The Sun is a star, and like all stars, has a life cycle. Our sun is currently 4.5 billion years old, and is in the middle of its life. In about five billion years, the sun will have used up all its fuel (converting hydrogen to helium through nuclear fusion), and start rapidly expanding into a red giant, growing to more than 100 times its current size, engulfing the inner planets, and likely the Earth too! But the Earth will be inhabitable long before that. In fact, in a billion years time, the sun will be about 10 per cent hotter than it is now, and vaporise all the water on Earth.