Structure of the Sun
The Core: The Sun's core has a tremendously high temperature and pressure. The temperature is roughly 15 million °C. At this temperature, nuclear fusion occurs, turning four hydrogen nuclei into a single helium nucleus plus a LOT of energy. This "hydrogen burning" releases gamma rays (high-energy photons) and neutrinos (particles with no charge and almost no mass).

The Radiative Zone (or radiation zone): The next layer out from the core is this zone which emits radiation. This radiation diffuses outwards. The temperature ranges from 15 million °C to one million °C. It may take photons of radiation millions of years to pass through the radiative zone, as they gradually make their way outwards.

The Convective Zone: In this next layer, photons continue to make their way outwards via convection (towards lower temperature and pressure). The temperature ranges from one million °C to 6,000 °C.

The Photosphere: This is the lower atmosphere of the Sun and the part that we see (since it emits light at visible wavelengths). This layer is about 300 miles (500km) thick. The temperature is about 5,500 °C.

The Chromosphere: This reddish layer is an area of rising temperatures. The temperature ranges from 6,000 °C (at lower altitudes) to 50,000 °C (at higher altitudes). This layer is a few thousand miles (or kilometers) thick. It appears red because hydrogen atoms are in an excite state and emit radiation near the red part of the visible spectrum.

The Chromosphere is visible during solar eclipses (when the moon blocks the Photosphere).

The Corona: This is the outer layer of the Sun's atmosphere. The corona extends for millions of miles and the temperatures are tremendous, reaching one million °C.

Holes in the corona occur where the Sun's magnetic field loops out into space. These coronal holes may be the source of the solar wind, a stream of energetic particles that permeate the Solar System.

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