The Sun still holds many mysteries. For example, the proton-proton reaction thought to be the source of most solar energy should also produce a certain number of the particles called neutrinos. Yet studies thus far have detected significantly fewer neutrinos than theory predicts. One radical suggestion is that the Sun produces fewer neutrinos than expected because it has an iron-plasma core that amounts to about 0.5% of its total mass. Other physicists have theorized that weakly interacting massive particles (WIMPs) - predicted by grand unification theories and sometimes invoked to account for the "missing matter" in the universe - might exist deep within the Sun and lower its temperature enough to explain the lack of neutrinos. Yet another proposal is that electron-type neutrinos in the Sun's core change on the way out into muon-type neutrinos unobservable by the detectors now in use. This proposal received strong support from neutrino research in the 1990s.

In the early 1960s, radial oscillations of the photosphere were detected that have since been explained as the resonant trapping of acoustic waves between certain layers of the convection zone of the Sun. Such studies are enabling scientists to probe the density, temperature, and velocity patterns of the Sun's layers hidden below the photosphere.

A sophisticated satellite, the Solar and Heliospheric Observatory ( SOHO), was launched on Dec. 2, 1995, to study the Sun during a relatively quiet period in the cycle of solar activity. As it circles the first stable point in Earth's orbital path around the Sun, its instruments have obtained the first images of the extended outer corona, along with views of gigantic "tornadoes" near the solar poles. The research program is being conducted jointly by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA); Japan is also taking part in this investigation into the physics of the Sun-Earth relationship.