Invisible Fires

ttere is more to the Sun than meets the eye! In addition to visible light, there are invisible gamma ray, X-ray, ultraviolet, infrared and radio waves, tte whole solar spectrum extends from short gamma rays: that are comparable to the size of an atom's nucleus, to long radio waves that are as broad as a mountain; and the Sun is so bright that it can be examined with precision in every spectral region. Observations at these invisible wavelengths have indeed broadened and sharpened our vision of the Sun.

However, our atmosphere effectively blocks most forms of invisible radiation including ultraviolet and X-ray radiation. Radio waves are the only kind of invisible radiation that is not absorbed in the Earth's atmosphere, so radio astronomy provided the first new window on the Sun.

FIG. 1.11 Very Large Array Each of the 27 radio telescopes of the Very Large Array, abbreviated VLA, measures 25 meters in diameter, or about the size of house, and weighs 235 tons (2.35 X 105 kilograms). These telescopes are placed along the arms of a Y-shaped array on a desert near Socorro, New Mexico, and interconnected electronically to provide a total of 351 pairs of telescopes. The telescopes can be rolled along tracks to change their configuration and create a radio zoom lens. When the telescopes are pushed to the outer ends of each arm and their output combined in a computer, the VLA creates a radio telescope with a diameter as large as 34 kilometers and an angular resolution that can be smaller than 1 second of arc. (Courtesy of NRAO, AUI and NSF.)

Astronomers use conventional radio telescopes to observe the Sun (Fig. 1.11); but radio telescopes do not really look at the Sun, they listen to it. Such telescopes usually have a metallic, dish-shaped, or parabolic, reflector that focuses the radio waves at a receiver, tte long, straight antenna on your automobile or home radio similarly intercepts radio signals. Moreover, the Sun is the brightest, noisiest radio object in the sky, and because the atmosphere does not distort radio signals we can observe the radio Sun on a cloudy day, just as your home radio works even when it rains or snows outside.

To look at the Sun through windows other than the radio or visible ones, we must loft telescopes above the atmosphere. Ms was done first by using balloons and sounding rockets, and then with satellites that orbit the Earth above the atmosphere. Satellite-borne telescopes now view the Sun at ultraviolet and X-ray wavelengths, above the Earth's absorbing atmosphere at places where night can be brief or non-existent.

ttus, astronomers now have new ways to extract previously unobtainable information about the Sun. ttey are aided by new telescopes and sophisticated computers that gather in an increasing wealth of unsuspected information. Computerized telescopes now operate, from the ground and in orbit, in each of the invisible domains of the electromagnetic spectrum, creating images that provide new insight to the Sun.

Much of this book describes the invisible Sun, an unseen world of perpetual change and cosmic violence that lies outside the visible solar disk. And as the title Sun, Earth and Sky suggests, our book also describes the Sun's interaction with planet Earth, mainly through invisible radiation and tiny, energetic particles that cannot be seen. It involves a global, space-age perspective that looks up at the Sun and down at the Earth, at both visible and invisible wavelengths, or directly samples the space outside Earth with orbiting satellites.

But before we begin our journey through these largely invisible realms, there is an equally fascinating world that lies hidden below everything we can see on the Sun; clever techniques are required to perceive this unseen interior of the Sun.

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