Hi Stu,

The Sun doesn't have layers the way an onion has layers. At some level the layers are regions defined and named by scientists. The different regions are selected or defined by the physics of what is going on. So the core is the region in which nuclear fusion is happening. The radiative zone is where the heat from the core is transferred up and out to the cooler regions by photons. The convective zone is where the energy is mostly transported by movement of the plasma. The sections from the photosphere to the corona are largely defined by the change in the behavior of the temperature (whether it is increasing, decreasing, or constant with increasing height).

Best,

Christina