Sunlight feels like it’s free, but it’s actually expensive - for the Sun. To make light, the Sun is constantly burning its own substance. About 600 million tons of hydrogen get fused into helium every second deep in the Sun’s core. About 4 million tons of that turns directly into pure energy, which streams out as sunlight.
Where does the mass go? It becomes light. Einstein’s famous equation E=mc² says that mass and energy are two forms of the same thing - and the Sun is showing it in real time. Every photon of sunlight that hits your face was once part of the Sun’s body. The Sun is, very slowly, throwing pieces of itself into space.
Over its entire life so far - about 4.6 billion years - the Sun has lost roughly the mass of the planet Saturn. That sounds like a lot until you realize the Sun is so big that this loss is barely a rounding error. It has another 5 billion years of steady burning ahead before it runs out of fuel.
