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In the night sky: Orion
In the night sky: Orion

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3.2 What is a galaxy?

An image of the Andromeda galaxy.
Figure 7 The Andromeda galaxy seen through a telescope.

A galaxy is a huge assemblage of stars, planets, nebulae, gas and dust. Objects within a galaxy are bound by gravity and orbit around a common centre of mass. It is estimated that there are probably more than 170 billion galaxies in the observable universe.

Most galaxies are believed to have supermassive black holes at their centre around which everything rotates. Our Galaxy, the Milky Way, is thought to have a supermassive black hole at the centre with a mass of four million times the Sun – you will come back to this a bit later.

The space between galaxies is known as ‘intergalactic space’ and is very sparse, with less than one atom per cubic metre.

You’ll learn about different types of galaxy in the next section.