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The spiral galaxy M100
Credit: STSci (HST)

A galaxy is a large collection of stars, dust and gas, all held together by gravity.

Galaxies come in all shapes and sizes. The smallest dwarf galaxies contain just a few million stars, whereas the largest giants can contain around 1 trillion stars - that's an incredible 1,000,000,000,000 stars. What's more, there are believed to be around 100 billion (100,000,000,000) galaxies in the observable Universe.

In fact, mathematicians have estimated that there are at least 100 stars in the Universe for every grain of sand on Earth's beaches - yes, every last one of them.

Our Solar System sits towards the edge of a large spiral galaxy called the Milky Way, and our Sun is about 26,000 light-years from its centre. Compare that to our nearest star, Alpha Centauri, which is just 4.4 light-years away.

It is thought that super-massive black holes may exist at the centre of many, if not all, galaxies. Indeed, observations of star movements close to the centre of the Milky Way suggest that it contains at least one such object.