Our Star, the Sun

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Our Star, the Sun

• Chapter Eighteen

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Guiding Questions

1. What is the source of the Suns energy?
2. What is the internal structure of the Sun?
3. How can astronomers measure the properties of the
   Suns interior?
4. How can we be sure that thermonuclear reactions
   are happening in the Suns core?
5. Does the Sun have a solid surface?
6. Since the Sun is so bright, how is it possible to
   see its dim outer atmosphere?
7. Where does the solar wind come from?
8. What are sunspots? Why do they appear dark?
9. What is the connection between sunspots and the
   Suns magnetic field?
10. What causes eruptions in the Suns atmosphere?

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Source of solar energy

• The sun produces 3.9 1026 joules/sec (watts).
• The earth receives 7 1017 watts or 2 billionths
  of suns energy.
• The sun has been shining for about 4.5 billion
  years.
• Theory of fusion energy source for suns
  radiation was developed 1928 1938.
• George Gamow, Hans Bethe, F. Houtermans, R.
  Atkinson, W. Pauli, W. Heisenberg developed the
  theory.
• 1968 solar neutinos observed by John N. Bahcall
  Raymond Davis.

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Neutrino experiments

• Brookhaven National Laboratory 100, 000 gallons
  of perchloroethylene (C2Cl4) 1968.
• Neutrino strikes Chlorine nucleus, turning it
  into Argon, can be separated from fluid.
• Only 1/3 as many neutrinos as predicted.
• Late 1980s M. Koshiba
• Kamiokande experiment in Japan
• Showed neutrinos indeed came from the sun.
• 1998 Super Kamiokande experiment
• Showed there were 3 types of neutrinos, explained
  missing 2/3 of the neutrinos.
• 1987 Supernova occurs in Magellenic Cloud
• Neutrinos observed in 2 different detectors.
• Product of supernova explosion.

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The Suns energy is generated by
thermonuclearreactions in its core

• The energy released in a nuclear reaction
  corresponds to a slight reduction of mass
  according to Einsteins equation E mc2
• Thermonuclear fusion occurs only at very high
  temperatures for example, hydrogen fusion occurs
  only at temperatures in excess of about 107 K
• In the Sun, fusion occurs only in the dense, hot
  core

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The Suns energy is produced by hydrogen fusion,
a sequence of thermonuclear reactions in which
four hydrogen nuclei combine to produce a single
helium nucleus
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Start of hydrogen fusion process in the suns
interior 2 protons collide.
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Step 2 in the fusion process involves a 3rd proton
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In the final step, the end products are helium
with 2 of the original 6 hydrogen atoms recycled.
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A theoretical model of the Sun shows how
energygets from its center to its surface

• Hydrogen fusion takes place in a core extending
  from the Suns center to about 0.25 solar radius
• The core is surrounded by a radiative zone
  extending to about 0.71 solar radius
• In this zone, energy travels outward through
  radiative diffusion
• The radiative zone is surrounded by a rather
  opaque convective zone of gas at relatively low
  temperature and pressure
• In this zone, energy travels outward primarily
  through convection

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Astronomers probe the solar interior usingthe
Suns own vibrations
Energy is transported by radiative diffusion from
center to 0.71 solar radii It takes 170,000 years
to traverse through the body of the sun to
surface.

• Helioseismology is the study of how the Sun
  vibrates
• These vibrations have been used to infer
  pressures, densities, chemical compositions, and
  rotation rates within the Sun

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Note that only 0.8 of the suns volume is lt .2
solar radii
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Internal solar densities and temperature, note
water is 1000 kg/m3
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Neutrinos reveal information about the
Sunscoreand have surprises of their own

• Neutrinos emitted in thermonuclear reactions in
  the Suns core have been detected, but in smaller
  numbers than expected
• Recent neutrino experiments explain why this is
  so

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The photosphere is the lowest of three main
layersin the Suns atmosphere

• The Suns 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 solar atmosphere

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Limb darkening because base of photosphere is
hotter than higher layers seen near solar limb.
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Convection in the photosphere produces granules
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The chromosphere is characterized by spikesof
rising gas

• 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

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Chromospheric Spectrum

• Chromospheric emission spectrum
• Emission lines with some matching wavelengths of
  photospheric absorption lines
• Bright yellow line produced by helium (He)
• Chromospheric temperature up to 30,000 K at
  highest level
• Gas density is lower than photosphere
• From this, one concludes that temperature must
  rise rapidly up through chromosphere

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• The outermost layer of the solar atmosphere, the
  corona, is made of very high-temperature gases at
  extremely low density
• The solar corona blends into the solar wind at
  great distances from the Sun

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The corona ejects mass into space to form the
solar wind
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Activity in the corona includes coronal mass
ejections and coronal holesUltraviolet image
taken from SOHO spacecraft.
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Sunspots are low-temperature regions inthe
photosphere
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Temperature in Umbra abt 4400 K, Penumbra abt
5000 K, 30 of light
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Tracking the suns rotation with sunspots 25 ¼
days at equator, 28.2 days at latitude 45, 34
days nearer the poles.
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Chromospheric Flares

• Flares - brief burst of X-rays and particle
• Observed in monochromatic light
• Lifetimes of about 20 minutes
• Size about 30,000 km
• Enhances particle density in solar wind and solar
  cosmic rays

Solar Flare Movie
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Bright areas or Plages form just before
appearance of sunspots Filaments appear as
prominences above solar limb
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Bright areas are called Plages,
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Sunspots are produced by a 22-year cyclein the
Suns magnetic field
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• The Suns 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

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Variations in solar activity

• 1610 Galileo observed sunspots
• From 1645 to 1715 very few sunspots were observed
• Historical records, Flamsteed in 1674 said that
  1st since 1664.
• Little Ice Age 1300 to 1850
• Glaciers advanced in Alps severe winters
• Greenland colony fails
• 1816, year without a summer follows Mt Tambora
  eruption
• Weather unstable unpredictable.
• Astronomical evidence that sunspot minima occurs
  about 20 of the time in a star like the sun
• 1958, Sunspot activity was the largest ever
  observed.
• Since 1900, sun is getting hotter, about 1/3 of
  global warming.

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• The magnetic-dynamo model suggests that many
  features of the solar cycle are due to changes in
  the Suns magnetic field

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These changes are caused by convection and the
Suns differential rotation
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Rotation of the Solar Interior center of sun
rotates uniformly
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The Suns magnetic field also produces
otherforms of solar activity

• A solar flare is a brief eruption of hot, ionized
  gases from a sunspot group
• A coronal mass ejection is a much larger eruption
  that involves immense amounts of gas from the
  corona

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Coronal Prominences

• Prominences - Chromospheric material extending
  upward into corona
• Seen against photospheric or chromospheric disk
  known as filaments
• Properties
• Much cooler than surrounding corona
• Sizes, if quiescent, height 30,000 km, length
  200,000 km, thickness 5000 km
• Exhibit motions associated with magnetic fields
  up to several hundred gauss
• Lifetimes up to 90 days

Prominence
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Soft X-RayJuly 7, 1998

• Holes - lower temperature and much lower density
  regions
• Sizes up to hundreds of thousands of km
• Magnetic field lines open out to interplanetary
  space
• Source of solar wind particles
• Changeable in periods of days to weeks
• Active regions - relatively hot and dense regions
  consisting of magnetic loop structures
• Sizes up to hundreds of thousands of km
• Magnetic field lines form large loop structures
• Occur over chromospheric plages
• Quiet regions - between coronal holes and coronal
  active regions
• Magnetic fields weak and roughly in loop
  structures

Pole
Coronal Quiet Region
Equator
Coronal Hole
Coronal Active Region
Pole
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Coronal Mass Ejection
Coronal mass ejections send bursts of energetic
charged particles out through the solar system.
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Coronal Mass Ejections

• Shows relatively quiet corona
• Black disk blocks photospheric and chromospheric
  radiation
• 16 minutes later, huge balloon-shaped volume of
  high-energy gas is ejected from corona
• Ejected material expands at typical velocities of
  400 km/s
• Ejection lasts several hours and contains
  trillions of tons of matter
• Often associated with solar flares, but not always

A.
B.
C.
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Coronal mass ejection of 1012 kg mass ( a billion
tons)
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Solar events also peak at 11 yr cycle

• Numbers of plages, filaments, solar flares and
  coronal mass ejects also follow the same 11-yr
  cycle as sunspots.
• Coronal mass ejections (CMJ) can occur at any
  point in the cycle. Flares are also
  unpredictable.
• Solar flares and CMJs produce showers of charged
  particles (electrons and protons).
• Can disrupt electrical grids, radio and TV
  transmissions.
• Dangerous for astronauts, satellites
• Can produce brilliant auroras, even at low
  latitudes.

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Space Weather

• Space weather study of variable emission of
  high-energy photons, particles, and magnetic
  fields and their interaction with the geosphere
• Earth influences
• Van Allen radiation belts
• Spacecraft and crews
• High-altitude aircraft
• Electric power grid
• Communications, land and satellite
• Major source of natural variability in
  terrestrial climate

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The Big Picture

• The Sun continues to shine, while it radiates
  away as its luminosity, by generating energy by
  thermonuclear fusion of hydrogen into helium.
• Gravitational and thermal equilibrium determine
  the Suns internal structure and its rate of
  energy generation.
• The Suns atmosphere displays its own version of
  weather and climate, governed by solar magnetic
  fields. Solar weather has important influences
  on the Earth.
• The Sun is important not only as our source of
  light and heat, but also because it is the only
  star near enough for us to study in great detail.
  In the coming chapters, we will use what weve
  learned about the Sun to help us understand other
  stars.

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Key Words

• 22-year solar cycle
• chromosphere
• CNO cycle
• conduction
• convection
• convective zone
• corona
• coronal hole
• coronal mass ejection
• differential rotation
• filament
• granulation
• granule
• helioseismology

• hydrogen fusion
• hydrostatic equilibrium
• limb darkening
• luminosity (of the Sun)
• magnetic-dynamo model
• magnetogram
• magnetic reconnection
• negative hydrogen ion
• neutrino
• neutrino oscillation
• photosphere
• plage
• plasma
• positron
• prominence