The interior of the Sun. Only a small part of the Sun, its core, actually generates energy. The rest of the gas just gets in the way as the heat tries to get out. Most of the Sun's interior is filled with transparent gas, and the energy passes through it in the form of radiation. But toward the surface, the gas is cooler and opaque, so radiation can't get through.
Convection takes over.
Figure 4 Fire burn and cauldron bubble. Each granule, small as it looks, is about the size of Texas. The granules flicker rapidly, making the Sun look like a burbling witch's brew. Granules are the top of convection cycles that bring material from deep within the Sun to the surface. Photo courtesy of Sacramento Peak Observatory.