The Core. The core of the Sun is where hydrogen turns into helium through the process of nuclear fusion. Every second, the Sun turns about 600 billion kg of hydrogen into helium in the core. That's the energy equivalent of about 100 billion megatons of TNT. The core accounts for about 20% of the solar radius and 99% of the energy production in the Sun. Its temperature is about 15 million K and its density is some 150 g/cm3. The density of lead, by comparison is about 11.3 g/cm3.

Most of the energy is released as tiny particles called neutrinos and as an energetic form of light called gamma rays. The neutrinos, which interact very weakly with matter, emerge from the core and out into space in a matter of seconds. The gamma rays take much longer to escape from the dense solar interior. They tend to scatter about for 100,000 years or so, losing energy all the while, before they finally emerge from the Sun as visible light. So the sunlight falling on your garden flowers was produced in the center of the Sun about 100,000 years ago, on average.

Radiative Zone. Above the core lies a zone where very little nuclear fusion occurs. Here, the atoms of hydrogen and helium are still ripped apart from their electrons because of the intense heat of some 2 million to 7 million K. Energy escapes outward here from the hotter lower layers to the cooler outer regions by thermal radiation, the same effect that causes your hand to lose heat when placed, for example, near a cold window. The radiative zone extends from about 20% to about 70% of the solar radius.

The Convective Zone. Moving out from the radiative zone, it becomes cool enough for some atoms to recombine with their electrons. This results in the formation of cells of convection, where hot gas rises in convective cells or bubbles, a little like bubbles in a pot of boiling soup. As hot gas rises, it releases heat and light to the outer layers of the Sun, cools and becomes denser, and falls back down to the lower layers of the convective zone.

The sharp division between the radiative and convective zones is called the tachocline. It lies about 200,000 km below the Sun's visible surface. The convective zone takes up most of the rest of the solar radius. You can glimpse the top layers of these deep convective zones in the Sun's photosphere. They look like circular granules on the visible surface of the Sun.