Chapter 15--Our Sun - Geological Sciences

large decrease in
rate of fusion
slight decrease in
core temperature
slight rise in
core temperature
large rise in
rate of fusion
Solar
Thermostat:
Gravitational
Equilibrium
Because the energy
supply is diminished,
gravity starts to
overcome thermal
pressure.
Increased energy
output enables
thermal pressure
to overcome
gravity.
Gravity compresses
the core, heats it up,
and restores fusion
rate to normal value.
Increased thermal
pressure causes the
core to expand and then
cool, which restores fusion
rate to normal value.
Figure 15.8 The solar thermostat. Gravitational equilibrium regulates the Sun’s core temperature. Everything
is in balance if the amount of energy leaving the core equals the amount of energy produced by
fusion. A rise in core temperature triggers a chain of events that causes the core to expand, lowering its
temperature to its normal value. A decrease in core temperature triggers the opposite chain of events,
also restoring the normal core temperature.
Sun, observations of “sun quakes,” and observations of
solar neutrinos.
Mathematical Models The primary way we learn about
the interior of the Sun and other stars is by creating mathematical
models that use the laws of physics to predict the
internal conditions. A basic model uses the Sun’s observed
composition and mass as inputs to equations that describe
gravitational equilibrium, the solar thermostat, and the
rate at which solar energy moves from the core to the photosphere.
With the aid of a computer, we can use the model
to calculate the Sun’s temperature, pressure, and density
at any depth. We can then predict the rate of nuclear fusion
in the solar core by combining these calculations with
knowledge about nuclear fusion gathered in laboratories
here on Earth.
Remarkably, such models correctly “predict” the radius,
surface temperature, luminosity, age, and many other
properties of the Sun. However, current models do not
predict everything about the Sun correctly. Scientists are
constantly working to discover what is missing from them.
Successful prediction of so many observed characteristics
of the Sun gives us confidence that the models are on the
right track and that we really do understand what is going
on inside the Sun.
Sun Quakes A second way to learn about the inside of
the Sun is to observe “sun quakes”—vibrations of the Sun
that are similar to the vibrations of the Earth caused by
earthquakes, although they are generated very differently.
Earthquakes occur when Earth’s crust suddenly shifts, generating
seismic waves that propagate through Earth’s interior
[Section 10.2].We can learn about Earth’s interior by recording
seismic waves on Earth’s surface with seismographs.
Sun quakes result from waves of pressure (sound waves)
that propagate deep within the Sun at all times. These
waves cause the solar surface to vibrate when they reach
it. Although we cannot set up seismographs on the Sun,
we can detect the vibrations of the surface by measuring
Doppler shifts [Section 6.5]. Light from portions of the surface
that are rising toward us is slightly blueshifted, while
light from portions that are falling away from us is slightly
redshifted. The vibrations are relatively small but measurable
(Figure 15.9).
504 part V • Stellar Alchemy