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Granulation and convection zone.This is a side view to show what is happening below the surface. Hotter gas is being brought up from below, producing the bright regions.The cooler gas, which produces the darker regions, is carried down to replace the gas that was brought up.

When we look on a scale of a few arc seconds, we see that the surface of the Sun does not have a smooth appearance (Fig. 6.7). We see a structure, called granulation, in which lighter areas are surrounded by darker areas. The darker areas are not really dark. They are just a little cooler than the lighter areas, and only appear dark in comparison to the light areas. The granules are typically about 1000 km across. The pattern of granulation also changes with time, with a new pattern appearing every 5 to 10 min.

We interpret this granulation as telling us about the underlying structure we cannot see directly. The granulation can be explained by circulating cells of material, called convection zones (Fig. 6.8). (Convection is the form of energy transport in which matter actually moves from one place to another. Strong convection on the Earth is responsible for the updrafts that produce thunderstorms. A pot of boiling water also has energy transport by convection.) The brighter regions are warmer gas rising up from below. The dark regions are cooler gas falling back down.

In addition to the granulation variations, there is also a variation called the five minute oscillation, in which parts of the photosphere are moving up and down. We think this convection results from sound waves in the upper layers of the convection zone. This type of oscillation is one of many that are studied for clues to the Sun's interior structure. This area of research is called solar seismology. (On the Earth, seismologists study motions near the surface to learn about the interior.)

One interesting question about the photosphere concerns the sharpness of the solar limb. The Sun is a ball of gas whose density falls off continuously as one moves farther from the center. There is no sharp boundary (like the surface of the Earth), yet we see a definite edge on the Sun. In Fig. 6.9, we see some lines of sight through the

Lines of sight through the solar limb. For clarity, we think of the Sun as being composed of a series of spherical shells.The density in each shell decreases as we move farther from the center.This decreasing density is indicated by the shading; two lines of sight are indicated. Note that most of each line of sight is in the densest layer through which that particular line passes. Even though line 2 is not shifted very far from line 1, line 1 passes through much more material.

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