Chapter 9
The Sun
Units of Chapter 9
The Sun in Bulk
The Solar Interior
The Solar Atmosphere
The Active Sun
The Heart of the Sun
Summary of Chapter 9
9.1 The Sun in Bulk
Interior structure
of the Sun
Outer layers are
not to scale.
The core is where
nuclear fusion
takes place.
Luminosity – total energy radiated by the Sun –
can be calculated from the fraction of that
energy that reaches Earth.
Total luminosity is
about 4 × 1026 W –
the equivalent of
10 billion 1megaton nuclear
bombs per
second.
9.2 The Solar Interior
Mathematical models, consistent with
observation and physical principles, provide
information about the Sun’s interior.
In equilibrium,
inward gravitational
force must be
balanced by
outward pressure.
Doppler shifts of solar spectral lines indicate a
complex pattern of vibrations.
Solar density and
temperature, according
to the standard solar
model.
Energy transport:
The radiation zone is relatively transparent; the
cooler convection zone is opaque.
The visible
top layer of
the
convection
zone is
granulated,
with areas of
upwelling
material
surrounded
by areas of
sinking
material.
9.3 The Solar Atmosphere
Spectral analysis can tell us what elements are
present, but only in the chromosphere and
photosphere.
The cooler chromosphere is above the
photosphere.
Difficult to see directly, as
photosphere is too bright,
unless Moon covers
photosphere and not
chromosphere during
eclipse
Small solar storms in chromosphere emit spicules.
Solar corona
can be seen
during eclipse
if both
photosphere
and
chromosphere
are blocked.
Corona is much hotter than layers below it – must
have a heat source, probably electromagnetic
interactions.
9.4 The Active Sun
Sunspots appear dark
because slightly cooler
than surroundings.
Sunspots
come and
go, typically
in a few
days.
Sunspots
are linked
by pairs of
magnetic
field lines.
The rotation of the Sun drags magnetic field
lines around with it, causing kinks.
The Sun has an 11-year
sunspot cycle, during
which sunspot
numbers rise, fall,
and then rise again.
This is really a 22-year cycle, because the spots
switch polarities between the northern and
southern hemispheres every 11 years.
Maunder minimum: few, if any, sunspots.
Areas around sunspots are active; large
eruptions may occur in photosphere.
Solar prominence is large sheet of ejected gas.
Solar flare is a
large explosion
on Sun’s
surface,
emitting a
similar amount
of energy to a
prominence, but
in seconds or
minutes rather
than days or
weeks.
A coronal mass ejection
emits charged particles
that can affect the Earth.
Solar wind escapes
Sun mostly through
coronal holes, which
can be seen in X-ray
images.
Solar corona
changes along
with sunspot
cycle; is much
larger and more
irregular at
sunspot peak.
9.5 The Heart of the Sun
Nuclear fusion requires
that like-charged nuclei
get close enough to
each other to fuse.
This can happen only
if the temperature is
extremely high – over
10 million K.
The process that powers most stars is a threestep fusion process.
Neutrinos are emitted directly from the core of
the Sun, and escape, interacting with virtually
nothing. Being able to observe these neutrinos
would give us a direct picture of what is
happening in the core.
Unfortunately, they are no more likely to interact
with Earth-based detectors than they are with the
Sun; the only way to spot them is to have a huge
detector volume and to be able to observe single
interaction events.
Neutrino observatories
The corona of the sun is only rarely
visible because
A. it is only visible during violent storms
in the sun.
B. it cannot be seen through the Earth's
atmosphere.
C. it is very faint compared to the sun's
surface.
D. it is too hot.
The 11 year solar cycle refers to
A. the rate of occupance of magnetic
storms on the surface of the sun.
B. the period of rotation of the sun.
C. the apparent motion of the sun
across the sky.
D. the nuclear reactions which occur in
the center of the sun.
For what reason do astronomers want
to observe and measure neutrinos from
the sun?
A. Neutrinos are more energetic than
photons from the sun.
B. Neutrinos are easier to detect than
photons.
C. Neutrinos give direct information about
thephotosphere.
D. Neutrinos give direct information about
the sun's core.
E. None of the above.
Two important characteristics of
sunspots are
A. low temperature, large magnetic
fields.
B. high temperature, large magnetic
fields.
C. high temperature, weak magnetic
fields.
D. low temperature, weak magnetic
fields.
E. north and south magnetic
polarities.
Which of these regions of the
sun has the highest
temperature?
A.
B.
C.
D.
Core
Photosphere
Chromosphere
Corona
Sunspots are caused by
A. interaction of the sun with the
planets.
B. irregularities in the sun's
magnetic field.
C. internal faults in the sun similar
to those which cause
Earthquakes.
D. no choice.
Differential rotation in the sun
makes the
A. equator move faster than higher
latitudes.
B. the higher latitudes move faster
than the equator.
C. the magnetic field rotate faster than
the surface.
D. none of these.
Combining two nuclei together
requires high temperatures to
A. remove the electrons which
surround the nuclei.
B. overcome the electric repulsion of
the nuclei.
C. make sure the nuclei thoroughly
mix when they collide.
D. force the nuclei to expand.
Which of the following statements
about the energy generating
process in our sun is true?
A. It can be duplicated efficiently on
Earth with our present technology.
B. Mass is converted to energy.
C. Hydrogen becomes helium in one
step.
D. We have detected many more
neutrinos from the process than we
expected to find.
Which of the following is true in the
proton-proton chain?
A. Protons are fused to produce
deuterium or heavy hydrogen.
B. Protons are split to form neutrons
and positrons.
C. Protons produce helium directly.
D. Protons decay into energy and
neutrinos.
The main nuclear reactions that
keep our sun shining begin with
which building blocks?
A.
B.
C.
D.
Three carbon nuclei.
Two electrons.
Two hydrogen nuclei.
A deuteron and a positron.
The heaviest element that can be
created in normal nucleosynthesis
is
A. iron.
B. helium.
C. uranium.
D. carbon.
E. bagelium.
Summary of Chapter 9
• The Sun is held together by its own gravity
and powered by nuclear fusion.
• Outer layers of the Sun: photosphere,
chromosphere, corona. The corona is very hot.
• Mathematical models and helioseismology
give us a picture of the interior of the Sun.
• Sunspots occur in regions of high magnetic
fields; darker spots are cooler.
Summary of Chapter 9, cont.
• Nuclear fusion converts hydrogen to helium,
releasing energy.
• Solar neutrinos come directly from the solar
core, although observations have told us more
about neutrinos than about the Sun.