The Sun Our Star Guidepost Outline Outline (continued

1
The Sun Our Star
2
Guidepost
The preceding programs described how we can get
information from a spectrum. In this program, we
apply these techniques to the sun, to learn about
its complexities. This program 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 program gives us our first detailed look at
a star. The programs that follow will discuss the
many kinds of stars that fill the heavens, but
this program shows us that each of them is both
complex and beautiful each is a sun.
3
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
4
Outline (continued)
III. Nuclear Fusion in the Sun A. Nuclear
Binding Energy B. Hydrogen Fusion C. The Solar
Neutrino Problem
5
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 Earths diameter

• 333,000 times Earths mass

• Consists entirely of gas (av. density 1.4
  g/cm3)

• Central temperature 15 million 0K

• Surface temperature 5800 0K

6

• THE SUN
• Distance from Earth 1.00 AU 1.496 X 108 km
• Maximum distance from Earth 1.0167 AU
  1.521 X 108 km
• Minimum distance from Earth 0.9833 AU 1.471 X
  108 km
• Average Angular Diameter
• from Earth 0.53 o 32 minutes of arc)
• Period of rotation 25.38 days at equator
• Radius R 6.96 X 105 km km
• 
  109 Rearth
• Mass 1.99 X 1030 kg
• 
  333,000 Mearth
• Average Density 1.409 g/cm3
• Surface Gravity 2.54 gearth
• Escape Velocity 617.7 km/sec 55.2 V earth

7
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.
8
The Solar Atmosphere
Heat Flow
Temp. incr. inward
Solar interior
9
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
10
Energy Transport in the Photosphere
Energy generated in the suns center must be
transported outward.
In the photosphere, this happens through
Bubbles last for 10 20 min.
Convection
Cool gas sinking down
Bubbles of hot gas rising up
1000 km
11
Granulation
is the visible consequence of convection
12
The Chromosphere

• Region of suns 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)
13
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.
14
The Layers of the Solar Atmosphere
Ultraviolet
Visible
Sun Spot Regions
Photosphere
Corona
Chromosphere
Coronal activity, seen in visible light
15
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
16
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)

17
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!
18
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!
19
Sun Spots (2)
Active Regions
Visible
Ultraviolet
20
Face of the Sun
Solar Activity, seen in soft X-rays
21
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
22
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.
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Magnetic Field Lines
Magnetic North Pole
Magnetic South Pole
Magnetic Field Lines
24
Star Spots?
Image constructed from changing Doppler shift
measurements
Other stars might also have sun spot activity
25
The Solar Cycle
After 11 years, North/South order of
leading/trailing sun spots is reversed
11-year cycle
gt Total solar cycle 22 years
Reversal of magnetic polarity
26
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.
27
The Suns 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.
28
Magnetic Loops
Magnetic field lines
29
The Suns 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
30
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
31
Magnetic Cycles on Other Stars
H and K line emission of ionized Calcium indicate
magnetic activity also on other stars.
32
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 suns
photosphere, flowing along magnetic loops
33
Eruptive Prominences
(Ultraviolet images)
Extreme events (solar flares) can significantly
influence Earths magnetic field structure and
cause northern lights (aurora borealis).
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36
Space Weather
5 minutes
Solar Aurora
Sound waves produced by a solar flare
Coronal mass ejections
37
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.
38
Energy Production
Energy generation in the sun (and all other
stars)
Binding energy due to strong force on short
range, strongest of the 4 known forces
electromagnetic, weak, strong, gravitational
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.
39
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).
T 107 0K 10 million 0K
4 protons have 0.04810-27 kg ( 0.7 ) more mass
than 4He.

• Energy gain Dmc2
• 0.4310-11 J
• per reaction.

Sun needs 1038 reactions, transforming 5 million
tons of mass into energy every second, to resist
its own gravity.
40
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
41
New Terms
sunspot granulation convection supergranule limb l
imb darkening transition region filtergram filamen
t spicule coronagraph magnetic carpet solar
wind helioseismology active region Zeeman
effect Maunder butterfly diagram differential
rotation
dynamo effect Babcock model prominence flare recon
nection aurora coronal hole coronal mass ejection
(CME) solar constant Maunder minimum weak
force strong force nuclear fission nuclear
fusion Coulomb barrier protonproton
chain deuterium neutrino
42
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.
43
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.
44
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.
45
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.
46
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.
47
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.
48
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.
49
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.
50
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.
51
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.
52
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.
53
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.
54
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.
55
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.
56
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.
57
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.
58
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.
59
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.
60
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.
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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.
62
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.