The Sun: A Garden Variety Star

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The Sun A Garden
-Variety Star
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The Sun

• Biggest object in the
  solar system
• diameter 1,392,000 km
• 109 x Earths diameter
• 10 x Jupiters diameter
• Most Massive
• 333,000 x Earths mass
• 1,000 x Jupiters mass
• so heavy, everything else orbits around it!
• so heavy, it makes its own heat and light
• Temperature of 15,000,000 K in its core
• nuclear power!

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The Suns Profile
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The Suns Composition

• The Sun contains the same elements as the Earth,
  but not in the same proportions
• About 73 of the Suns mass is comes from
  hydrogen, and another 25 from helium
• Other chemical elements make up the rest 2
• The fact that the Sun are mostly made up of H and
  He was first shown by Cecilia Payne-Gaposchkin
• The 1st woman to get a PhD in astronomy in the
  U.S.

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The parts of the Sun
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The Sun's Interior

• From inside out
• Core
• Radiative zone
• Convection zone
• Photosphere
• Chromosphere
• Transition region
• Corona

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The Sun's Core

• The core
• is the innermost 10 of the Sun's mass
• generates energy from nuclear fusion
• has the highest temperature and density
• temperature 10 million K
• density 160 x density of water 20 x density
  of iron
• at this temperature, the core is a gas
• no molten interior

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How does Heat from the Core Reach Us?

• Three ways to transfer heat
• Conduction direct contact
• A spoon in a hot cup of coffee gets warm
• Convection moving currents in a fluid
• Hot air rises
• Hot current in boiling water
• Radiation electromagnetic waves emitted by a
  heat source and absorbed by a cooler material
• Electric stove
• Heat from the Sun reaches us through the EM waves
  it emits

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Radiative Zone

• Radiation transfers heat from the interior of the
  Sun to its "cooler" outer layers
• The core radiation zone make up 85 of the Sun
• The temperature drops from 10 million K at the
  inner side of the radiative zone to 2 million K
  at its edge
• The energy generated in the core is carried by
  photons that bounce from particle to particle
  through the radiative zone
• The photons are too energetic to
  be absorbed by atoms
• Each photon bounces so many
  times that it is estimated
  to take
  one million years to reach the
  outer edge of the
  region

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Convection Zone

• Matter at the base of the convection zone is
  cool enough (2 million K) for the atoms to
  absorb energy and hold on to it
• Convection occurs in this region
• The hotter material near the top of the radiation
  zone (the bottom of the convection zone) rises
  while the cooler material sinks heated below
  like a pot of boiling water
• It takes a week for the hot material to carry its
  energy to the top of the convection zone

convection zone
radiative zone
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Photosphere

• This is the Suns deepest layer that one
  can see from the outside
• Photosphere means light sphere
• It is the visible surface of the Sun
• From this layer, photons can finally escape to
  space
• The surface is not something one could land or
  float on
• The photosphere is about 500 km thick
• The gas is so dense that you could not see
  through it
• The gas emits a continuous spectrum of light
• It features sunspots

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Temperature of Photosphere

• The photosphere temperature is about 5,800 K
• The sunspots appear darker because they are
  cooler than their surroundings
• The center of a typical sunspot has a temperature
  of 4,000 K
• The spectrum and energy output of the radiation
  emitted from the photosphere obey Wiens Law and
  Stefan-Boltzmann law

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Features of Photosphere

• Sunspots
• dark spots, 1500 K, cooler than surroundings
• glow by themselves

granules tops of convection cells 700 to 1000 km
diameter last 10 minutes centers 100 K hotter
than edges
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Sunspots

• Discovered by Galileo Galilei
• Sun's surface sprinkled with small dark regions -
  sunspots
• Sunspots are darker because they are cooler by
  1000 to 1500 K than the rest of the photosphere
• Spots can last a few days or as long as a few
  months 
• Galileo used the longer-lasting sunspots to map
  the rotation patterns of the Sun
• Sunspots number varies in a cycle with an average
  period of 11 years
• Cycle starts with minimum and most of them are
  at around 35 from the solar equator
• At solar maximum (number peaked), about 5.5
  years later, most of the sunspots are within
  just 5 of the solar equator

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Sunspots and Magnetic Field

• Sunspots regions of strong magnetic fields
• Found by observation of Zeeman effect

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Sun Rotates

• Galileo
• discovered sunspots
• sunspots moved ? sun rotates
• Rotation speed depends on latitude
• equator once/25 days
• 30º N once/26.5 days
• 60º N once/30 days
• Jupiter also does this

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Chromosphere

• Visible during solar eclipses as a thin pink
  layer at the edge of the dark Moon
• Colorful layer color sphere
• Color due to hydrogen bright emission line
• Also shows yellow emission due to helium
  discovered in 1868 new element previously not
  seen on Earth
• Helium was found on Earth in 1895
• The chromosphere is only 2,000 to 3,000 km thick
• Temperature rises outward away from the
  photosphere from 4,500 K to 10,000 K

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Transition Region

• Its a thin region (about 10 km thick) in the
  Suns atmosphere where temperature changes from
  10,000 K to nearly 1,000,000 K

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Solar Weather

• The Sun has complex and violent weather patterns
• Chromosphere contains jet-like spikes of gas
  called spicules
• Spicules rise vertically through the chromosphere
• Last 10 minutes
• Consist of gas jets, at 30 km/s
• Rise to heights of 5000 to 20000 km
• T chromosphere

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Corona (1)

• The outermost part of the Suns
  atmosphere is called the corona
• It is visible during total solar
  eclipses as a
  pearly-white glow
  around the dark Moon 
• The corona has a very high
  temperature of 1-2
  million K
• It is known to be very hot because it contains
  multiply ionized atoms
• At very high temperatures, atoms like iron can
  have 9 to 13 electrons ejected (the atoms become
  ionized)
• 9-times ionized iron is only produced at a
  temperature of 1.3 million K
• 13-times ionized iron means the temperature gets
  up to 2.3 million K! 

Total solar eclipse in 1973
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Corona (2)

• Most of the corona is trapped close to Sun by
  loops of magnetic field lines
• In X-rays, those regions appear bright
• Some magnetic field lines do not loop back to
  the Sun and will appear dark in X-rays
• These are called coronal holes
• More details visible at short wavelengths

A solar eclipse photographed in the extreme
ultraviolet taken by the SOHO spacecraft 
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X-rays from the Corona
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Prominences

• Bright clouds of gas forming above the sunspots
• Quiet prominences
• 40,000 km above surface
• Last days to several weeks
• Eruptive prominences
• 700 km/s
• Rare
• Surge prominences
• Last up to a few hours
• Shoot gas up to 300,000 km
• Gas speed 1300 km/s

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Prominences follow magnetic-field loops
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Solar Flares

• Solar flares are eruptions more powerful than
  surge prominences
• Flares last from a few minutes to a few hours
• A lot of ionized material is ejected in a flare
• Unlike the material in prominences, the
  solar-flare material moves with enough energy to
  escape the Sun's gravity
• When such a burst of ions reaches the Earth, it
  interferes with radio communication
• Sometimes a solar flare will cause voltage pulses
  or surges in power and telephone lines
• Brownouts or blackouts may result
• Humans traveling outside the protection of the
  Earth's magnetic field will need to have
  shielding from the powerful ions in a flare

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Solar Wind

• Fast-moving charged particles (mostly protons and
  electrons) can escape the Sun's gravitational
  attraction
• The stream of particles is called the solar wind
• They move outward at a speed of about 400 km/s
• They can reach the farthest reaches of the solar
  system
• Solar-wind particles passing close to a planet
  with a magnetic field are deflected around the
  planet
• Some are deflected to the planet's magnetic poles
• As the particles hit the planet's atmosphere,
  they cause the molecules in the atmosphere to
  produce beautiful curtains of light called the
  auroras
• Aurora borealis in the northern hemisphere
• Aurora australis in the southern hemisphere

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Is the Sun a Variable Star?

• What is more certain than that the Sun will rise
  tomorrow?
• Weve already seen that sunspots follow an
  11-year cycle
• On longer time scales, the Sun undergoes changes
  in overall activity
• Changes are only about 0.1!
• Yet this is enough to affect our climate
• In the mid 1600s the Suns output was
  particularly low ? the Little Ice Age
• Other stars are seen to vary by 0.3, up to 1