The Sun is a medium-sized star. It is powered by nuclear reactions that occur in its hot, pressurized core. In temperatures of about 20 million degrees, hydrogen nuclei are fused together to become heavier helium atoms, releasing tremendous amounts of energy. It is this energy that gives us heat and light.
In the 4.5 billion years since the Sun formed, it has used up about
half of its initial hydrogen supply. The Sun will start to die in about 4-5 billion years.