The Interior of the Sun

We shall consider the interior structure of the Sun in more detail when we consider solar energy production. In this section we consider only the general properties of the Sun's interior.

Relative Volume of the Sun

The radius of the Sun is about 109 times that of the Earth. The volume of the Sun would hold approximately 1.3 million Earths. However, the average density of the Sun is much smaller than that of the Earth:

Density of Sun ~ 1.4 g/cm3

Density of Earth ~ 5.5 g/cm3

This is because the Sun's composition is dominated by the elements hydrogen and helium (similar to the Gas Giant planets), while that of the terrestrial planets is dominated by heavier metals and their compounds.

Interior Zones of the Sun

These zones are illustrated in the figure above and may be characterized by their ranges of temperature and density and by the mode of energy transport through them.

1. The core is the hot, dense central region in which the nuclear reactions that power the Sun take place. It comprises about 25 percent of the interior radius and about 10 percent of the Sun's mass.

2. The radiative zone is comprised of the interior from about 25 percent to 70 percent of the solar radius. It is called the radiative zone because here (and in the core) the primary transport of energy is by photons (electromagnetic radiation).

3. The convective zone starts at about 70 percent of the solar radius and extends to just below the surface. It is a region where the change in temperature with increasing radius is so rapid that the Sun becomes unstable to convection (rapid up-and-down motion of large packets of gas), much as the Earth's atmosphere becomes convectively unstable on a hot summer day and produces thunderstorms.