The Sun is a smooth and continuously varying ball of gas which reduces in density moving outward until it gradually fades away into space. We see an edge which is called the photosphere, but why do we see an edge at all if the Sun is smoothly changing in temperature density and pressure? The interior of the Sun is opaque. The opacity is high. Photons are always colliding with atoms and particles and cannot travel freely, but moving outward in the Sun at a point where the temperature reduces below six thousand degrees Kelvin there’s a thin zone where the density has reduced to the point where photons no longer interact with particles. At this point they travel freely through space, and we see the Sun as an edge. This is just like the situation in a cloud. The density, pressure, and temperature in a cloud are not much different from outside the cloud, but the pressure of water vapor is slightly higher so that water vapor can condense. When you go in and out of a cloud in an airplane there is no hard or sharp edge. In fact the density is just slightly higher, sufficient that light bounces around inside the cloud; the cloud is opaque. The edge of the cloud is the place where light travels freely. Thus the photosphere is the edge of the Sun where the opacity is reduced to the point where light travels freely. Radiation that has taken hundreds of thousands of years to reach the surface from the core travels in only eight minutes to the Earth.