Fig. 11.12. The Wolf-Rayet star WR 104 photographed by the 10-m Keck telescope. The spiral is dust and gas which is thrown out from the rotating binary system. The spiral "pinwheel" is seen to make a full revolution in about 220 days. (Photo U.C. Berkeley Space Sciences Laboratory/W.M. Keck Observatory)

of hydrogen fuel has been consumed. The Sun will go on shining like a normal main sequence star for another 5000 million years, before there will be any dramatic change.

Some problems remain in regard to the observations. One is the solar neutrino problem. The neutrinos produced by solar nuclear reactions have been observed since the beginning of the 1970's by the techniques described in Sect. 3.7. Only the neutrinos formed in the relatively rare ppIII reaction are energetic enough to be observed in this way. Their observed number is too small: whereas the models predict about 5 units, the observations have consistently only registered 1-2.

The discrepancy may be due to a fault in the observational technique or to some unknown properties of the neutrinos. However, if the solar models are really in error, the central temperature of the Sun would have to be about 20% lower than thought, which would be in serious conflict with the observed solar luminosity. One possibility is that some of the electron neutrinos change

to other, unobservable particles during their passage to Earth. (See also Sect. 12.1.)

A second problem is the observed abundance of lithium and beryllium. The solar surface contains a normal abundance of beryllium, but very little lithium. This should mean that during its contraction, the Sun was still fully convective when the central temperature was high enough to destroy lithium (3 x 106 K), but not beryllium (4 x 106 K). However, according to the standard solar evolution models, convection ceased in the centre already at a temperature of 2 x 106 K. One suggested explanation is that the convection has later carried down lithium to layers where the temperature is high enough to destroy it.

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