The center of the Sun
The center of the sun is very hot (about 15 million degrees Celsius) and the pressure is immense (about 100 billion times the airpressure here on Earth).
Because of that, atoms come so close to eachother that they fuse.
In every second, the Sun spends 700 billion tons of protons (or: Hydrogen) in this way. And only a small fraction (0.7 percent) is turned into light.
Right now, about half of the amount of Hydrogen in the core of the Sun has been fused into Helium. This took the sun about 4,5 billion years.