Building a detector the size of a 10-story building two kilometers underground is a strange way to study solar phenomena. Yet that has turned out to be the key to unlocking a decades-old puzzle about the physical processes occurring inside the sun. English physicist Arthur Eddington suggested as early as 1920 that nuclear fusion powered the sun, but efforts to confirm critical details of this idea in the 1960s ran into a stumbling block: experiments designed to detect a distinctive by-product of solar nuclear fusion reactions--ghostly particles called neutrinos--observed only a fraction of the expected number of them. It was not until last year, with the results from the underground Sudbury Neutrino Observatory (SNO) in Ontario, that physicists resolved this conundrum and thereby fully confirmed Eddington's proposal.

Like all underground experiments designed to study the sun, SNO's primary goal is to detect neutrinos, which are produced in great numbers in the solar core. But unlike most of the other experiments built over the previous three decades, SNO detects solar neutrinos using heavy water, in which a neutron has been added to each of the water molecules' hydrogen atoms (making deuterium). The additional neutrons allow SNO to observe solar neutrinos in a way never done before, by counting all three types, or "flavors," of neutrino equally. Using this ability, SNO has demonstrated that the deficit of solar neutrinos seen by earlier experiments resulted not from poor measurements or a misunderstanding of the sun but from a newly discovered property of the neutrinos themselves.

This article was originally published with the title Solving the Solar Neutrino Problem.

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