SOLAR ANATOMY. Helioseismology has enabled researchers to create images of the invisible structure and processes deep inside the sun.

"I have heard the mermaids singing," the poet T. S. Eliot wrote. "Yes, but we have heard the sun's soft ringing," might be the reply of astronomers such as Douglas O. Gough of the University of Cambridge. Gough is a pioneer in the emerging field of helioseismology.

Over the past two decades, scientists have slowly learned to read the subtle acoustic oscillations of the sun--analogous to the tones and overtones of a clanging bell--to probe the unseen interior structure of our nearest star. The effort has just received a tremendous boost from two new experiments, the ground-based Global Oscillation Network Group (GONG) and instruments on the space-faring Solar and Heliospheric Observatory (SOHO). A flurry of press releases, presentations and a special section of papers in the May 31, 1996 issue of Science documents how helioseismology has finally achieved the unthinkable: a comprehensive understanding of the invisible structure and processes deep inside the sun.

GONG consists of identical observatories at six locations around the world; they began active duty in October 1995. SOHO is even younger, having been launched in December, 1995. The oscillations that these experiments seek out are extremely subtle. A typical acoustic wave produces a velocity change on the order of one centimeter per second, superimposed on the overall roiling of the 6,000-degree Celsius solar surface; the oscillations have periods ranging from minutes to hours.

These motions can be detected by the way they change the wavelength of the light emitted by the sun (a phenomenon known as the Doppler shift). It is also possible to detect the variations in the local brightness of the sun induced by acoustic waves; these, too, are minuscule, roughly one part in 10 million. Gough notes that even in the late 1970s, many astronomers scoffed at the notion that helioseismology could produce any usable information about the sun's interior. Yet now astronomers can confidently describe the density and pressure throughout the sun to an accuracy of 1 percent.

And helioseismology can do much more than paint a general picture of the physical state of the sun. It also allows unprecedented insight into how stars shine. In one early triumph, T. L. Duvall, of the National Aeronautics and Space Administration Goddard Space Flight Center deduced that the solar interior must be significantly more opaque than expected at temperatures of between two million and four million degrees C--a fundamental high-temperature physics discovery that would be nearly impossible to make in the laboratory. Such information has led astronomers to revise their models of how energy from the nuclear reactions in the core of the sun wends its way to the surface.

More detailed knowledge of the solar interior has actually deepened some other scientific mysteries. For nearly 25 years, scientists have noted the flux of neutrinos (uncharged, nearly massless particles created as a by-product of nuclear fusion) from the sun is significantly less than expected. Either something is missing from our theories of particle physics, or else the interior of the sun, where fusion takes place, has different properties than researchers have assumed. Helioseismology makes it possible to examine the physical conditions deep inside the sun; J¿rgen Christensen-Dalsgaard of ¿rhus University in Denmark reports elsewhere in Science that the GONG results "suggest that no solution of the neutrino problem can be found by modifying the computation of solar models." This is an exciting finding for physicists, one that affirms other indications of the existence of a fundamentally new process affecting subatomic particles

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