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Title: DTU 8e Chap 10 The Sun


1
Neil F. Comins William J. Kaufmann III
Discovering the Universe Ninth Edition
CHAPTER 10 The Sun Our Extraordinary Ordinary
Star
2
WHAT DO YOU THINK?
  1. What percentage of the solar systems mass is in
    the Sun?
  2. Does the Sun have a solid and liquid interior
    like Earth?
  3. What is the surface of the Sun like?
  4. Does the Sun rotate? If so, how fast?
  5. What makes the Sun shine?
  6. Are matter and energy conserved?

3
In this chapter you will discover
  • why the Sun is a typical star
  • how todays technology has led to new
    understanding of solar phenomena, from sunspots
    to the powerful ejections of solar matter that
    sometimes enter our atmosphere
  • that some features of the Sun generated by its
    varying magnetic field occur in cycles
  • how the Sun generates the energy that makes it
    shine
  • new insights into the nature of matter from solar
    neutrinos

4
The Sun emits most of its visible light from a
thin layer of gas, called the photosphere, as
shown. Although the Sun has no solid or even
liquid region, we see the photosphere as its
surface. It is actually the top of the Suns
convective zone. Astronomers always take great
care when viewing the Sun by using extremely dark
filters or by projecting the Suns image onto a
screen.
5
Limb Darkening
The Suns edge, or limb, appears distinctly
darker and more orange than does its center, as
seen from Earth. This occurs because we look
through the same amount of solar atmosphere at
all places. As a consequence, we see higher in
the Suns photosphere near its limb than when we
look at its central regions. The higher
photosphere is cooler and, because it is a
blackbody, darker and more orange than the lower,
hotter region of the photosphere.
6
Solar Granulation
High-resolution photographs of the Suns surface
reveal a blotchy pattern, called granulation.
Granules, which measure about 1000 km across, are
convection cells in the Suns photosphere. Inset
Gas rising upward produces the bright granules.
Cooler gas sinks downward along the darker,
cooler boundaries between granules. This
convective motion transports energy from the
Suns interior outward to the solar atmosphere.
7
Solar Granulation
At lower resolution, the Suns surface appears
relatively smooth. Inset Viewed near the Suns
limb, granules are seen to bulge upward at their
centers as a result of the convection that
creates them.
8
The Chromosphere
This photograph of the chromosphere was taken by
the Hinode (Japanese for sunrise) satellite.
The dark bumps are the tops of granules, and the
light regions are hotter gases in spicules. The
spicules on the edge or limb of the Sun give a
sense of the height of these gas jets.
9
Spicules and Supergranules
Spicules appear in this photograph of the Suns
chromosphere. The Sun appears rose-colored in
this image because it was taken through an Ha
filter that passes red light from hydrogen and
effectively blocks most of the photospheres
light. Surrounded by spicules, supergranules are
regions of rising and falling gas in the
chromosphere. Each supergranule spans hundreds of
granules in the photosphere below. Inset This is
a view of spicules from above.
10
Spicules and Supergranules
The spicules are jets of gas that surge upward
into the Suns outer atmosphere. This schematic
diagram shows a spicule and its relationship to
the solar atmospheres layers. The photosphere is
about 400 km thick. The chromosphere above it
extends to an altitude of about 2000 km, with
spicules jutting up to nearly 10,000 km above the
photosphere. The outermost layer, the corona,
extends millions of kilometers above the
photosphere.
11
The Solar Corona
(a) This visible-light photograph was taken
during the total solar eclipse of July 11, 1991.
Numerous streamers are visible, extending
millions of kilometers above the solar surface.
(b) This X-ray image of the Suns corona, taken
by the Yohkoh satellite in 1999, provides hints
of the complex activity taking place on and in
the Sun. The million-degree gases in the corona
emit the X rays visible here.
12
The Solar Corona
This graph shows how temperature varies with
altitude in the Suns chromosphere and corona and
in the transition region between them (white).
Note that both the height and temperature scales
are nonlinear.
13
Sunspots
(a) This dark region on the Sun is a typical
isolated sunspot. Granulation is visible in the
surrounding, undisturbed photosphere.
(b) This high-resolution photograph shows a
sunspot group in which several sunspots overlap.
14
The Sunspot Cycle
The number of sunspots on the Sun varies with a
period of about 11 years. The most recent sunspot
maximum occurred in 2001, and the most recent
sunspot minimum occurred in 2007 (and continued
through 2010).
15
The Sunspot Cycle
The active Sun has many sunspots (this photo was
taken in 1979).
The Sun has many fewer sunspots when it is not
active (this photo, showing a time with no
sunspots, was taken in 1989).
16
The Suns Rotation
This series of photographs taken in 1999 shows
the same sunspot group over one-third of a solar
rotation. Note how the sunspot groups have
changed over this time. By observing a group of
sunspots from one day to the next in this same
manner, Galileo found that the Sun rotates once
in about 4 weeks. Sunspot activity also reveals
the Suns differential rotation The equatorial
regions rotate faster than the polar regions.
17
Locations of Sunspots Throughout the Sunspot
Cycle
This butterfly diagram of sunspot locations
shows that they occur at changing latitudes
throughout each cycle. From most common locations
to least, the diagram is color coded yellow,
orange, and black.
18
Zeeman Splitting by a Sunspots Magnetic Field
(a) The black line drawn across the sunspot
indicates the location toward which the slit of
the spectroscope was aimed. (b) In the resulting
spectrogram, one line in the middle of the normal
solar spectrum is split into three components by
the Suns magnetic field. The amount of splitting
between the three lines is used to determine the
magnetic fields strength. Typical sunspots have
magnetic fields some 5000 times stronger than
Earths magnetic field.
19
Helioseismology
(a) This computer-generated image shows one of
the myriad ways in which the Sun vibrates because
of sound waves resonating in its interior. The
regions that are moving outward are blue those
moving inward are red. The cutaway shows how deep
these oscillations are believed to extend. (b)
This cutaway picture of the Sun shows how the
rate of solar rotation varies with depth and
latitude. Red and yellow denote
faster-than-average motion blue regions move
more slowly than average. The pattern of
differential surface rotation, which varies from
25 days at the equator to 35 days near the poles,
persists at least 19,000 km down into the Suns
convective layer. Sunspots preferentially occur
on the boundaries between different rotating
regions. Earthlike jet streams and other wind
patterns have also been discovered in the Suns
atmosphere.
20
Babcocks Magnetic Dynamo
Differential rotation wraps a magnetic field
around the Sun. Convection under the photosphere
tangles the field, which becomes buoyant and
rises through the photosphere, creating sunspots
and sunspot groups. Insets In each group, the
sunspot that appears first as the Sun rotates has
the same polarity as the Suns magnetic pole in
that hemisphere. The Suns magnetic fields are
revealed by the radiation emitted from the gas
they trap. These ultraviolet images show coronal
loops up to 160,000 km (100,000 mi) high, with
gases moving along the magnetic field lines at
speeds of 100 km/s (60 mi/s).
21
Active Sun in Ha
This photograph shows the chromosphere and corona
during a solar maximum, when sunspots are
abundant. The image was taken through a filter
allowing only light from Ha emission to pass
through. The hot, upper layers of the Suns
atmosphere are strong emitters of Ha photons. A
few large sunspots are evident. Most notable are
features that do not appear at the solar minimum,
such as the snakelike features shown here called
filaments, bright areas called plages, and
prominences (filaments seen edge on) observed at
the solar limb.
22
Prominences
(a) A huge prominence arches above the solar
surface. The radiation that exposed this picture
is from singly ionized helium at a wavelength of
30.4 nm, corresponding to a temperature of about
50,000 K.
(b) The gas in these prominences was so energetic
that it broke free from the magnetic fields that
shape and confine it. This eruptive prominence
occurred in 1999 (and did not strike Earth, which
is shown for size).
23
A Coronal Hole
This X-ray picture of the Suns corona was taken
by the SOHO satellite. A huge coronal hole
dominates the lower right side of the corona. The
bright regions are emissions from sunspot groups.
24
A Flare
Solar flares, which are associated with sunspot
groups, produce energetic emissions of particles
from the Sun. This image, taken in 2000 by SOHO,
shows a twisted flare (upper left part of figure)
in which the Suns magnetic field lines are still
threaded through the region of emerging particles.
25
Origin of a Solar Flare
This Hinode satellite image of the Sun shows a
developing sunspot colliding with a preexisting
sunspot. The interacting magnetic fields funnel
hot gases rapidly away from the Sun as a solar
flare.
26
A Snapshot of the Suns Global Magnetic Field
By following the paths of particles emitted by a
solar flare, astronomers have begun mapping the
solar magnetic field outside the Sun. The field
guides the outflowing particles, which, in turn,
emit radio waves that indicate the position of
the field. These data were collected by the
Ulysses spacecraft in 1994. Ulysses was the first
spacecraft to explore interplanetary space from
high above the plane of the ecliptic.
27
A Coronal Mass Ejection
An X-ray image of a coronal mass ejection from
the Sun taken by SOHO.
28
A Coronal Mass Ejection
Two to 4 days later, the highest-energy gases
from the ejection reach 1 AU. If they come our
way, most particles are deflected by Earths
magnetic field (blue). However, as shown, some
particles leak Earthward, causing aurorae,
disrupting radio communications and electric
power transmission, damaging satellites, and
ejecting some of Earths atmosphere into
interplanetary space.
29
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30
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31
Hydrostatic Equilibrium
(b) When the forces on the divers in water are in
hydrostatic equilibrium, they neither sink nor
rise.
(a) Matter deep inside the Sun is in hydrostatic
equilibrium, meaning that upward and downward
forces on the gases are balanced.
32
The Solar Model
Thermonuclear reactions occur in the Suns core,
which extends to a distance of 0.25 solar radius
from the center. In this model, energy from the
core radiates outward to a distance of 0.7 solar
radius. Convection is responsible for energy
transport in the Suns outer layers.
33
The Solar Model
The Suns internal structure is displayed here
with graphs that show how the luminosity, mass,
temperature, and density vary with the distance
from the Suns center. A solar radius (the
distance from the Suns center to the
photosphere) equals 696,000 km.
34
A Solar Neutrino Experiment
Located 2703 m (6800 ft) underground in the
Creighton nickel mine in Sudbury, Canada, the
Sudbury Neutrino Observatory is centered around a
tank that contains 1000 tons of water.
Occasionally, a neutrino entering the tank
interacts with one or another of the particles
already there. Such interactions create flashes
of light, called Cerenkov radiation. Some 9600
light detectors sense this light. The numerous
silver protrusions are the back sides of the
light detectors prior to their being wired and
connected to electronics in the lab (seen at the
bottom of the photograph).
35
Summary of Key Ideas
36
The Suns Atmosphere
  • The thin shell of the Suns gases we see are from
    its photosphere, the lowest level of its
    atmosphere. The gases in this layer shine nearly
    as a blackbody. The photospheres base is at the
    top of the convective zone.
  • Convection of gas from below the photosphere
    produces features called granules.
  • Above the photosphere is a layer of hotter, but
    less dense, gas called the chromosphere. Jets of
    gas, called spicules, rise up into the
    chromosphere along the boundaries of
    supergranules.
  • The outermost layer of thin gases in the solar
    atmosphere, called the corona, extends outward to
    become the solar wind at great distances from the
    Sun. The gases of the corona are very hot, but
    they have extremely low densities.

37
The Active Sun
  • Some surface features on the Sun vary
    periodically in an 11-year cycle. The magnetic
    fields that cause these changes actually vary
    over a 22-year cycle.
  • Sunspots are relatively cool regions produced by
    local concentrations of the Suns magnetic field
    protruding through the photosphere. The average
    number of sunspots and their average latitude
    vary in an 11-year cycle.
  • A prominence is gas lifted into the Suns corona
    by magnetic fields. A solar flare is a brief, but
    violent, eruption of hot, ionized gases from a
    sunspot group. Coronal mass ejections send out
    large quantities of gas from the Sun. Coronal
    mass ejections and flares that head our way
    affect satellites, communication, and electric
    power, and cause aurorae.
  • The magnetic dynamo model suggests that many
    transient features of the solar cycle are caused
    by the effects of differential rotation and
    convection on the Suns magnetic field.

38
The Suns Interior
  • The Suns energy is produced by the thermonuclear
    process, called hydrogen fusion, in which four
    hydrogen nuclei release energy when they fuse to
    produce a single helium nucleus.
  • The energy released in a thermonuclear reaction
    comes from the conversion of matter into energy,
    according to Einsteins equation, E mc2.
  • The solar model is a theoretical description of
    the Suns interior derived from calculations
    based on the laws of physics. The solar model
    reveals that hydrogen fusion occurs in a core
    that extends from the center to about a quarter
    of the Suns visible radius.

39
The Suns Interior
  • Throughout most of the Suns interior, energy
    moves outward from the core by radiative
    diffusion. In the Suns outer layers, energy is
    transported to the Suns surface by convection.
  • Neutrinos were originally believed to be
    massless. The electron neutrinos generated and
    emitted by the Sun were originally detected at a
    lower rate than is predicted by our model of
    thermonuclear fusion. The discrepancy occurred
    because electron neutrinos have mass, which
    causes many of them to change into other forms of
    neutrinos before they reach Earth. These
    alternative forms are now being detected.

40
Key Terms
hydrostatic equilibrium limb (of the Sun) limb
darkening magnetic dynamo neutrino photosphere pla
ges plasma positron prominence radiative
zone solar cycle
solar flare solar luminosity solar model solar
wind spicule sunspot sunspot maximum sunspot
minimum supergranule thermonuclear
fusion transition zone Zeeman effect
Cerenkov radiation chromosphere convective
zone core (of the Sun) corona coronal
hole coronal mass ejection differential
rotation filament granule helioseismology hydrogen
fusion
41
WHAT DID YOU THINK?
  • What percentage of the solar systems mass is in
    the Sun?
  • The Sun contains about 99.85 of the solar
    systems mass.

42
WHAT DID YOU THINK?
  • Does the Sun have a solid and liquid interior
    like Earth?
  • No. The entire Sun is composed of hot gases.

43
WHAT DID YOU THINK?
  • What is the surface of the Sun like?
  • The Sun has no solid surface. Indeed, it has no
    solids or liquids anywhere. The level we see, the
    photosphere, is composed of hot, churning gases.

44
WHAT DID YOU THINK?
  • Does the Sun rotate? If so, how fast?
  • The Suns surface rotates differentially, varying
    between once every 35 days near its poles and
    once every 25 days at its equator.

45
WHAT DID YOU THINK?
  • What makes the Sun shine?
  • Thermonuclear fusion in the Suns core is the
    source of the Suns energy.

46
WHAT DID YOU THINK?
  • Are matter and energy conserved?
  • By themselves, they are not always conserved.
    Nuclear fusion converts matter into energy.
    Energy can also be converted into matter. The
    sum of the matter (multiplied by c2) and energy
    is always conserved.
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