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Chapter 17 - The Sun
CHAPTER 17
THE SUN
CHAPTER OUTLINE AND LECTURE NOTES
INTRODUCTION
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Regulus, the painting that opens this chapter is a remarkable example of J. W. M. Turner’s ability to represent light.
It is almost difficult to stare at the position of sunrise in the painting. Regulus was a Roman general who was
captured by the Carthaginians. When Carthage sent Regulus back to Rome with an offer of peace he promised to
return to Carthage if the Romans refused peace. Regulus then urged the Roman Senate not to make peace and they
didn’t. He returned to Carthage where he was executed. There is a story that he was tortured by removing his
eyelids and fixing his head so that he had to stare at the rising Sun. Turner painted the scene as it would have been
viewed by Regulus.
The Internal Structure of the Sun
The study of solar oscillations has made a tremendous difference in our knowledge of the internal structure of the
Sun. One of the Sun’s oscillation modes is shown in Figure 17.14.
The Sun’s Energy
The reactions in the most common form of the proton-proton chain are shown in an animation on the CD that
accompanies this textbook. There are several other, less important ways that helium 3 can be, ultimately, converted
to helium 4. One of these alternatives is the one that produces the high energy neutrinos detected by the Homestake
and Kamiokande neutrino telescopes. The discovery of neutrino oscillations and the solar neutrino measurements at
the Sudbury Neutrino Observatory seem to finally have resolved the solar neutrino problem.
The Outer Layers of the Sun
Videos of solar prominences are included on the CD that accompanies this textbook. The spectacular results
from the SOHO spacecraft can be viewed at http://sohowww.nascom.nasa.gov/
There are indications that the Voyager I spacecraft, now 90 AU from the Sun, has entered the disturbed region
just inside the heliopause. The heliopause may already have been detected remotely. My former colleagues Don
Gurnett and Bill Kurth have low frequency radio wave experiments on the Voyagers. They have detected radio
radiation that may originate at the heliopause.
The Sunspot Cycle
The polarity of sunspots during a complete 22-year cycle is shown in Figure 17.36.
Solar Mysteries
From time to time, as in this short section, I like to remind my students that astronomers (like other scientists) don’t
have all the answers. Of course, if we did there wouldn’t be any point in having astronomers.
KEY TERMS
active region — A region of the Sun’s surface layers that has a large magnetic field and in which
sunspots, flares, and prominences preferentially occur.
chromosphere — The part of the Sun’s atmosphere between the photosphere and the corona.
convection zone — The outer part of the Sun’s interior in which convection occurs.
coronal hole — A low density, dim region in the Sun’s corona. Coronal holes occur in regions of
open magnetic field lines where gases can flow freely away from the Sun to form the solar
wind.
coronal mass ejection — A blast of gas moving outward through the Sun’s corona and into
interplanetary space following the eruption of a prominence.
deuterium — An isotope of hydrogen. The nucleus of a deuterium atom is a deuteron.
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Chapter 17 - The Sun
differential rotation — Rotation in which the rotation period of a body varies with latitude.
Differential rotation occurs for gaseous bodies like the Sun or for planets with thick
atmospheres.
filament — A dark line on the Sun’s surface when a prominence is seen projected against the solar
disk.
granule — A bright convective cell or current of gas in the Sun’s photosphere. Granules appear
bright because they are hotter than the descending gas that separates them.
heliopause — The boundary of the heliosphere, where the solar wind merges into the interstellar
gas.
helioseismology — A technique used to study the internal structure of the Sun by measuring and
analyzing oscillations of the Sun’s surface layers.
heliosphere — The region of space dominated by the solar wind and the Sun’s magnetic field.
Kelvin-Helmholtz time — The time it would take a star to contract from infinite diameter down to
the main sequence while radiating away the gravitational energy released during contraction.
limb – The apparent edge of the disk of an astronomical body.
limb darkening — The relative faintness of the Sun’s edge of the Sun’s disk (limb) compared with
the center of the Sun’s disk.
Maunder minimum — A period of few sunspots and low solar activity that occurred between 1740
and 1710.
mode of oscillation — A particular pattern of vibration of the Sun.
neutrino — A particle with no charge and little mass that is produced in nuclear reactions.
Neutrinos pass freely through matter and travel nearly at the speed of light.
opacity — The ability of a substance to absorb radiation. The higher the opacity, the less
transparent it is.
penumbra — The outer part of a sunspot. The penumbra consists of dark radial features.
photosphere — The visible region of the atmosphere of the Sun or another star.
polarity — The property of a magnet that causes it to have north and south magnetic regions.
prominence — A region of cool gas embedded in the corona. Prominences are bright when seen
above the Sun’s limb, but appear as dark filaments when seen above the Sun’s disk.
proton-proton chain — A series of nuclear reactions through which stars like the Sun produce
energy by converting hydrogen to helium. Named because the first reaction in the series is the
reaction of one proton with another.
solar flare — An explosive release of solar magnetic energy.
spicule — A hot jet of gas moving outward through the Sun’s chromosphere.
sunspot — A region of the Sun’s photosphere that appears darker than its surroundings because it is
cooler.
sunspot cycle — The regular waxing and waning of the number of spots on the Sun. The amount of
time between one sunspot maximum and the next is about 11 years.
sunspot group — A cluster of sunspots.
supergranulation — The pattern of very large (15,000 to 30,000 km in diameter) convective cells in
the Sun’s photosphere.
termination shock – The point where the solar wind slows significantly as it interacts with the
interstellar gas. The termination shock is located on a surface whose distance from the Sun
varies with direction and solar wind speed
umbra — The dark central portion of a sunspot.
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Zeeman effect — The splitting of a spectral line into two or more components when the atoms or
molecules emitting the line are located in a magnetic field.
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Chapter 17 - The Sun
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
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It would have no effect at all because the rate of energy production is limited by the rate at which protons react with
protons, which is much slower than the rate at which deuterons react with protons.
Most of the Sun’s hydrogen is too far from the core and too cool to participate in fusion reactions.
Photons in the Sun travel such short distances that many absorptions and reemissions must occur before radiative
diffusion can carry the Sun’s energy to the surface.
It is at that distance from the center that opacity becomes large enough for convection to occur.
Not noticeable at all because the one week deficit would be spread over more than 100,000 years
It is underground to shield the detector from spurious detections due to cosmic rays. It can operate around the
clock because neutrinos can pass freely through the Earth.
They turn into other kinds of neutrinos.
Only the inner 1.5% of the Sun’s mass is hot enough for significant fusion of hydrogen to occur.
It would make the Sun’s limb much fuzzier than it is.
That is the average temperature of the gases we see when we view the Sun’s disk.
It would tell us that temperature increases with height in the photosphere.
Bright granules are the hot tops of rising convection cells. The dark spaces between granules are cooler,
descending gas. The same pattern occurs on a larger scale for supergranules.
They are cooler than the rest of the Sun’s surface.
The magnetic fields in sunspots are thought to inhibit convection and the outward flow of energy in the sunspots.
A spicule is a rapidly rising jet of gas in the chromosphere.
The gases in the corona are highly ionized.
A small rate of energy input is sufficient to keep the corona hot because its gases are very inefficient at radiating
away energy to cool the corona.
Coronal holes do not occur over active regions.
Gas in coronal holes streams away from the Sun to produce the solar wind.
Quiescent prominences are located away from active regions while flares and active prominences are located above
or in active regions. Flares are more energetic than prominences.
Because the magnetic field lines in the solar wind are attached to the Sun, which rotates and stretches the field lines
into a spiral pattern.
The heliopause is thought to be nearer in the direction toward which the Sun is moving because the heliosphere is
compressed by the interstellar gas through which the heliosphere moves.
The Maunder minimum, a period within which few sunspots were seen, coincided with a time of cold climate.
It tells us about the numbers and latitudes of sunspots throughout the sunspot cycle.
The cycle begins with a few spots at about 40° N and S latitudes. The leading spots in groups in the northern and
southern hemisphere have different magnetic polarity. Over the next 11 years the average latitudes of spots shifts
toward the equator. After about 11 years new spots begin to appear at 40° N and S latitude. This time, however,
the polarity of the spots in the two hemispheres is different than it was during the first 11 years of the cycle.
Differential rotation twists the solar magnetic field lines, increasing the strength of the solar magnetic field. When
the twisted ropes of field lines get strong enough, they emerge through the solar surface to produce spot groups.
Problems
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1.2 × 1021 W, absolute magnitude = 18.6
About 4.3 hours
35,000 years
Figure-based Questions
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