The three layers of the sun's atmosphere are the chromosphere, the transition layer and the corona. The corona is both the outer layer and the hottest layer of the atmosphere, as it can reach 2 million degrees Fahrenheit.
The atmosphere of the sun surrounds its surface, which is separated from the core by radiative and convective layers. The core is the hottest part of the sun, as it reaches about 27 million degrees Fahrenheit.
The sun does not have a surface like a planet; rather, it is composed entirely of gas. It is mostly hydrogen, though it contains about 8 percent helium.