Artist’s impression of the Earth scorched by our Sun as it enters its Red Giant Branch phase. Image Credit: Wikimedia Commons.
The Sun is about 4.5 billion years old. During this time, it used almost half of its fuel (hydrogen – the sun is steadily converting hydrogen into helium through fusion). So, right now, it is midway through life, and in about 5 billion years from now, it will begin to die – here’s how:
First, the sun will run low on hydrogen – its main fuel. Its core will begin collapsing in on itself, and get denser and hotter – until it suddenly starts the second type of nuclear fusion: turning helium atoms into carbon and oxygen. This extra energy will push out the outer layers of the sun. Once a star starts burning helium, it can be considered a red giant. The sun will spend around a billion years burning helium in its core. During this phase, the core will contract and the outer layers will expand, cool, and become less bright. The sun will expand about 250 times its original size. It will probably swallow the Earth.
Eventually, the helium also will run out, and the core of the Sun will begin to collapse again. But this time, it will never get hot enough to start the next fusion chain. Its outer layers will get “puffed” off into space as a planetary nebula and the sun itself will become a white dwarf.