Daytime Star

All stars are suns, kin to our own daytime star. Indeed, the Sun is just one of about one hundred billion stars in our Galaxy, the Milky Way, and countless billions of galaxies stretch out in the seemingly boundless Universe. But the Sun is a special star; it is our only daytime star! Nothing else in the Universe is so critically important to us. As the Victorian English poet Francis William Bourdillon (1852-1921) wrote:

"tte night has a thousand eyes, And the day but one; Yet the light of the bright world dies, With the dying Sun.

"tte mind has a thousand eyes, And the heart but one: Yet the light of a whole life dies When love is done3

tte Sun is a quarter million times closer to us than the next nearest star. Because of this closeness, the Sun is about a hundred billion times brighter than any other star, tte Sun's brilliance provides ample light for the most exacting studies of its chemical constituents, magnetic fields, and oscillations, ttis blessing can also be a curse, for the Sun's heat can melt mirrors or burn up electronic equipment when focused to high intensity. For this reason, special mirror configurations are used to reduce the concentration of visible sunlight, while still producing large images that contain fine detail (Fig. 1.5).

FIG. 1.5 Eyes on the Sun Scattered sunlight colors the McMath solar telescope a stunning red, while stars mark trails across the evening sky (top). A moveable heliostat, perched atop this telescope, follows the Sun and directs its light downward through the long fixed shaft of the telescope (bottom). A figured mirror at the shaft bottom reflects and focuses the sunlight toward the observation room. The shaft's axis is parallel to the rotation axis of the Earth, and about three fifths of it is underground. It is kept cool by pumping cold water through tubes in the exterior skin, thereby reducing turbulence in the air inside and keeping the Sun's image steady. (Courtesy of William C. Livingston, NOAO.)

FIG. 1.5 Eyes on the Sun Scattered sunlight colors the McMath solar telescope a stunning red, while stars mark trails across the evening sky (top). A moveable heliostat, perched atop this telescope, follows the Sun and directs its light downward through the long fixed shaft of the telescope (bottom). A figured mirror at the shaft bottom reflects and focuses the sunlight toward the observation room. The shaft's axis is parallel to the rotation axis of the Earth, and about three fifths of it is underground. It is kept cool by pumping cold water through tubes in the exterior skin, thereby reducing turbulence in the air inside and keeping the Sun's image steady. (Courtesy of William C. Livingston, NOAO.)

tte Sun's proximity allows a level of detailed examination unique among stars. While most other stars appear only as unresolved spots of light in the best telescopes, the Sun reveals its features in exquisite detail. Most ground-based optical telescopes can resolve structures on the Sun's visible disk that are about 750 kilometers across, about the distance from Boston to Washington, D.C. and about three-quarters the size of France; that is comparable to seeing the details on a coin from one kilometer away.

Yet, the resolution of ground-based telescopes is limited by turbulence in the Earth's atmosphere; it reduces the clarity of the Sun's image at visible wavelengths. Similar variations cause the stars to twinkle at night, tte best visible images with even finer detail can be obtained using adaptive optics that correct for the changing atmosphere, or from the unique vantage point of outer space using satellite-borne telescopes unencumbered by the limits of our atmosphere.

tte other stars are so far away that their surfaces remain unresolved with even the largest telescopes, tte Sun therefore permits examination of physical phenomena and processes that cannot be seen in detail on other stars. Furthermore, the Sun's basic properties provide benchmarks and boundary conditions for the study of stellar structure and evolution.

So, all astronomers do not work in the dark. Many of them closely scrutinize our daytime star, deciphering some of the most fundamental secrets of nature.

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