Chapter 15--Our Sun - Geological Sciences

Key
model
10 7 data
Key
100 model
data
10
temperature (K)
10 6
core
radiation
zone
convection
zone
density (g/cm 3 )
1
0.1
core
radiation
zone
convection
zone
0.01
0 0.2 0.4 0.6 0.8 1.0
fraction of the Sun’s radius
a Temperature at different radii within the Sun.
0 0.2 0.4 0.6 0.8 1.0
fraction of the Sun’s radius
b Density at different radii within the Sun. (The density of water is
1 g/cm 3 .)
Figure 15.10 Agreement between mathematical models of solar structure and actual measurements of
solar structure derived from “sun quakes.” The red lines show predictions of mathematical models of the
Sun. The blue lines show the interior structure of the Sun as indicated by vibrations of the Sun’s surface.
These vibrations tell us about conditions deep within the Sun because they are produced by sound waves
that propagate through the Sun’s interior layers.
Solar Neutrinos Another way to study the solar interior
is to observe the neutrinos coming from fusion reactions in
the core. Don’t panic, but as you read this sentence about a
thousand trillion solar neutrinos will zip through your body.
Fortunately, they won’t do any damage, because neutrinos
rarely interact with anything. Neutrinos created by fusion
in the solar core fly quickly through the Sun as if passing
through empty space. In fact, while an inch of lead will stop
an X ray, stopping an average neutrino would require a slab
of lead more than 1 light-year thick! Clearly, counting neutrinos
is dauntingly difficult, because virtually all of them
stream right through any detector built to capture them.
THINK ABOUT IT
Is the number of solar neutrinos zipping through our bodies
significantly lower at night? (Hint: How does the thickness of
Earth compare with the thickness of a slab of lead needed
to stop an average neutrino?)
Nevertheless, neutrinos do occasionally interact with
matter, and it is possible to capture a few solar neutrinos
with a large enough detector. Neutrino detectors are usually
placed deep inside mines so that the overlying layers of
rock block all other kinds of particles coming from outer
space except neutrinos, which pass through rock easily. The
first major solar neutrino detector, built in the 1960s, was
located 1,500 meters underground in the Homestake gold
mine in South Dakota (Figure 15.11).
The detector for this “Homestake experiment” consisted
of a 400,000-liter vat of chlorine-containing dry-cleaning
fluid. It turns out that, on very rare occasions, a chlorine
nucleus can capture a neutrino and change into a nucleus
of radioactive argon. By looking for radioactive argon in
the tank of cleaning fluid, experimenters could count the
number of neutrinos captured in the detector.
From the many trillions of solar neutrinos that passed
through the tank of cleaning fluid each second, experimenters
expected to capture an average of just one neutrino
per day. This predicted capture rate was based on measured
properties of chlorine nuclei and models of nuclear fusion
in the Sun. However, over a period of more than two
decades, neutrinos were captured only about once every
3 days on average. That is, the Homestake experiment detected
only about one-third of the predicted number of
neutrinos. This disagreement between model predictions
and actual observations came to be called the solar neutrino
problem.
The shortfall of neutrinos found with the Homestake
experiment led to many more recent attempts to detect solar
neutrinos using more sophisticated detectors (Figure 15.12).
The chlorine nuclei in the Homestake experiment could
506 part V • Stellar Alchemy