Transcript: Knowing the size, composition, and energy source of the Sun, astronomers can calculate the physical conditions at any point within its volume. This is called the standard solar model. Fusion occurs within the core, the inner quarter of the Sun’s radius. The temperature at the very center is fifteen million Kelvin. One quarter of the radius out has dropped to ten million, the edge of the fusion zone, and at half the radius is down to about five million Kelvin. The density at the core is ten to the fifth kilograms per cubic meter. This drops by a factor of two out to the edge of the fusion core and then down to about ten to the three kilograms per cubic meter halfway out the Sun. The fusion core, one quarter of the radius, one sixty-fourth of the volume, contains half the Sun’s mass and generates ninety-nine percent of its energy. Remember that these statements are based on a model of the Sun’s behavior. We cannot see directly into the Sun’s core.