There is no real "surface" to the sun. There is instead hot gas that gets denser and denser as you get closer to the core. The photosphere is about 500 kilometers thick and is the innermost layer of the sun's atmosphere. When we refer to the size of the sun, we are referring to the region surrounded by the photosphere. The photosphere is also where the sun's radiation can escape and reach earth as sunlight as well as where sunspots form which are darker spots on the sun that form due to strong magnetic activity. They can last from hours to months, range in size, and are 1500 K cooler than the photosphere. The Chromosphere is the second layer of the sun's atmosphere. It is about 2000 kilometers deep and the density here decreases as you move further away from the photosphere. The corona is the outermost layer of the sun's atmosphere and it extends for millions of miles into space. During a solar eclipse, the corona shines brightly like a crown around the sun. Most of the radiation emitted from the corona is X-ray and ultraviolet wavelengths.