We have seen how the Solar System came to be, and how it has changed in the billions of years since it was born. Now it is time to take a different journey – a journey into the future of the Solar System. This is the subject of Part 4.

We think of the Sun as all-powerful and everlasting. Indeed, on a human timescale it is. Deep within its fiery interior, though, the numbers speak for themselves. On the main sequence the Sun converts a phenomenal 600 million tonnes – the mass of a small mountain – of hydrogen into helium every second, just to keep itself balanced against gravity. At the moment, there is no need for us to worry about this alarming appetite. For the Sun has enough hydrogen to keep its nuclear fires stoked for a good few billion years into the future – long after mankind has vanished. But the day will come when the Sun's fuel heap will run dry, and its useable hydrogen has been totally consumed. When that happens, the Sun will start to die – and with it, the rest of the Solar System.