The Sun
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Guiding Questions
1. Does the Sun have a solid surface?
2. Since the Sun is so bright, how is it
possible to see its dim outer
atmosphere?
3. Where does the solar wind come from?
4. What are sunspots? Why do they appear
dark?
5. What is the connection between
sunspots and the Sun’s magnetic field?
6. What causes eruptions in the Sun’s
atmosphere? 2
Introduction
Table 13-1
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Photosphere
The photosphere is the lowest of three main layers
in the Sun’s atmosphere
Fig. 13-1: The Photosphere
• The Sun’s atmosphere
has three main layers: the
photosphere, the
chromosphere, and the
corona
• Everything below the
solar atmosphere is
called the solar interior
• The visible surface of the
Sun, the photosphere, is
the lowest layer in the
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solar atmosphere
Convection in the photosphere produces granules
Fig. 13-3: Granules
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Fig. 13-4: Supergranules
Chromosphere
The chromosphere is characterized by spikes
of rising gas
Fig. 13-5: Chromosphere
• Above the
photosphere is a
layer of less dense
but higher
temperature gases
called the
chromosphere
• Spicules extend
upward from the
photosphere into the
chromosphere along
the boundaries of
supergranules 7
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Fig. 13-6: The Solar Atmosphere
Corona
The corona ejects mass into space to form the solar wind
Fig. 13-8: The Solar Corona
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Activity in the corona includes coronal mass ejections and coronal holes
Fig. 13-9: The Ultraviolet Corona
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Sunspots
Sunspots are low-temperature regions in
the photosphere
Fig. 13-10: The Sunspots
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Fig. 13-11: The Sun’s
Rotation with Sunspots
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Fig. 13-12: Sunspot Maximum and Minimum
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Sunspots are produced by a 22-year cycle
in the Sun’s magnetic field
Fig. 13-13: The Sunspot Cycle
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Fig. 13-14: Variations in the Average Latitude of Sunspots
• The Sun’s surface features vary in an 11-year cycle
• This is related to a 22-year cycle in which the surface magnetic field
increases, decreases, and then increases again with the opposite polarity
• The average number of sunspots increases and decreases in a regular cycle
of approximately 11 years, with reversed magnetic polarities from one 11-
year cycle to the next
• Two such cycles make up the 22-year solar cycle 15
Sun’s Magnetic Fields
Fig. 13-15: Mapping the Sun’s Magnetic Field
The magnetic-dynamo model suggests that many
features of the solar cycle are due to changes in
the Sun’s magnetic field
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Fig. 13-16: Sunspots and Magnetic Fields
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Fig. 13-18: Rotation of the Solar Interior
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Other Magnetic Activities
The Sun’s magnetic field also produces other
forms of solar activity
• Solar Flares: A solar
flare is a brief
eruption of hot,
ionized gases from a
sunspot group
• Coronal Mass
Ejection: A coronal
mass ejection is a
much larger eruption
that involves
immense amounts of
gas from the corona Fig. 13-19: Magnetic Arches in the Corona
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Prominences, Filaments, Plages
Fig. 13-20a: Prominences, etc - I. 20
Fig. 13-20b: Prominences, etc - II. 21
Fig. 13-21: Coronal Mass Ejection 22
Internal
Solar
Layers
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Nuclear Fusion !
• At 15 million degrees Celsius in the center
of the star, fusion ignites !
• 4 (1H) --> 4He + 2 e+ + 2 neutrinos +
energy
• Where does the energy come from ?
• Mass of four 1H > Mass of one 4He
E = mc2
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A Balancing Act
• Energy released from nuclear fusion
counter-acts inward force of gravity.
Throughout its life, these
two forces determine the
stages of a star’s life.
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Key Words
• 22-year solar cycle • Fusion
• chromosphere • granule
• conduction • photosphere
• Convection • plasma
• Convection zone
• Prominence
• core
• Radiation zone
• corona
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