The composition of the Sun varies with depth. In the photosphere, it is about 73–74% by mass, the rest being primarily, which is the same composition as the of, and the primordi. Approximately 3.7×10 (), or roughly 600 million tonnes of hydrogen, are converted into every second, releasing energy at a rate of 3.86×10 joules per second. The core produc. The rate of nuclear fusion depends strongly on density. Therefore, the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and slightly against the. The high-energy () released in fusion reactions take indirect paths to the Sun's surface. According to current models, random scattering from free electrons in the solar radiative zone (the zone within 75%.