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Diagram illustrating the structure of the Sun. Energy generated by nuclear reactions in the core is transported out by radiation in the radiative envelope and by convection in the convective envelope. From Layers of the Sun. |
3. INTERIOR STRUCTURE
The Sun's interior structure is determined by an exact balance between the inward force of the Sun's own gravity and the outward force of the pressure of the hot gases (mostly hydrogen and helium) in its interior. This balance is called hydrostatic equilibrium. If the Sun's interior were much colder than it is, it would collapse within an hour; if much hotter, it would expand within an hour.
The principle of hydrostatic equilibrium, expressed mathematically, enables astronomers to calculate the run of temperature (T) and density inside the Sun (which we cannot measure directly) from observations of two quantities that we can measure: the Sun's mass Msun, and radius Rsun. Such calculations tell us that that the temperature at the center of the Sun must be about 16 x 106 K (16 million degrees Kelvin -- the modern temperature scale is named after Lord Kelvin, who was one of the first people to calculate the interior temperature of the Sun, near the beginning of this century). The interior temperature decreases outward, to about 6,000 K at the photosphere.
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Diagram illustrating the structure of the Sun. Energy generated by nuclear reactions in the core is transported out by radiation in the radiative envelope and by convection in the convective envelope. From Layers of the Sun. |
Heat leaks out from the Sun's hot interior to its relatively cool photosphere by two mechanisms. Most of the interior of the Sun is stable, so the heat energy is carried out through the matter by photons (mainly X-rays). Moving at the speed of light, a photon could travel from the center of the Sun to the photosphere in only about 2 seconds if there was nothing to stop it. But actually, a photon can travel only a few centimeters through the Sun's interior before it will be deflected or absorbed by an electron or atom. Thus, instead of travelling straight out from the Sun's interior, a photon will rattle around for thousands of years before it eventually finds its way out. This process is called radiative diffusion. Thus, the Sun's envelope serves as a very effective insulating blanket that lets the intense heat of the Sun's core leak out only very slowly. That's a good thing, because without this insulation, the radiation from the Sun's core would melt the Earth's surface in a very short time.
In the outer 30% of the Sun's radius, the envelope is literally boiling. Hot gases at the bottom become buoyant and rise to the top, causing an overturning motion called convection. The heat is carried from the bottom to the top of the convective layer by the motion of the rising hot gas. The top of the convective layer is the photosphere, where we can see the overturning motion of the convective cells. (Click here for a movie (source) showing a simulation of solar convection on a supercomputer.)
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Last modified January 24, 2002
Copyright by Richard McCray
