The composition of the Sun has gradually changed due to 4.5 billion years of processing Hydrogen into Helium. An interesting byproduct of this nuclear burning is that the Sun has gotten brighter over time; it was once only about 70% as bright as it is today. The reason for this is linked to the balance between gravity pulling inward and gas pressure pushing outward. Each time four Hydrogen nuclei are fused into one Helium nucleus, there are fewer particles to provide outward pressure. The Sun's core contracts slightly which heats up the remaining particles, speeds up nuclear fusion. More intense fusion at the core causes the Sun to brighten and its outer layers to expand. The Sun will continue to expand and brighten over time, and things will get really hot for planet Earth in another billion years or so. Watch this animation of the Life of the Sun to find out what will happen next.