Every second, the Sun converts 620 million metric tons of hydrogen into helium through thermonuclear fusion. At its core, the temperature reaches 15 million Kelvin and the pressure is 265 billion atmospheres. Hydrogen nuclei are forced so close together that the Strong Nuclear Force overcomes their electromagnetic repulsion, fusing them into helium and releasing a burst of pure energy described by Einstein's equation: E = mc². This is the engine that powers every star in the universe.