Chapter 15--Our Sun - Geological Sciences

Figure 15.11 This tank of dry-cleaning fluid (visible underneath
the catwalk), located deep within South Dakota’s Homestake mine,
was a solar neutrino detector. The chlorine nuclei in the cleaning
fluid turned into argon nuclei when they captured neutrinos from
the Sun.
a Scientists inspecting individual detectors within Super-Kamiokande.
capture only high-energy neutrinos that are produced by
one of the rare pathways of step 3 in the proton–proton
chain (not shown in Figure 15.7). More recent experiments
can detect lower-energy neutrinos, including those produced
by step 1 of the proton–proton chain, and therefore offer a
better probe of fusion in the Sun. To date, all these experiments
have found fewer neutrinos than current models of
the Sun predict. This discrepancy between model and experiment
probably means one of two things: Either something
is wrong with our models of the Sun, or something is
missing in our understanding of how neutrinos behave.
THINK ABOUT IT
Although the observed number of neutrinos falls short of
theoretical predictions, experiments like Homestake have
shown that at least some neutrinos are coming from the
Sun. Explain why this provides direct evidence that nuclear
fusion really is taking place in the Sun right now. (Hint: See
Figure 15.7.)
For the moment, many physicists and astronomers
are betting that we understand the Sun just fine and that
the discrepancy has to do with the neutrinos themselves.
One intriguing idea arises from the fact that neutrinos come
in three types: electron neutrinos, muon neutrinos, and
tau neutrinos [Section S4.2].
Fusion reactions in the Sun produce only electron neutrinos,
and most solar neutrino detectors can detect only
b An image of the Sun created by tracing the paths of
neutrinos detected by Super-Kamiokande back to the Sun.
Figure 15.12 The Super-Kamiokande experiment in Japan is one
of the world’s premier neutrino detectors.
chapter 15 • Our Star 507