The spiral galaxy M100
The spiral galaxy M100
Credit:STSci (HST)

A galaxy is a large collection of stars, dust and gas. They are surrounded by dark matter, and are held together by gravity.

Galaxies come in all shapes and sizes. The smallest galaxies, known as dwarfs, contain just a few million stars. The largest giant galaxies can contain around 1 trillion stars - that's an incredible 1,000,000,000,000 stars. It is thought that there are around 100 billion (100,000,000,000) galaxies in the Universe.

You can try to recreate the process astronomers went through to work this out using our 'Weighing the Universe' Workshop.

Scientists have estimated that there are at least 100 stars in space for every grain of sand on Earth's beaches!

Our own galaxies is called the Milky Way. It is a large spiral galaxy. Our Solar System sits towards the edge it, about 26,000 light-years from the centre. The closest star to our Solar System is Alpha Centauri, which is just 4.4 light-years away.

It is thought that super-massive black holes live at the centre of most galaxies. This includes our Milky Way. Astronomers have found it by looking at the movement of stars close to the centre of the galaxy.