The Sun is about 4.5 billion years old. it has used up about half of its nuclear fuel (hydrogen). In about 5 billion years from now, the sun will begin to die. As the Sun grows old, it will expand. As the core runs out of hydrogen and then helium, the core will contact and the outer layers will expand, cool, and become less bright. It will become a red giant star and consume earth. Anyone residing on earth will be toasted.
This is my greatest concern regarding the future of mankind. 5 billion years will be here before you know it. We need to start preparing now.