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Chapter 8
The Sun – Our Star
Guidepost
The preceding chapter described how we can get
information from a spectrum. In this chapter, we apply
these techniques to the sun, to learn about its
complexities.
This chapter gives us our first close look at how
scientists work, how they use evidence and hypothesis
to understand nature. Here we will follow carefully
developed logical arguments to understand our sun.
Most important, this chapter gives us our first detailed
look at a star. The chapters that follow will discuss the
many kinds of stars that fill the heavens, but this chapter
shows us that each of them is both complex and
beautiful; each is a sun.
Outline
I. The Solar Atmosphere
A. Heat Flow in the Sun
B. The Photosphere
C. The Chromosphere
D. The Solar Corona
E. Helioseismology
II. Solar Activity
A. Sunspots and Active Regions
B. The Sunspot Cycle
C. The Sun's Magnetic Cycle
D. Magnetic Cycles on Other Stars
E. Chromospheric and Coronal Activity
F. The Solar Constant
Outline (continued)
III. Nuclear Fusion in the Sun
A. Nuclear Binding Energy
B. Hydrogen Fusion
C. The Solar Neutrino Problem
General Properties
• Average star
• Spectral type G2
• Only appears so bright because it is so close.
• Absolute visual magnitude = 4.83 (magnitude if it
were at a distance of 32.6 light years)
• 109 times Earth’s diameter
• 333,000 times Earth’s mass
• Consists entirely of gas (av. density = 1.4 g/cm3)
• Central temperature = 15 million 0K
• Surface temperature = 5800 0K
Very Important Warning:
Never look directly
at the sun through
a telescope or
binoculars!!!
This can cause permanent eye
damage – even blindness.
Use a projection technique or a special
sun viewing filter.
The Solar Atmosphere
Apparent surface
of the sun
Heat Flow
Only visible
during solar
eclipses
Solar interior
Temp.
incr.
inward
The Photosphere
• Apparent surface layer of the sun
• Depth ≈ 500 km
• Temperature ≈ 5800 oK
• Highly opaque (H- ions)
• Absorbs and re-emits radiation produced in the solar interior
The solar corona
Energy Transport in the
Photosphere
Energy generated in the sun’s center must be transported outward.
In the photosphere, this happens through
Convection:
Cool gas
sinking down
Bubbles of hot
gas rising up
≈ 1000 km
Bubbles last for
≈ 10 – 20 min.
Granulation
… is the visible consequence of convection
The Chromosphere
• Region of sun’s atmosphere
just above the photosphere.
• Visible, UV, and X-ray
lines from highly ionized
gases
• Temperature increases
gradually from ≈ 4500 oK to ≈
10,000 oK, then jumps to ≈ 1
million oK
Filaments
Transition region
Chromospheric
structures visible in Ha
emission (filtergram)
The Chromosphere (2)
Spicules: Filaments
of cooler gas from
the photosphere,
rising up into the
chromosphere.
Visible in Ha
emission.
Each one lasting
about 5 – 15 min.
The Layers of the Solar
Atmosphere
Visible
Sun Spot
Regions
Ultraviolet
Photosphere
Corona
Chromosphere
Coronal activity,
seen in visible
light
The Magnetic Carpet of the Corona
• Corona contains very low-density, very hot (1 million oK) gas
• Coronal gas is heated through motions of magnetic
fields anchored in the photosphere below (“magnetic
carpet”)
Computer
model of
the
magnetic
carpet
The Solar Wind
Constant flow of particles from the sun.
Velocity ≈ 300 – 800 km/s
 Sun is constantly losing mass:
107 tons/year
(≈ 10-14 of its mass per year)
Helioseismology
The solar interior is opaque
(i.e. it absorbs light) out to
the photosphere.
 Only way to investigate
solar interior is through
Helioseismology
= analysis of vibration
patterns visible on the
solar surface:
Approx. 10 million
wave patterns!
Sun Spots
Cooler regions of the
photosphere (T ≈ 4240 K).
Only appear dark against the
bright sun. Would still be
brighter than the full moon when
placed on the night sky!
Sun Spots (2)
Active Regions
Visible
Ultraviolet
Face of the Sun
Solar Activity, seen in soft X-rays
Magnetic Fields in Sun Spots
Magnetic fields on the photosphere can be measured
through the Zeeman effect
 Sun Spots are related to magnetic
activity on the photosphere
Sun Spots (3)
Magnetic field in sun spots is about 1000 times
stronger than average.
Magnetic North Poles
Magnetic
South
Poles
In sun spots, magnetic field lines emerge out of the
photosphere.
Magnetic Field Lines
Magnetic
North
Pole
Magnetic
South
Pole
Magnetic
Field
Lines
Star Spots?
Image
constructed
from changing
Doppler shift
measurements
Other stars might also have sun spot activity:
The Solar Cycle
After 11 years, North/South
order of leading/trailing sun
spots is reversed
11-year cycle
Reversal of magnetic
polarity
=> Total solar cycle
= 22 years
The Solar Cycle (2)
Maunder Butterfly Diagram
Sun spot cycle starts out with spots at higher
latitudes on the sun
Evolve to lower latitudes (towards the
equator) throughout the cycle.
The Sun’s Magnetic Dynamo
The sun rotates faster at the equator than
near the poles.
This differential rotation might be responsible for
magnetic activity of the sun.
Magnetic Loops
Magnetic field lines
The Sun’s Magnetic Cycle
After 11 years, the magnetic
field pattern becomes so
complex that the field
structure is re-arranged.
 New magnetic field
structure is similar to the
original one, but reversed!
 New 11-year cycle starts
with reversed magnetic-field
orientation
The Maunder Minimum
The sun spot number also fluctuates
on much longer time scales:
Historical data indicate a very quiet phase of the
sun, ~ 1650 – 1700: The Maunder Minimum
Magnetic Cycles on Other Stars
H and K line
emission of
ionized Calcium
indicate magnetic
activity also on
other stars.
Prominences
Relatively cool gas
(60,000 – 80,000 oK)
May be seen as dark
filaments against the
bright background of
the photosphere
Looped Prominences: gas ejected from the sun’s
photosphere, flowing along magnetic loops
Eruptive Prominences
(Ultraviolet
images)
Extreme events (solar
flares) can significantly
influence Earth’s
magnetic field structure
and cause northern lights
(aurora borealis).
~ 5 minutes
Space Weather
Solar Aurora
Sound
waves
produced
by a
solar
flare
Coronal mass ejections
Coronal Holes
X-ray images of
the sun reveal
coronal holes.
These arise at
the foot points of
open field lines
and are the
origin of the
solar wind.
Energy Production
Energy generation in the sun
(and all other stars):
Nuclear Fusion
= fusing together 2 or more
lighter nuclei to produce
heavier ones.
Nuclear fusion can
produce energy up to
the production of iron;
For elements heavier than
iron, energy is gained by
nuclear fission.
Binding energy
due to strong
force = on short
range, strongest
of the 4 known
forces:
electromagnetic,
weak, strong,
gravitational
Energy Generation in the Sun: The
Proton-Proton Chain
Basic reaction:
4 1H  4He + energy
Need large proton speed ( high
temperature) to overcome
Coulomb barrier (electromagnetic
repulsion between protons).
4 protons have
0.048*10-27 kg (= 0.7 %)
more mass than 4He.
 Energy gain = Dm*c2
= 0.43*10-11 J
per reaction.
Sun needs 1038 reactions, transforming 5 million
tons of mass into energy every second, to resist
its own gravity.
T ≥ 107 0K =
10 million 0K
The Solar Neutrino Problem
The solar interior can not be
observed directly because it
is highly opaque to radiation.
But neutrinos can penetrate
huge amounts of material
without being absorbed.
Early solar neutrino
experiments detected a much
lower flux of neutrinos than
expected ( the “solar
neutrino problem”).
Recent results have proven that
neutrinos change (“oscillate”)
between different types (“flavors”),
thus solving the solar neutrino
problem.
Davis solar neutrino
experiment
New Terms
sunspot
granulation
convection
supergranule
limb
limb darkening
transition region
filtergram
filament
spicule
coronagraph
magnetic carpet
solar wind
helioseismology
active region
Zeeman effect
Maunder butterfly
diagram
differential rotation
dynamo effect
Babcock model
prominence
flare
reconnection
aurora
coronal hole
coronal mass ejection
(CME)
solar constant
Maunder minimum
weak force
strong force
nuclear fission
nuclear fusion
Coulomb barrier
proton–proton chain
deuterium
neutrino
Discussion Questions
1. What energy sources on Earth cannot be thought of
as stored sunlight?
2. What would the spectrum of an auroral display look
like? Why?
3. What observations would you make if you were
ordered to set up a system that could warn astronauts
in orbit of dangerous solar flares? Such a warning
system exists.
Quiz Questions
1. What effect does the formation of negative hydrogen ions in
the Sun's photosphere have on solar observations?
a. We can view the Sun's interior through special filters set to
the wavelength of the absorption lines created by such ions.
b. Concentrations of such ions form sunspots that allow us to
track solar rotation.
c. It divides the Sun's atmosphere into three distinct, easily
observable layers.
d. The extra electron absorbs different wavelength photons,
making the photosphere opaque.
e. These ions produce the "diamond ring" effect that is seen
during total solar eclipses.
Quiz Questions
2. What evidence do we have that the granulation seen on the
Sun's surface is caused by convection?
a. The bright centers of granules are cooler than their dark
boundaries.
b. The bright centers of granules are hotter than their dark
boundaries.
c. Doppler measurements indicate that the centers are rising
and edges are sinking.
d. Both a and c above.
e. Both b and c above.
Quiz Questions
3. Which layer of the Sun's atmosphere contains the cooler low
density gas responsible for absorption lines in the Sun's
spectrum?
a. The photosphere.
b. The chromosphere.
c. The corona.
d. The solar wind.
e. All of the above.
Quiz Questions
4. Which of the following is true about granules and
supergranules?
a. They are both about the same size.
b. Granules and supergranules each fade in about 10 to 20
minutes.
c. They are both due to convection cells in layers below.
d. Both a and c above.
e. Both b and c above.
Quiz Questions
5. What is revealed by observing the Sun at a very narrow
range of wavelengths within the 656-nanometer hydrogen
alpha line?
a. The structure of the photosphere.
b. The structure of the chromosphere.
c. The structure of the corona.
d. We can see the electrons make the transition from energy
level 3 to level 2.
e. Nothing is seen; all light is absorbed at this wavelength.
Quiz Questions
6. What are the general trends in temperature and density from
the photosphere to the chromosphere to the corona?
a. The temperature increases and density decreases.
b. The temperature increases and density increases.
c. The temperature decreases and density decreases.
d. The temperature decreases and density increases.
e. The temperature and density remain constant.
Quiz Questions
7. What physical property of the Sun is responsible for "limb
darkening"?
a. The chromosphere is hotter than the photosphere.
b. The chromosphere is cooler than the photosphere.
c. The lower photosphere is cooler than the upper
photosphere.
d. The lower photosphere is hotter than the upper photosphere.
e. Both a and d above.
Quiz Questions
8. The spectrum of the corona has bright spectral lines of highly
ionized elements. What does this reveal?
a. The corona is a very hot, high density gas.
b. The corona is a very hot, low density gas.
c. The corona is very irregular in shape.
d. The corona extends out to 20 solar radii.
e. Both b and d above.
Quiz Questions
9. What heats the chromosphere and corona to high
temperatures?
a. Long-wavelength electromagnetic radiation emitted by layers
below.
b. Visible light emitted by layers below.
c. Short-wavelength electromagnetic radiation emitted by
layers below.
d. Sungrazing comets, giving up their energy of motion as they
vaporize in these two layers.
e. Fluctuating magnetic fields from below that transport energy
outward.
Quiz Questions
10. How are astronomers able to explore the layers of the Sun
below the photosphere?
a. Short-wavelength radar pulses penetrate the photosphere
and rebound from deeper layers within the Sun.
b. Long-wavelength radar pulses penetrate the photosphere
and rebound from deeper layers within the Sun.
c. Highly reflective space probes have plunged below the
photosphere and sampled the Sun's interior.
d. By measuring and modeling the modes of vibration of the
Sun's surface.
e. By observing solar X-rays and gamma rays with space
telescopes. These shorter wavelengths are emitted from hotter
regions below the photosphere.
Quiz Questions
11. What is responsible for the Sun's surface and atmospheric
activity?
a. The Sun's magnetic field.
b. Many comets impacting the Sun.
c. Gravitational contraction of the Sun.
d. The Sun sweeping up interstellar space debris.
e. Gravitational interactions between the Sun and the planets.
Quiz Questions
12. What is the source of the Sun's changing magnetic field?
a. The differential rotation of the Sun.
b. Convection beneath the photosphere.
c. The Sun's large iron core.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
13. What evidence do we have that sunspots are magnetic?
a. The spectral lines of sunspots are split by the Zeeman
Effect.
b. Observations show that the north pole and south pole
sunspots attract one another and move closer together over
time.
c. Observations at far ultraviolet show material arched above
the Sun's surface from one sunspot to another.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
14. Which active feature in the Sun's atmosphere, seen from a
different point of view, corresponds precisely to the dark
filaments that are observed with an hydrogen alpha filter?
a. A sunspot.
b. A solar flare.
c. A prominence.
d. A spicule.
e. A coronal hole.
Quiz Questions
15. What does a Maunder Butterfly Diagram show?
a. During the 11-year sunspot cycle, the spots begin at high
latitude and then form progressively closer to the equator.
b. Between the years 1645 and 1715 the low activity on the
Sun correlates with the Little Ice Age.
c. The Sun's magnetic field is simple at the beginning of a
sunspot cycle and grows progressively more complex due to
differential rotation.
d. Planetary nebulae do not all have spherical symmetry.
e. When a butterfly flaps its wings in Brazil it affects the climate
worldwide.
Quiz Questions
16. How constant is the solar constant; that is, by how much
has the solar constant of 1360 joules per square meter per
second been observed to vary over a few years?
a. About 20%.
b. About 10%.
c. About 5%.
d. About 1%.
e. About 0.1%.
Quiz Questions
17. How does the Sun maintain its energy output?
a. Gravitational contraction.
b. Fusion of hydrogen nuclei.
c. The impact of small meteoroids.
d. Coal burning in pure oxygen.
e. Fission of Uranium 235.
Quiz Questions
18. Why does nuclear fusion require high temperatures?
a. Protons have positive charge, and like charges repel one
another.
b. Two protons must get close enough together to overcome
the Coulomb barrier.
c. Two protons must get close enough for the strong force to
bind them together.
d. Both a and b above.
e. All of the above.
Quiz Questions
19. What happens to the neutrinos that are produced in the
proton-proton chain?
a. They collide immediately with other particles, thus adding to
the gas pressure that supports the Sun against gravitational
contraction.
b. They combine with antineutrinos and form a pair of gamma
rays.
c. They head out of the Sun at nearly the speed of light.
d. They are blocked by the Coulomb barrier and remain inside
the Sun.
e. They spiral out along magnetic field lines to become cosmic
rays.
Quiz Questions
20. What solved the solar neutrino problem?
a. The discovery that neutrinos oscillate between three different
types.
b. The standard model of energy production within the Sun was
modified.
c. It was discovered that electron neutrinos do not penetrate
rock as easily as expected.
d. Some of the radioactive argon gas was found leaking out of
the neutrino detector undetected.
e. The finding that chlorine does not interact with electron
neutrinos as predicted.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
d
e
a
c
b
a
d
b
e
d
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
a
d
e
c
a
e
b
e
c
a