The Sun appears to be a burning ball of fire, but it has an internal structure. The visible surface is known as the photosphere, and it heats up to approximately 6,000 degrees Kelvin. The convective zone lies beneath that, where heat slowly moves from the inner Sun to the surface and cooled material falls back down in columns. This region begins at 70% of the radius of the Sun. The radiative zone lies beneath the convection zone. Heat can only travel through radiation in this zone. The Sun's core extends from the Sun's centre to a distance of 0.2 solar radii. Temperatures reach 13.6 million degrees Kelvin here, and hydrogen molecules fuse to form helium.