The core of the Sun is considered to extend from the center to about 0.2 solar radii. It has a mass of up to 150,000 kg/m3 (150 times the density of water on Earth) and a temperature of close to 13,600,000 kelvins (by contrast, the surface of the Sun is close to 5,785 kelvins (1/2350th of the core)). Through most of the Sun's life, energy is formed by nuclear fusion through a series of steps called the p-p (proton-proton) chain; this process converts hydrogen into helium. The core is the only spot in the Sun that produces an substantial amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy formed by fusion in the core must travel through many consecutive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.