Chapter 16. The Solar Interior

16.1 Introduction

The Sun's Internal Structure

The goals of this module: After completing this exercise, you should be able to:

1. Describe hydrostatic equilibrium and thermal equilibrium.
2. Explain how energy is transported through the different regions of the solar interior.

In this module you will explore:

1. The principles of hydrostatic equilibrium and thermal equilibrium.
2. How hydrostatic equilibrium and thermal equilibrium apply to the Sun.
3. How the energy produced in the Sun's core travels through its interior and escapes from its surface.

Why you are doing it: The concepts of hydrostatic equilibrium and thermal equilibrium are keys to understanding the structure and evolution of all stars, including our own Sun. Learning how energy is transported throughout the interior of a star is crucial to an understanding of how the Sun remains balanced.

16.2 Background

Visual image of the Sun

When we look at a visual image of the Sun, we see only the photosphere - the surface layer of the Sun. Below the photosphere, there are three layers, which make up the Sun's interior: the core, the radiative zone, and the convective zone. The core lies at the center of the Sun and is the site of all its energy production. The energy produced in the core travels through the radiative zone and the convective zone as it journeys outward through the Sun, eventually leaving the photosphere into space. To understand this journey, we must first look at the conditions that govern the interior zones in the Sun.

Our Sun is in equilibrium, or balanced, at every point in its interior. In other words, there isn't much motion up or down. We can illustrate this with a simple example: a fish floating freely in water. In the picture below, we see that the fish is in equilibrium because the forces (shown by the arrows) pushing down on the fish are exactly balanced by the force pushing up on it. The fish is neither sinking to the bottom of the ocean nor rising up to the water's surface; it can remain at its location effortlessly.

Just like the fish in the picture, all of the particles in the Sun's interior - in its core, radiative zone, and convective zone - are balanced at their respective positions and temperatures. This balance makes the Sun stable, and explains why it appears basically the same from day to day. In order to understand the interior of the Sun, you need to understand what is being balanced inside the Sun and how this balance is maintained.

A fish in equilibrium. (Note that the width of the arrows indicates the strength of the forces.)

16.3 Hydrostatic Equilibrium

The first balance in the solar interior is called hydrostatic equilibrium. Look at the small piece of material marked in the figure below. Hydrostatic equilibrium for this piece is a balance between 3 forces:

1. the force of gravity, which pulls the material downward (toward the center of the Sun) - this is the weight of the piece itself
2. pressure from solar material above the piece, which is pushing it downward
3. pressure from solar material below the piece, which is trying to push it upward

If one of the forces is too large or too small, the balance is broken, and the piece of solar material will move.

16.4 Thermal Equilibrium

The second balance in the solar interior is called thermal equilibrium. Thermal equilibrium means that the temperature remains constant at each point within the Sun. This does not mean that the temperature is the same everywhere inside the Sun - it's not! In fact, the further you go inside the Sun, the hotter the temperature becomes. Thermal equilibrium just means that the temperature at each point doesn't fluctuate back and forth.

Look at the first picture of the Sun below.

16.5 Radiative Zone

In order for the Sun to remain in equilibrium, all of the energy produced in the core must travel through the interior and be radiated away at the surface. In the Sun, energy travels via two methods: radiative diffusion and convection.

From the center of the Sun out to about 71% of its radius, the energy produced in the core is transported directly by photons; this process is called radiative diffusion. As photons travel through the gases in the radiative zone, they are absorbed and re-emitted. Each time they are re-emitted, they leave traveling in a completely random direction. For this reason, the path of a photon through the radiative zone is known as a random walk. The photons travel on a zigzag path, but eventually do make it out of the radiative zone.

The temperature in the radiative zone is so high that basically all of the atoms are completely ionized; in other words, they have lost all of their electrons. However, as the edge of the radiative zone is approached, the temperature has dropped to a point where electrons and nuclei have recombined into atoms. Energy no longer can be transported by radiative diffusion, since these atoms absorb the energy and the whole region begins to heat.

Click on play to see how photons move through the radiative zone.

16.6 Convective Zone

As the distance from the core increases, the density, temperature, and pressure all decrease. Finally, at a distance of about 71% of the Sun's radius, the temperature is low enough that a large number of nuclei and electrons have bonded together to form atoms. These atoms absorb the incoming photons very efficiently, effectively halting the radiative transfer of energy. At this point, convection takes over as the main energy transfer mechanism.

Convection differs from radiative diffusion in that the energy is transferred via the bulk movement of material instead of by the photons themselves. In the Sun, gas that absorbs the incoming photons heats up and rises to a cooler layer where it deposits the absorbed energy. The region of gas then sinks down to its original location, and the cycle repeats. Each cycle is called a convective cell. The animation below shows a very simplified picture of convection in the Sun. Keep in mind that in the real convective zone there are many, many layers of convective cells.

16.7 Energy Transport Summary

Once the energy reaches the surface of the Sun (the photosphere), transport switches back to radiative diffusion, but for a different reason than in the radiative zone. In the cool photosphere, densities are so low that the photons can escape relatively easily. The figure below summarizes energy transport throughout the solar interior.

16.8 Quick Check Quiz

Indepth Activity: The Solar Interior