Astronomy
A BEGINNER’S GUIDE
TO THE UNIVERSE
EIGHTH EDITION
CHAPTER 9
The Sun
Lecture Presentation
© 2017 Pearson Education, Inc.
Chapter 9 The Sun
The Big Picture
The Sun is our star—the main
source of energy that powers
weather, climate, and life on
Earth.
Imagine our planet without the
Sun—no light, no heat, no
comforting “parent” in the sky.
Although we take it for granted
each and every day, the Sun is
vitally important to us in the
cosmic scheme of things.
Simply put, without the Sun, we
would not exist.
Units of Chapter 9
• 9.1 The Sun in Bulk
• 9.2 The Solar Interior
• 9.3 The Solar Atmosphere
• 9.4 The Active Sun
• 9.5 The Heart of the Sun
• Summary of Chapter 9
9.1 The Sun in Bulk
The Sun is made up of hot gas, there isn't really a "surface.” Instead, as you move
from space toward the Sun's core, the gas gets denser and denser.
The Sun’s bulk properties—mass, radius, temperature,
and luminosity.
Figure 9.1 The Sun (composite, filtered image).
The inner part shows a sharp edge, although our star, like all stars, is composed of a
gradually thinning gas. The edge appears sharp because the solar photosphere is so thin.
The outer portion of the image is the solar corona, normally too faint to be seen, but visible
during an eclipse, when the light from the solar disk is blotted out. (Note the blemishes; they
are sunspots.)
9.1 The Sun in Bulk – Photosphere
The layer of the Sun that we get the most
information from is the one we can most
easily see – the photosphere – describes
the Sun’s temperature, luminosity and
chemical composition.
Solar photosphere (“sphere of light”)
is the surface of the Sun that emits
radiation.
• Most of the Sun's radiation escapes from the
photosphere and is detected as sunlight that we
observe here on Earth.
• The solar photosphere is a thinner, cooler layer
than its neighboring layers.
• It is thin compared to the other
atmospheric regions – making the Sun
appear to have a distinct edge.
• About 500 km (300 miles) wide.
• The surface temperature of the Sun is 5780 K
or ~ 5506.85 C.
• Features observed: sunspots, granules,
suppergranules, large scale flows and pattern of
waves and oscillation.
9.1 The Sun in Bulk – Atmosphere Layers
• Outer Layers of the Sun:
•
Chromosphere (“sphere of color”) is
the Sun’s lower atmosphere.
–
–
Above the photosphere, it is about 1500 km
thick.
It is only visible during a solar eclipse, as it is
dimmer than the photosphere.
• The “color” is produced by the H-alpha line,
and the layer looks slightly pink.
•
Transition zone is a region where
temperature rises dramatically that
separates the Sun’s chromosphere from
the corona.
•
Solar corona (“crown”) is a wide layer
that extends out into space, eventually
turning into the solar wind.
–
–
•
Above 10,000 km, and stretching far beyond, is
a thin, hot upper atmosphere.
It is also only visible during a total solar eclipse.
Solar wind flows away from the Sun and
permeates the entire solar system, consists
mainly of protons and electrons in a state
known as a plasma.
Image of the solar
corona (white) and
chromosphere
(pink) during a total
solar eclipse on
Monday, August 21,
2017 above
Madras, Oregon.
9.1 The Sun in Bulk
• Interior structure of the Sun:
– Outer layers are not to scale.
– Below the photosphere,
extending down some 200,000
km, is the convection zone, a
region where the material of
the Sun is in constant
convective motion.
– Below the convection zone lies
the radiation zone, where solar
energy is transported toward
the surface by radiation rather
than by convection.
– The term solar interior is often
used to mean both the radiation
and convection zones.
– The core is where nuclear
fusion reactions that generate
the Sun’s enormous energy
output take place, roughly
200,000 km in radius.
9.1 The Sun in Bulk
• Luminosity—total energy
radiated by the Sun—can
be calculated from the
fraction of that energy
that reaches Earth.
• Total luminosity of the Sun
is about 4 ×026 W—the equivalent
of 10 billion 1-megaton
nuclear bombs per second.
– If we draw an imaginary sphere around the Sun
so that the sphere’s surface passes through Earth’s
center, then the radius of this imaginary sphere is 1 AU.
– The “solar constant” is the amount of power
striking a 1-m2 detector at Earth’s distance from
the Sun, as suggested in the inset,
is approx. 1400 watts per square meter (W/m2).
– The Sun’s luminosity is then determined by
multiplying the sphere’s surface area by the solar
constant.
•
Surface area is 4π3 (1 AU)2, or approximately
2.8 ×1023 m2.
Figure 9.3 Solar Luminosity
9.2 The Solar Interior – Modeling the Structure of the Sun
• Mathematical models, consistent with observation and
physical principles, provide information about the Sun’s
interior.
– In equilibrium, inward
pull of gravitational force must
be balanced by outward
pressure of hot gas.
Figure 9.4 Stellar Balance
In the interior of a star such as the Sun,
the outward pressure of hot gas
balances the inward pull of gravity.
This is true at every point within the
star, guaranteeing its stability.
9.2 The Solar Interior – Modeling the structure of the Sun
• Doppler shifts of solar spectral lines indicate a complex pattern of vibrations.
– These vibrations are the result of internal pressure (“sound”) waves that reflect off the
photosphere and repeatedly cross the solar interior.
• These waves penetrate deep inside the Sun, and analysis of their surface patterns allows
scientists to study conditions far below the Sun’s surface.
Figure 9.5 Solar Oscillations
(a) By observing the motion of the solar surface, scientists can determine the wavelengths and
frequencies of the individual waves and deduce information about the Sun’s complex vibrations.
(b) Waves contributing to the observed oscillations can travel deep inside the Sun, providing vital
information about the solar interior.
9.2 The Solar Interior
• Solar density and temperature,
according to the standard solar
model:
– As the distance from the center
increases, the density decreases
faster than the temperature.
Figure 9.6 Solar Interior
Density and temperature distributions in the
interior of the Sun.
Parts (b) and (c) show the large variations in
solar density and temperature, relative to a
cutaway diagram of the Sun’s interior (a).
9.2 The Solar Interior – Energy Transport
• The core is the area where temperatures are high enough for the energy
production to occur.
• Energy transport:
– The radiation zone is relatively transparent; the convection zone is cooler and opaque.
• In the radiation zone, the temperatures are lower, but still high enough that all the atoms are ionized
and the radiation travels freely.
• In the convection zone, the temperatures are even lower, and electrons are now bound to nuclei;
atoms absorb the photons so the energy can no longer be transported through radiation.
Figure 9.7 Solar Convection
Energy is physically transported in the Sun’s convection zone, which is here visualized as a boiling, seething sea of gas.
As drawn, the convective cell sizes become progressively larger at greater depths.
9.2 The Solar Interior – Granulated solar photosphere
• Granulation is evidence of
convection in the solar interior.
• The visible top layer of
the convection zone is
granulated, with areas
of upwelling material
surrounded by areas
of sinking material.
– A photograph of the granulated
solar photosphere, taken with the
1-m Swedish Solar Telescope,
shows solar granules comparable
in size to Earth’s continents or a
large U.S state.
– The bright portions are regions
where hot material is upwelling
from below.
– The dark regions are cooler gas
that is sinking back down into the
Figure 9.8 Solar Granulation
interior.
9.2 The Solar Interior – Granulated solar photosphere
• Supergranulation occurs on a
much larger scale than
granulation, so it is thought to
be due to convective flow at a
deep level within the Sun's
interior.
– These features also cover the
entire Sun and are continually
evolving.
– Individual supergranules last for a
day or two and have flow speeds
of about 0.5 km/s (1000 mph).
– The fluid flows observed in
supergranules carry magnetic field
bundles to the edges of the cells
where they produce the
chromospheric network.
Supergranules
9.3 The Solar Atmosphere
• Spectral analysis of absorption lines can tell us what elements are
present, but only in the chromosphere and photosphere.
A detailed visible spectrum
of our Sun shows
thousands of dark
absorption lines,
indicating the presence of
67 different elements in
various stages of
excitation and ionization
in the lower solar
atmosphere.
Each element absorbs light
at characteristic
wavelengths, resulting in a
unique pattern of absorption
lines in the spectrum.
The numbers give
wavelengths in nanometers.
Figure 9.9 Solar Spectrum
9.3 The Solar Atmosphere
Table 9.2 The
Composition of
the Sun
Lists the 10 most
common elements
in the Sun based
on spectral
analysis, and it is
what we will find for
the universe as a
whole. Hydrogen
is by far the most
abundant
element, followed
by helium.
9.3 The Solar Atmosphere
• Colourful eclipse composite captured the chromospheric or flash spectrum
of the Sun, on August 21, 2017 (Madras, Oregon):
• Individual eclipse images at each wavelength of light emitted by atoms along the
thin solar chromosphere.
–
The brightest
images are due to
Hydrogen atoms.
• Red hydrogen alpha
emission is at the far
right.
–
–
Blue and purple
hydrogen series
emission to the left.
• The highest
energy and
shortest
wavelength light is
given off by the
electrons that fall
the farthest.
In between, the
brightest yellow
emission is caused
by atoms of Helium.
9.3 The Solar Atmosphere
• The cooler
chromosphere is above
the photosphere.
– It is difficult to see
directly, as the
photosphere is too
bright, unless the Moon
covers the photosphere
and not the
chromosphere during
eclipse.
Figure 9.10 Solar Chromosphere
This photograph of a total solar eclipse shows
the solar chromosphere a few thousand
kilometers above the Sun’s surface. Note the
prominence at left.
9.3 The Solar Atmosphere – The Chromosphere
• Small solar storms in chromosphere emit spicules.
–
–
–
Every few minutes, small solar storms erupt, expelling spikes of hot matter known as spicules
into the Sun’s upper atmosphere.
Spicules tend to accumulate around the edges of supergranules, and the Sun’s magnetic field
is also somewhat stronger than average in those regions.
Scientists think that the downwelling material there amplifies the solar magnetic field and that
spicules are the result of magnetic disturbances in the Sun’s churning outer layers.
Figure 9.11 Solar Spicules
Short-lived, narrow jets of gas that typically last mere minutes can be seen sprouting up from
the chromosphere in this ultraviolet image of the Sun.
These so-called spicules are the thin spikelike regions where gas leaves the surface at
speeds of about 100 km/s, reaching thousands of kilometers above the photosphere.
9.3 The Solar Atmosphere
• Solar corona can be seen during eclipse if both
photosphere and chromosphere are blocked.
The solar wind is hot coronal
gas escaping the gravity of the
Sun.
The density and temperature in
the solar corona are much
lower than in the photosphere.
https://eclipse2017.nasa.gov/science
9.3 The Solar Atmosphere
• Corona is much hotter than layers below it. It must
have a heat source, probably electromagnetic
interactions.
Figure 9.13 Solar
Atmospheric Temperature.
The change of gas
temperature in the lower solar
atmosphere is dramatic.
The temperature, indicated by
the blue line, reaches a
minimum of 4500 K in the
chromosphere and then rises
sharply in the transition zone,
finally leveling off at around
3 million K in the corona.
9.4 The Active Sun
• Sunspots appear dark because they are slightly cooler
than the brighter, background photosphere.
– Sunspots cluster at high latitudes
when solar activity is at a minimum.
– They appear in larger numbers at
about the time of the solar maximum,
at roughly 11-year intervals.
– They typically measure about
10,000 km across.
Figure 9.14 Sunspots
This photograph of the entire Sun, taken
during a period of maximum solar activity,
shows several groups of sunspots. The
largest spots in this image are more than
20,000 km across, nearly twice the
diameter of Earth.
9.4 The Active Sun
• Sunspots appear dark because they are slightly cooler
than the brighter, background photosphere.
Figure 9.15 Sunspots, Up
Close
(a) An enlarged photo of the
largest pair of sunspots
in shows how each spot
consists of a cool,
dark umbra
surrounded by a
warmer, brighter
penumbra.
(b) A high-resolution image
of a single typical
sunspot shows details
of its structure as well
as the granules
surrounding it.
9.4 The Active Sun – Solar magnetism
• Sunspots come and go, typically in a few days.
• Sunspots are linked by pairs of magnetic field lines.
Figure 9.16 Sunspot Magnetism
(a) The Sun’s magnetic field lines emerge from the surface through one member of a
sunspot pair and reenter the Sun through the other member. If the magnetic field lines are
directed into the Sun in one leading spot, they are inwardly directed in all other leading
spots in that hemisphere. The opposite is the case in the southern hemisphere.
(b) A far-ultraviolet image taken by NASA’s Transition Region and Coronal Explorer
(TRACE) satellite, showing magnetic field lines arching between two sunspot groups.
9.4 The Active Sun
• The rotation of the Sun drags magnetic field lines around with it,
causing kinks.
Figure 9.17 Solar Rotation
(a, b) The Sun’s differential rotation wraps and distorts the solar magnetic field.
(c) Occasionally, the field lines burst out of the surface and loop through the lower atmosphere,
thereby creating a sunspot pair. The underlying pattern of the solar field lines explains the
observed pattern of sunspot polarities.
9.4 The Active Sun
• The Sun has an 11-year sunspot cycle, during which sunspot numbers
rise, fall, and then rise again.
(a) The monthly number of sunspots during the 20th century clearly displays the (roughly) 11-year solar cycle. At the time of minimum
solar activity, hardly any sunspots are seen. About 4 years later, at maximum solar activity, about 100 spots are observed per month.
Figure 9.18 Sunspot Cycle
(b) Sunspots cluster at high latitudes when solar activity is at a minimum. They appear at lower and lower latitudes as the number of
sunspots peaks. They are again prominent near the Sun’s equator as solar minimum is again approached.
9.4 The Active Sun
• This is really a 22-year solar cycle because the spots switch polarities
between the Northern and Southern Hemispheres every 11 years.
• Maunder minimum: There are few, if any, sunspots.
Figure 9.19 Maunder Minimum
Peaks and troughs of the sunspot cycle over the past four centuries.
Note the near total absence of spots during the late 17th century.
https://www.swpc.noaa.gov/products/solar-cycle-progression
9.4 The Active Sun
• Areas around sunspots are active; large eruptions may occur in the photosphere.
• Solar prominence is a large sheet of ejected gas.
– Prominences are loops or sheets of glowing gas ejected from an active region on the solar surface.
– Prominences move through the inner parts of the corona under the influence of the Sun’s magnetic field.
•
Magnetic field is involved in producing sunspots, flares, and prominences.
(a) This particularly large solar prominence
was observed by ultraviolet detectors aboard
the SOHO spacecraft in 2002.
(b) Like a phoenix rising from the solar surface, this filament of hot gas
measures more than 100,000 km in length.
• Dark regions in this TRACE image have temperatures less than 20,000 K;
• the brightest regions are about 1 million kelvins.
• Most of the gas will subsequently cool and fall back into the photosphere.
9.4 The Active Sun
• Solar flare is a large explosion on the Sun’s surface, emitting an
amount of energy similar to a prominence, but in seconds or minutes
rather than days or weeks.
– Flares are caused by magnetic
disturbances in the lower
atmosphere of the Sun.
Figure 9.21 Solar Flare
Much more violent than a
prominence, a solar flare is an
explosion on the Sun’s surface that
sweeps across an active region in a
matter of minutes, accelerating solar
material to high speeds and blasting
it into space.
https://sdo.gsfc.nasa.gov/
https://soho.nascom.nasa.gov/home.html
9.4 The Active Sun
• A coronal mass ejection emits charged particles
that can affect Earth.
Figure 9.22 Coronal Mass Ejection
(a) A few times per week, on average, a
giant magnetized “bubble” of solar
material detaches itself from the Sun
and rapidly escapes into space, as
shown in this SOHO image taken in
2002. The circles are artifacts of an
imaging system designed to block out
the light from the Sun itself and
exaggerate faint features at larger radii.
https://www.swpc.noaa.gov/
(b) Should such a coronal mass
ejection encounter Earth with its
magnetic field oriented opposite to
our own, as illustrated, the field lines
can join together as in part (c),
allowing high-energy particles to enter
and possibly severely disrupt our
planet’s magnetosphere.
9.4 The Active Sun
• Solar wind escapes the Sun mostly through coronal holes, which can be
seen in X-ray images. The coronal hole is the dark V-shaped region.
Figure 9.23 Coronal Hole
(a) Images of X-ray emission from the Sun observed by the Yohkoh
satellite. Note the dark, V-shaped coronal hole traveling from left to
right, where the X-ray observations outline in dramatic detail the
abnormally thin regions through which the high-speed solar wind
streams forth.
(b) Charged particles follow magnetic field lines that compete with
gravity. When the field is trapped and loops back toward the
photosphere, the particles are also trapped; otherwise, they can
escape as part of the solar wind.
9.4 The Active Sun
• Solar corona changes along with the sunspot cycle.
It is much larger and more irregular at sunspot peak.
9.5 The Heart of the Sun
• Nuclear fusion requires that like-charged nuclei get close enough
to each other to fuse.
• This can happen only if the temperature is extremely high—over
10 million kelvins.
– The helium produced from the fusion of hydrogen has less mass than the hydrogen
that goes into its formation. The missing matter is converted into energy.
Figure 9.24 Proton Interactions
(a) Since like charges repel, two low-speed
protons veer away from one another, never
coming close enough for fusion to occur.
(b) Higher-speed protons may succeed in overcoming
their mutual repulsion, approaching close enough for the
strong force to bind them together—in which case they
collide violently, triggering nuclear fusion that ultimately
powers the Sun.
9.5 The Heart of the Sun
• The process that powers most stars is a three-step fusion process, the
proton–proton chain.
Figure 9.25 Solar
Fusion
A total of six protons
(and two electrons) are
converted to two
protons, one helium-4
nucleus, and two
neutrinos.
The two leftover
protons are available
as fuel for new
proton–proton
reactions, so the net
effect is that four
protons are fused to
form one helium-4
nucleus.
Energy, in the form of
gamma rays, is
produced at each
stage.
I. Two protons combine,
as forming a deuteron
and releasing a positron
and a neutrino.
III. Finally, two helium-3
II. The deuteron then combines with
another proton to create a type of helium nuclei combine to form
called helium-3. which contains only one helium-4 and two protons.
neutron, while the positron annihilates with
an electron, producing gamma rays. The
neutrino escapes into space
9.5 The Heart of the Sun
• Neutrinos are emitted directly from the core of the
Sun and escape, interacting with virtually nothing.
Being able to observe these neutrinos gives us a
direct picture of what is happening in the core.
• They are no more likely to interact with Earth-based
detectors than they are with the Sun; the only way to
spot them is to have a huge detector volume and to
be able to observe single interaction events.
• The "solar neutrino problem” is that too few
neutrinos are detected on Earth compared to the
number the solar model predicts.
9.5 The Heart of the Sun
• Neutrino observatories
(a) The Super Kamiokande neutrino detector is
buried beneath a mountain near Tokyo, Japan.
It is filled with 50,000 tons of purified water and
contains 13,000 individual light detectors to sense
the telltale signature—a brief burst of light—of a
neutrino passing through the apparatus.
(b) The Sudbury Neutrino Observatory, some 2 km
underground in Ontario, Canada, is similar in design
to the Kamiokande facility, but is also sensitive to
other neutrino types. It contains 10,000 light-sensitive
detectors arranged on the inside of the large sphere
shown here.
Summary of Chapter 9
• The Sun is held together by its own gravity and
powered by nuclear fusion.
• Outer layers of the Sun are the photosphere,
chromosphere, and corona.
• The photosphere is the visible “surface” of the Sun.
The corona is very hot.
• Mathematical models and helioseismology give us a
picture of the interior of the Sun.
• Sunspots occur in regions of high magnetic fields;
darker spots are cooler.
© 2017 Pearson Education, Inc.
Summary of Chapter 9, cont.
• Nuclear fusion converts hydrogen to helium,
releasing energy.
• Solar neutrinos come directly from the solar core,
although observations have told us more about
neutrinos than about the Sun.
© 2017 Pearson Education, Inc.