2. Energizing the Sun
How the energy gets out
All of the Sunís energy is produced by nuclear fusion reactions deep down inside a dense, high-temperature core, which extends from the Sunís center to about one quarter of its radius, or 1.74 x 108 meters out. It thus accounts for only 1.6 percent of the Sunís volume Ė but about half its mass since the gas is so compacted down there.
Energy moves from the core to the rest of the Sun through two spherical shells that surround the core like nested Russian dolls (Fig. 2.3). The inner shell is called the radiative zone, and the outer one is called the convective zone. Radiation and convection are the two ways that energy can travel from one place to another inside a star.
Although light is the fastest thing around, radiation does not move quickly through the radiative zone. Instead, it diffuses slowly outward in a haphazard zig-zag pattern, called a random walk, that resembles a drunk staggering through a crowd of people. An insulating shroud of charged particles in the radiative zone controls the flow, reducing the energy content of the radiation by repeatedly absorbing, re-radiating and deflecting it. Each time the energy is re-emitted, it comes on the average from a layer at a slightly lower temperature, so the energy of the radiation is degraded as it works its way out.
Because of this continuing ricocheting in the radiative zone, it takes about 170,000 years, on average, for radiation to work its way out from the Sunís core to the bottom of the convection zone.
The cool, opaque material at the bottom of the convective zone absorbs great quantities of radiation without re-emitting it. This causes the material to become hotter than it would otherwise be. The heated material expands and becomes less dense than the gas in the overlying layers. Due to its low density, the hot gas rises, just as a balloon does. On Earth you can see hot air rising when watching the smoke above a fire or hawks riding on upward currents of heated air.
The heated material carries energy through the convection zone, from bottom to top, in about 10 days. The hot material then cools by radiating sunlight into space, and sinks back down to become reheated and rise again. Such wheeling convective motions occur in a kettle of boiling water, with hot rising bubbles and cooler sinking material.
High-resolution images of the Sun taken in white light, or in all the visible colors of the Sun combined, show a granular pattern that marks the top of the convective zone (Fig. 2.4). This solar granulation exhibits a non-stationary, overturning motion, a visible manifestation of convection. The bright center of each granule, or convection cell, is the highest point of a rising column of hot gas. The dark edges of each grain are the cooled gas beginning its descent to be heated once again.
A larger cellular pattern, called the supergranulation, has typical sizes of 10 to 30 million meters, but it is not visible in white light. The supergranulation is instead observed as the difference of two monochromatic images at closely spaced wavelengths (Fig. 2.5). About 2,500 supergranules are seen on the visible solar disk, each persisting for one or two days. Like the ordinary granulation, the supergranulation is generally accepted as convection. Whereas the small granules form and disperse within minutes, supergranules last, on average, about a day before they are replaced by other supergranules. The material in these large-scale convection cells rises in the center at about 100 meters per second, moves away from the centers with horizontal velocities of about 400 meters per second, and sinks down again at about 200 meters per second.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University