Why is the center of the Sun’s disk brighter than its edge?

The phenomenon where the center of the Sun’s disk appears brighter than its edge is known as limb darkening.12

This occurs because the solar atmosphere increases in temperature with depth. When observing the center of the solar disk, your line of sight penetrates deeper into the photosphere, reaching the warmer, denser layers that emit more intense light.12

Conversely, when looking toward the edge, or limb, of the Sun, your line of sight is angled such that it only reaches the upper, cooler regions of the atmosphere. Because these higher layers are at a lower temperature, they produce less radiation, resulting in the observed decrease in brightness. Scientists use these variations in brightness to study the temperature structure of the solar atmosphere.12