Why does the visible edge of the Sun appear sharp despite its gaseous atmosphere?

The Sun does not have a solid surface; rather, it is composed of plasma, a gas of ionized atoms. The appearance of a sharp edge is an optical effect caused by the vast distance between the Sun and Earth.1

In reality, the visible "surface" is the photosphere, a layer approximately 400 km thick. This region is highly opaque due to the strong absorption of light by hydrogen ions. Scientists define this "surface" as the specific region above which most photons can escape into space without being absorbed or reemitted.1

While the edge appears sharp to the naked eye, it actually exhibits limb darkening, where the disk appears dimmer toward the edge. This occurs because, at the center of the solar disk, an observer looks deeper into the hotter, brighter layers, whereas at the limb, the line of sight only reaches the cooler, higher layers of the atmosphere.21