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For all the nitty-gritty details of the solar neutrino problem as it appeared before being solved, see John Bahcall's authoritative book (Bahcall 1989), complemented by his more recent updates (Bahcall 1997a; Bahcall & Krastev & Smirnov 1998), and with a more accessible introduction in Bahcall (1990).

One of the leading theories of the formation of the sun was (and is) the 18th-century nebular theory of Kant and Laplace, in which the sun formed through the gravitational contraction of a large gas cloud.

The potential gravitational energy of the cloud would be released as heat, as it contracted, and Hermann Helmholtz realized that this was a possible energy source for the sun, provided that it was still in the contracting phase.

In this , I will first describe the standard view of the sun's internal workings, and then go on to neutrinos, what they are, and what we expect of them from the sun.

After that I will discuss various possible solutions to the solar neutrino problem.

It was rapidly realized that none of our terrestrial energy sources would be adequate.

If the sun, for example, consisted of some fuel that could burn, it would burn out in a matter of centuries, nowhere near enough to illuminate all of recorded history, and never mind the long prehistory of the earth.

Neutrino detectors on earth do find neutrinos coming from the sun, and the observed flux is of the right order of magnitude, confirming that fusion is actually taking place in the sun.

But when detailed calculations of the expected neutrino flux were confronted with measurements, about 30 years ago, a significant discrepancy was found.

Speculations about the nature of the sun are as old as recorded history, but I shall not dwell on the more fanciful versions of old, as they are not pertinent to the solar neutrino issue.

If we restrict ourselves to serious study of the sun, the mid-19 The issue of where the sun's energy came from turned out to be a thorny one.

William Thomson (better known as Lord Kelvin) elaborated and promulgated this theory during the last decades of the 19 century.