The Sun is fuelled by hydrogen, fusing 620 million tons of hydrogen nuclei into helium each second. This reaction produces solar radiation, which we experience as light and heat, but which also showers us with neutrinos – tiny particles that can pass through matter.
In fact, at this very second billions of neutrinos are passing through your body. The Sun is about 4.5 billion years old and, after comparing it to similar stars in our galaxy, astrophysicists reckon it is about halfway through its hydrogen burning stage. That leaves us another 5 billion years before its fuel begins to run low.