The Interior
of the Sun

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

Relative Volume of the Sun

The Sun is enormous compared with other objects in the Solar System, as illustrated in the following image.

The Sun and planets drawn to scale


Thus, for example, the radius of the Sun is about 109 times that of the Earth, which implies that the volume of the Sun would hold approximately 1.3 million Earths (since the volume goes as the cube of the radius). However, the average density of the Sun is much smaller than that of the Earth: about 1.4 g/cm3 compared with about 5.5 g/cm3 for the Earth. As we shall see, this is because the composition of the Sun is dominated by the light 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

It is useful to divide the interior of the Sun into three interior regions. These are summarized in the following table.

Interior Zones of the Sun

Zone R/R(0) Temperature ( K) Density (g/cm3) Energy Transport

Core 0.0 - 0.25 ~ 15,000,000 - 8,000,000 ~ 160 - 10 Radiative
Radiative ~ 0.25 - 0.85 ~ 8,000,000 - 500,000 ~ 10 - 0.01 Radiative
Convective ~ 0.85 - 1.00 ~ 500,000 - 10,000 < 0.01 Convective


These zones may be characterized by their ranges of temperature and density, and also 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% of the interior radius.

  2. The radiative zone is comprised of the interior from about 25% to 85% 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 85% of the solar radius and extends to just below the surface. It is a region in which 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.
We will take up the issue of energy production in the core of stars and the transport of that energy to the star's surface in the section on stellar energy production.

Helioseismology

One way to study the solar interior is through helioseismology. In helioseismology, one attempts to learn about the properties of the Sun by studying the propagation of waves in its body (which, for example, cause small oscillations of the surface that are observable) in a manner similar to geologists learning about the interior of the Earth by studying seismic waves.

The left image illustrates a computer generated model of acoustic waves in the body of the Sun (Source). The right image illustrates a similar calculation (Ref) of a standing wave with red and blue indicating displacements of opposite sign. Presently, helioseismology is placing strong constraints on our theories of the solar interior. This information is important, for example, in the discussion of the solar neutrino problem.


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