Lecture Slides
CHAPTER 11: Our Star: The Sun
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
Our Star: The Sun
 Describe the structure
of the Sun’s interior
and atmosphere.
 Explain how energy is
produced and
transported by the Sun.
 Understand solar
activity cycles.
Solar Interior
 We only see the outer
layers of the Sun, a G2 star.
 Physics tells us about the
interior.
 The structure of the Sun is
caused by a balance
between forces due to
pressure and gravity.
Solar Interior: Hydrostatic Equilibrium
 The Sun must be in
balance to exist for billions
of years.
 Hydrostatic equilibrium:
outward pressure = inward
force of gravity at every
point in the Sun.
Solar Interior: Sun’s Surface
 Density, pressure, and
temperature decrease
away from the center of
the Sun.
 Energy production peaks
in the center – the core of
the Sun.
Energy Production
 Recall: Nuclei may
consist of protons
and neutrons.
• Protons: positive
electrical charge.
• Neutrons: no
electrical charge.
 Protons are kept
apart by electric
repulsion.
Energy Production: Nuclear Fusion
 The strong nuclear
force binds the
nucleus together.
 Fusion requires
ramming protons
together at high speed
(i.e., at high
temperature).
 Fusion also requires
high densities to
insure collisions.
Energy Production: Nuclear Fusion (Cont.)
Energy Production: Nuclear Fusion (Cont.)
•They get close
enough for the strong
nuclear force to
overpower electric
repulsion
Energy Production: Nuclear Fusion (Cont.)
Energy Production: Sun’s Lifetime
 The Sun has been around a long time, about 4.6
billion years.
 The Sun must therefore generate a lot of energy over
a long time.
 Source: nuclear fusion of hydrogen to helium in the
core of the Sun.
 Fusion takes place in the core, where it is hot and
dense enough.
•
More facts about the Sun
The core of the Sun extends from the center to
about 20–25% of the Sun’s radius.
•
The Sun’s radius is 109 times bigger than the
Earth’s radius!
•
The core of the sun has a temperature of about
15.7 million Kelvin!
•
The core of the sun has a density of 150 g/cm3
(but the average density of the Sun is only
about 1.4g/cm3 ). Note that the density of water
is 1g/cm3.
Energy Production: The Proton-Proton Chain
 Fusion process: proton-proton
chain.
 Net result: 4 hydrogen nuclei
turn into 1 helium nucleus, plus
neutrinos, positrons and gamma
rays! Positrons are just like
electrons but have positive
charge. Neutrinos have mass
but have almost no interaction
with matter—they easily go right
through the Earth!
 Why is energy produced?
Energy Production: Hydrogen Burning Process
 Mass of 4 H nuclei is slightly greater than 1 He
nucleus. It’s about 0.7% greater.
 Thus, a little bit of mass is converted into
energy as 4 H nuclei fuse into 1 He nucleus.
 Relativity: mass and energy are equivalent:
E = mc2
 Difference in mass is released as energy.
 This nuclear fusion process is often called
hydrogen burning.
 This happens for all main-sequence stars.
Class Question
In which layer of the Sun does nuclear
fusion occur?
A.
B.
C.
D.
Convective Zone
Core
Corona
Radiative Zone
Energy Transport: Radiation Zone
 Radiative zone: layer just outside of the core of
the Sun.
 Radiative transfer: photons travel from hotter to
cooler regions.
Energy Transport: Convection Zone
 Convection:
rising/falling of
hot/cool gas.
 Convective
zone: layer in
between the
radiative zone
and the surface
of the Sun.
Energy Transport
 We can see evidence for
the convective zone by
looking at the surface of
the Sun.
 Convection is visible as
bubbling of the surface.
 Bubbles are large – the
size of countries on Earth!
Energy Transport: Helioseismology
 Helioseismology: sound
waves move through the
Sun, making surface and
interior waves.
 Doppler shifts give the
speed of wave motion.
Energy Transport: Interior of the Sun
 Speeds depend on the
Sun’s composition and the
depth of the convection
zone.
 Observations agree with
models of the solar
interior.
Surface
 Photosphere: layer where light is
emitted into space = apparent
surface of the Sun.
 Average temperature: 5800 K
 Limb darkening: because we look
through less material at the
edges, it appears darker.
Surface (Cont.)
Surface (Cont.)
Atmosphere
 Atmosphere: where
the density drops
very rapidly with
increasing altitude.
 The solar
atmosphere is
much less dense
than the
atmosphere of the
Earth.
Atmosphere: The Solar Spectrum
 Cooler outer layers of the
Sun absorb some of the
light from hotter,
deeper layers.
 This produces a complex
absorption spectrum with
more than 70 elements
identified.
Atmosphere: Chromosphere
 Chromosphere: layer
directly above the
photosphere.
 Higher temperature than
the photosphere.
 Gives off a reddish
emission-line spectrum
due to hydrogen.
Atmosphere: Chromosphere (Cont.)
Atmosphere: Chromosphere (Cont.)
Atmosphere: Corona
 Corona: layer above the
chromosphere.
 Very hot: T = 1 to 2
million K.
 Emits X-rays.
 Can extend for several
solar radii.
 This picture was taken
during a total solar eclipse
(the only time you can see
the corona with your eyes)
Class Question
Rank these layers of the Sun
in the correct order, from coolest to hottest.
A.
B.
C.
D.
Core, Corona, Photosphere
Photosphere, Core, Corona
Corona, Photosphere, Core
Photosphere, Corona, Core
Magnetic Field
 Sun’s magnetic field affects the
structure of the atmosphere.
 The solar wind: charged
particles flowing away from the
Sun through coronal holes,
where magnetic field lines
extend away from the Sun.
Magnetic Field: The Solar Wind Streams
 Coronal material
flows along the
magnetic field
away from the
Sun.
 This is the solar
wind, which
extends about
100 AU.
 The Voyager 1
spacecraft is
traveling across
this boundary.
Solar Activity: Sunspots
 Sunspots: cooler areas in the photosphere.
 Sunspot structure: dark umbra with
surrounding penumbra.
 Sunspots are caused by the solar magnetic field.
Solar Activity: Sunspots (Cont.)
Solar Activity: Sunspots (Cont.)
Solar Activity: Sunspots (Cont.)
 The number of sunspots varies as a function of time.
 The latitude of sunspots also varies as a function
of time.
Solar Activity: Sunspots (Cont.)
Solar Activity: Sunspots (Cont.)
Solar Activity: 11-year Sunspot Cycle
 Sun shows an 11-year sunspot cycle.
 Solar maximum: most sunspots and activity.
 The Maunder minimum showed a distinct lack of
sunspots between 1645 and 1715.
Class Question
Why does the previous plot only date back to 1600?
A. The Sun did not have sunspots pre-1600.
B. There were previously too many sunspots
to count.
C. Sunspots were difficult to observe before the
invention of the telescope.
Solar Activity: Magnetic Flip
 The Sun’s magnetic field flips every 11 years =>
Solar magnetic field shows a 22 year cycle.
Solar Activity: Magnetic Flip (Cont.)
Solar Activity: Magnetic Flip (Cont.)
Solar Activity: Solar Prominences and Flares
 Prominences: hot rising gas in the
chromosphere constrained by
magnetic fields.
 Solar flares and coronal mass
ejections are highly energetic
eruptions.
Solar Activity: Solar Prominences and Flares (Cont.)
Solar Activity: Solar Prominences and Flares (Cont.)
Solar Activity: Solar Prominences and Flares (Cont.)
 The explosive behavior
of the Sun – prominences,
flares, coronal mass
ejections – is tied to the
sunspot cycle.
 Activity is greater at
solar maximum.
Solar Activity: Solar Prominences and Flares (Cont.)
Solar Activity: Solar Prominences and Flares (Cont.)
Solar Activity: Solar Prominences and Flares (Cont.)
Solar Activity: Changes Over Time
 Solar activity changes over time.
 Solar storms can disrupt electric power grids and
satellites and cause brilliant auroras.
Chapter Summary
 The Sun’s structure is maintained by hydrostatic
equilibrium.
 Nuclear reactions converting hydrogen to helium are
the source of the Sun’s energy.
 The Sun has multiple layers, each with a different
densities, temperatures, and pressures.
 Sunspot observations led to the discovery of 11- and
22-year cycles in solar activity.
Astronomy in Action
Random Walk
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(Requires an active Internet connection)
Astronomy in Action
Inverse Square Law
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(Requires an active Internet connection)
AstroTour
The Solar Core
Click the image to launch the AstroTour Animation
(Requires an active Internet connection)
Nebraska Applet
Proton-Proton Animation
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(Requires an active Internet connection)
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
This concludes the Lecture Slides for
CHAPTER 11: Our Star:
The Sun
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company