continually change, some quickly and violently, others more slowly and moderately," wrote Galileo of his sighting.

Since then, the science of solar astronomy has largely evolved as an extension of Galileo's first effort. What is known about the sun essentially comes from examination of its outer features, although modern-day instruments, both on the ground and in space, have revealed a solar surface more turbulent and varied than seventeenth-century astronomers could ever have imagined: High-speed streams of solar particles emanate from dark coronal "holes"; solar prominences, immense arches of glowing gas, soar for hundreds of thousands of kilometers above the solar surface; and solar flares, lightning-like cataclysmic explosions, can flash across a region of the sun in a matter of minutes.

Nearly all these effects reflect complicated and tumultuous activities inside the sun itself. But an exact description of what lies beneath the sun's fiery surface has been based more on conjecture than explicit measurement. The sun may be the star closest to Earth, yet at the same time, it is very remote. Its center lies some 700,000 kilometers from its surface, and all that fiery hot material in between acts as an effective shield, keeping solar astronomers from directly viewing the sun's interior. The laws of physics, however, do enable scientists to make some educated guesses on what they would see. Theoretical modeling and computer simulations have established that the sun is powered at its core, the inner 20 percent, by the thermonuclear conversion of hydrogen into helium. The resulting energy slowly makes its way out of the core, first by radiative diffusion, and then, starting about seven-tenths of the way out, by convection as the heated gases physically flow upward within the sun's outer layers. The gases subsequently release their energy at the surface, bursting through like bubbles in a pot of boiling fudge, only to recirculate downward to be heated once again. In this way a regular pattern of convection cells—columns of hot gas rising, cooling off, and then descending—is created within the sun (see Figure 1.1).

Many elements in this description, however, are far from secure. Certainty can arrive only if the sun is probed directly. Given the very nature of the sun—its 1.5-million-kilometer width and scorching temperatures—such an endeavor always seemed like an impossible dream—but no longer. In recent decades, solar astronomers have noticed that the sun quivers and shakes. It continually rings, in fact, like a well-hit gong. These reverberations, which carry information about the sun's deep

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