A galaxy is a giant collection of stars that are bound together because of gravity. A galaxy can have anywhere between a hundred million stars (that’s 1 with eight 0s after it), or a hundred trillion stars! (that’s 1 with fourteen 0s after it!)

Galaxies come in many types. The most common type is a spiral galaxy. Spiral galaxies consist of a flat disk of stars that orbit around a central ‘bulge’. Bulges are more 3-dimensional than the flat disk, and contain older stars than the rest of the galaxy.

Another type of galaxy is an elliptical galaxy. These look like smooth, bright ellipses (squashed circles). However, they are actually 3D ellipses (or ellipsoids). The stars in elliptical galaxy are generally older than those found in spiral galaxies. Unlike stars in spiral galaxies, stars in elliptical galaxies have fairly random orbits around the galaxy centre.

The final main type of galaxy is an irregular galaxy. These galaxies have quite a chaotic appearance, and are quite small compared to other galaxy types.

Nearly all galaxies contain dark matter. Like ordinary (everyday) matter, dark matter has a gravitationally pull on other matter. However, unlike ordinary matter, dark matter cannot be seen at all! It does not emit, reflect or absorb light (which is why it’s dark).
Currently, no one knows what dark matter is actually made of — even though it makes up about 85% of the total matter in the universe!

The Solar System is in a spiral galaxy called the Milky Way.

If you’re somewhere without much light pollution (and it’s not cloudy), you can actually see the Milky Way with your own eyes!

(Okay, I guess we’re always looking at the Milky Way: we’re in it! But you know what I mean.)

Because we’re inside the Milky Way, we can only see it side-on. To get the full circular view we’d need to be far outside it. (Imagine a chocolate chip near the edge of a cookie. From its point of view, all the other chocolate chips would appear in a straight line!)

Our view of the Milky Way is what gives it its name: it is called the Milky Way because it looks like milk that’s been poured over the sky. You might find this name silly for something so big and grand — but hey, there’s no use crying over spilt milk.

Nearly all large galaxies have a black hole in the centre.

Black holes are what’s left when a very massive star collapses under its own gravity (often the outer parts of the star will explode at the same time, in an event called a supernova).

Black holes are made up of an enormous mass within a tiny space, so there is a very strong gravitational force near a black hole.

This gravitational pull is so strong that nothing can escape a black hole’s grasp! Anything that gets close enough to a black hole (including light) is doomed to be sucked into it. (This is what makes black holes black!)

This point-of-no-return near a black hole is called an event horizon. No matter how fast you are going, if you are inside an event horizon there is no way out.

The Milky Way has its own black hole in the centre, called Sagittarius A*. It has a mass 4 million times as big as the Sun!

In 2019, the first image of a real black hole was formed. This is from the centre of the elliptical galaxy Messier 87. The black hole has a mass billions of times the mass of the Sun.