Question

Why does the surface of the Sun appear to have a sharp edge?

Answer 1

The Sun appears to have a sharp edge because it is distant and the photosphere is very bright.

Explanation:

The Sun's photosphere is very bright. The outer layers of the Sun are the chromosphere and corona which have very distinct features. These features can only be seen during a solar eclipse or using specialist equipment.

The photograph below was taken with a digital camera using a mylar film which reduces the intensity of light by a factor of several million. Notice that the Sun appears circular with a sharp edge. The only visible features are Sun spots.

The reason that other features are not visible is that reducing the amount of light to see the photosphere reduces their light to levels where they can't be seen.

Now things are different during a solar eclipse. The photosphere is completely covered by the Moon. This makes the chromosphere, solar flares and the solar corona visible. The photograph shows these features.

Using specialist filters such as Hydrogen `alpha` filters allows other features of the Sun to become visible.