Diluting The Radiation

All the Sun's nuclear energy is released deep down inside its high-temperature core, and no energy is created in the cooler regions outside it. tte energy-generating core extends to about one quarter of the distance from the center of the Sun to the visible disk, accounting for only 1.6 percent of the Sun's volume. But about half the Sun's mass is packed into its dense core.

Because we cannot see inside the Sun, astronomers combine basic theoretical equations, such as those for equilibrium and energy generation or transport, with observed boundary conditions, such as the Sun's mass and luminous output, to create models of the Sun's internal structure, ttese models consist of two nested spherical shells that surround the hot, dense core, like Russian dolls (Fig. 2.3). In the innermost shell, called the radiative zone, energy is transported by radiation; it reaches out from the core to 71.3 percent of the distance from the center of the Sun to the visible solar disk, tte radiative zone is encompassed by a higher layer known as the convective zone, where turbulent motion, called convection, transports energy.

Even though light is the fastest thing around, radiation does not move quickly from the center of the Sun to its visible surface, tte energy made inside the Sun's pressurized core slowly trickles out to finally escape as the light we see.

tte solar core is so dense that a single gamma ray produced by nuclear fusion at the center of the Sun cannot move even a fraction of a millimeter before banging into a subatomic particle, where the radiation is scattered or absorbed and re-emitted with less energy. Ms radiation quickly collides with another particle in the radiative zone, and is eventually re-radiated at yet lower energy, tte process continues over and over again countless times, as the radiation moves outward on a haphazard, zigzag path, steadily losing energy at each encounter.

As a result of this continued ricocheting and innumerable collisions in the radiative zone, it takes about 170 thousand years, on the average, for radiation to work its way out from the Sun's core to the bottom of the convective zone. By this time, the radiation has shed so much energy that it emerges on the other side of the convective

FIG. 2.3 Anatomy ofthe Sun The Sun is powered by the nuclear fusion of hydrogen in its core at a central temperature of 15.6 million kelvin. Energy produced by these fusion reactions is transported outward, first by countless absorptions and emissions within the radiative zone, and then by turbulent motion in the outer convective zone. The visible disk of the Sun, called the photosphere, has a temperature of 5,780 kelvin. Just above the photosphere is the thin chromosphere and a transition region to the rarefied, million-degree outer atmosphere of the Sun. They are represented by an image at the extreme ultraviolet wavelength of 30.38 nanometers wavelength, emitted by singly ionized helium, denoted He II, at a temperature of 60,000 kelvin, taken from the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOIarandHeliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium and NASA. SOHO is a project of international cooperation between ESA and NASA.)

FIG. 2.3 Anatomy ofthe Sun The Sun is powered by the nuclear fusion of hydrogen in its core at a central temperature of 15.6 million kelvin. Energy produced by these fusion reactions is transported outward, first by countless absorptions and emissions within the radiative zone, and then by turbulent motion in the outer convective zone. The visible disk of the Sun, called the photosphere, has a temperature of 5,780 kelvin. Just above the photosphere is the thin chromosphere and a transition region to the rarefied, million-degree outer atmosphere of the Sun. They are represented by an image at the extreme ultraviolet wavelength of 30.38 nanometers wavelength, emitted by singly ionized helium, denoted He II, at a temperature of 60,000 kelvin, taken from the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOIarandHeliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium and NASA. SOHO is a project of international cooperation between ESA and NASA.)

zone as relatively unenergetic visible light. In contrast, sunlight moves freely through interplanetary space, taking only eight minutes to travel from the Sun to the Earth.

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