154 PART 2^ |^ THE STARS
the sun produce only one fl avor, and the Davis experiment was
designed to detect (taste) that fl avor. But during the 8-minute
journey from the sun’s core to Earth, the neutrinos oscillated so
much that they were evenly distributed among the three diff erent
fl avors by the time they arrived at Earth. Th at’s why the Davis
experiment detected only one-third of the number predicted.
Th e missing neutrinos have been detected by more sophisti-
cated detectors (Figure 8-11b). In 2007, scientists announced
that a supersensitive experiment in a tunnel under the Italian
Alps had detected 50 neutrinos a day coming from the sun. Th e
neutrinos have lower energies that those caught by the Davis
experiment and are produced by a side reaction that produces
beryllium-7. Th e number of neutrinos detected matches the
prediction of models of nuclear fusion in the sun.
Th e center of the sun seems forever beyond human experi-
ence, but counting solar neutrinos provides evidence to confi rm
the theories. Th e sun makes its energy through nuclear fusion.
SCIENTIFIC ARGUMENT
Why does nuclear fusion require that the gas be very hot?
This argument has to include the basic physics of atoms and ther-
mal energy. Inside a star, the gas is so hot it is ionized, which
means the electrons have been stripped off the atoms leaving bare,
positively charged nuclei. In the case of hydrogen, the nuclei are
single protons. These atomic nuclei repel each other because of
their positive charges, so they must collide with each other at highvelocity if they are to overcome that repulsion and get close enough
together to fuse. If the atoms in a gas are moving rapidly, then
the gas must have a high temperature, so nuclear fusion requires
that the gas be very hot. If the gas is cooler than about 4 million
K, hydrogen can’t fuse because the protons don’t collide violently
enough to overcome the repulsion of their positive charges.
It is easy to see why nuclear fusion in the sun requires high
temperature, but now expand your argument. Why does it require
high density?Solar Activity
The sun is not quiet. It is home to slowly changing spots larger
than Earth and vast eruptions that dwarf human imagination.
All of these seemingly diff erent forms of solar activity have one
thing in common—magnetic fi elds. Th e weather on the sun is
magnetic.Observing the Sun
Solar activity is often visible with even a small telescope, but you
should be very careful if you try to observe the sun. Sunlight is
intense, and when it enters your eye it is absorbed and converted
into thermal energy. Th e infrared radiation in sunlight is espe-
cially dangerous because your eyes can’t detect it. You don’t sense
how intense the infrared is, but it is converted to thermal energy
in your eyes and can burn and scar your retinas.8-3
ab■ Figure 8-11
(a) The Davis solar neutrino exper-
iment used cleaning fl uid and
could detect only one of the three
fl avors of neutrinos. (Brookhaven
National Laboratory) (b) The Sudbury
Neutrino Observatory is a 12-me-
ter-diameter globe containing
water rich in deuterium (heavy
hydrogen) in place of ordinary
hydrogen. Buried 2100 m (6800
ft) deep in an Ontario mine, it can
detect all three fl avors of neutri-
nos and confi rms that neutrinos
oscillate. (Photo courtesy of SNO)