of the sun? Astronomers from around the world are proposing to build the Large Earth-based Solar Telescope (LEST), a 2.4-m solar telescope in the Canary Islands. The hallmark of LEST is unprecedented angular resolution, obtained by the use of special optics that will remove the distorting effect of the atmosphere. The principal aim of LEST is to observe the solar surface with sufficient acuity to reconstruct the three-dimensional structure of solar magnetic fields.

In conjunction with LEST, orbiting satellites equipped with visible, ultraviolet, and x-ray telescopes will study the radiation from the sun. Other x-ray satellites, such as the Advanced X-ray Astrophysics Facility (AXAF) already under construction, will monitor the coronal emission from other stars to help understand our own sun.

Astronomers have little direct data from the interior of the sun or any other star. The light we see comes from its surface. However, the surface motions contain clues about conditions far below. A major new field of study called helioseismology measures these vibrations of the solar interior. Just as the intensity and intervals of terrestrial earthquakes tell us about conditions deep within the earth, so also do the vibrations of the sun's surface inform us about the density, temperature, and rate of rotation in the deep interior (Plate 2.4).

A better knowledge of the interior of the sun might resolve another problem that has worried astronomers for years. The number of subatomic particles called neutrinos emitted by the sun and detected on the earth is much smaller than predicted. Neutrinos are produced in nuclear reactions at the center of the sun, and their rate of production has been calculated from our theories of nuclear physics and presumed knowledge of the conditions of temperature and density in the sun. Since neutrinos interact extremely weakly with other matter, almost every neutrino produced at the sun's center should escape. For the last 20 years, American physicists have counted neutrinos emitted from the sun and found fewer than one-third of the predicted number. The discrepancy is serious. Either the interior conditions of the sun are not what we think, or the neutrino has some property that allows it to change form and avoid detection once emitted. The former explanation, if correct, could alter our theories of the structure of stars. The latter would have significant implications for our understanding of subatomic physics. Planned for the 1990s are several new, more sensitive experiments to monitor neutrinos emitted from the sun.

The Formation of Stars

It takes two things to form a star—matter, and a mechanism to compress the matter to high density. Matter is plentiful in space in the form of diffuse hydrogen gas along with traces of other elements and small particles of dust. A dense clump within a gas cloud can pull itself together by gravity, becoming even denser. When the inward pull of gravity is sufficiently strong to overcome



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