Hello Gina,

This is an interesting question. Although the Sun is essentially a large ball of gas we typically define the surface to be that region where the light can flow outwards essentially unimpeded. This is called the photosphere and is marked by the yellow disk one sees in the sky. Above this surface is the atmosphere which consists of the chromosphere, a region where the temperature increases from about 4,000K to about 50,000K - typically the chromosphere extends to a few thousand kilometers above the surface. There is then a small region called the transition region, about 500 kms thick over which the temperature increases to about a million degrees. Above this is the corona, which extends all the way out to the very edge of the solar system. So technically, the atmosphere of the Sun extends all the way out to where its pressure is balanced by the pressure of the nearby stars.

In terms of depth, we can now probe below the Sun;s surface using the relatively new techniques of helioseismology which studies how the Sun "shakes" to infer the properties of the interior. To very good precision we can now determine the properties of the bulk of the solar interior. The Sun's radius is 700,000 km.

David