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
Seismic studies also offer a look into the distant future of the sun. In a few billion years, the sun will exhaust the hydrogen fuel in its core, triggering a sequence of events that will eventually lead to its death as a collapsed white dwarf. Exactly when that will happen depends on the amount of mixing that occurs between the solar core and its outer layers: the more mixing, the longer the sun's life (because more hydrogen can find its way to the central nuclear fires). Gough and a large team of co-authors report finding hints in the GONG data that the amount of mixing may be greater than generally thought. That may be good news for those of us inhabiting the earth, but it could spell trouble for cosmology. Already the age of the oldest stars is uncomfortably close to the estimated age of the universe itself. If stars actually age more slowly than expected, scientists may have a full-blown paradox on their hands.
One of the great advantages of GONG and SOHO over earlier instruments is that they allow continuous coverage of the sun: GONG's eyes are spaced across the globe, and SOHO orbits the sun far from the earth, where it never plunges into night. That continuity allows the collection of extremely accurate data on the very subtle acoustic oscillations that probe deep into the sun. M. J. Thompson of the University of London and his colleagues have used these data to explore the way that the interior of the sun rotates. Astronomers had long assumed that the dense core of the sun rotates more quickly than its outer layers, in part a relic of the early days when the sun (like most young stars) was a fast spinner. GONG has shown that assumption to be wrong, raising new questions about how stars lose their angular momentum. (Incidentally, it is a good thing that they do; fast-rotating stars produce powerful flares and other outbursts that could be devastating to life-forms like us.)
Helioseismology is still a young field, so it is not surprising that so many questions remain to be answered. GONG will provide years of uninterrupted solar observations, which may illuminate the poorly understood mechanism that causes the 11-year cycle of solar activity. Working with just four months of data, a SOHO team led by David H. Hathaway of the NASA Marshall Space Flight Center has already detected huge convective flows in the solar surface.
Further down the road, SOHO's remarkable precision should ultimately allow scientists to observe an even more delicate kind of solar oscillation: buoyancy waves, which can penetrate to the very heart of the sun's energy-producing core. Such a detection may still be year away. But even now, astronomers are marveling at the clarity with which they can hear the sun ringing out its innermost secrets.