galaxy is a gravitationally bound system of stars, gas, dust and dark matter. Ours is the Milky Way. Estimates of how many galaxies exist keep moving upward as instruments improve, which is itself a useful thing for a child to know about science.

The easy mistake

Careful here. A galaxy is not the universe and the universe is not a galaxy. The nesting order worth learning is planet, star, galaxy, universe.

The full explanation

What Is the Universe?

A glossary entry is the short version. This one has the child explanation, the grown-up explanation, the questions and an activity.

Read it

Words next to this one

Universe Everything there is, including you. Most of it is empty space. Star An enormous ball of burning gas, so far away it looks like a dot. Our sun is one. Gravity The pull that everything with weight has on everything else. Bigger things pull harder. Orbit Going round and round something because it keeps pulling you in and you keep missing it.