Chapter 11: Our Sun

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Chapter 11 Our Sun

• 2 Opposing forces in star
• Inward pressure - gravity
• Outward pressure due to fusion inside star
• These 2 forces balance each other
• known as gravitational or hydrostatic equilibrium

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Plunging into sun

• Total power flux Luminosity
• 3.8 x 1026 watts
• If we could capture and store all the suns
  output for 1 second it would supply energy for
  next 500,000 years
  
• Data Earth Sun difference between
• Radius 6378 Km 695,000 Km 109
• Mass 6x 1024 Kg 2x 1030 Kg 333000
• Sun 70 H, 28 He, 2 other
• rotates around every 27 days

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Inside the sun
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Inside the sun
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Inside the sun

• 1) Corona
• Extreme outer edge of sun
• May be seen during a solar eclipse

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Inside the sun

• 2) Chromosphere
• 0.7 Rsun -1.0 Rsun
• Temp 6,000 K
• 3) Convection Zone
• 0.3 Rsun 0.7 Rsun
• 10 million K
• warm gas rising to chromosphere, cool gas falling
  onto radiation zone to be heated again

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Inside the sun

• 5) Radiation zone or Solar Core
• 15 million K
• Density 100 times water
• Pressure 200 billion times that on Earth

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Nuclear Fusion

• Remember Nucleus held together by strong force
• Need High Temperatures of core to overcome this
  force
  
• Fission- splitting of Nucleus
• Nuclear Bomb
• Nuclear Power
• Fusion -2 nuclei combine
• Sun
• Hydrogen Bomb

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Hydrogen FusionProton-Proton Chain

• 1)
• d- deuterium (Hydrogen-2)
• Q- how did we get a neutron? Is charge
  balanced)
  
• positron annihilates with electron - 2 gamma
• 2)
• 3)
• 4) back to 1

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Hydrogen FusionProton-Proton Chain
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Hydrogen FusionProton-Proton Chain

• You gain energy in this process because He-3 is
  a bit less massive then 4 protons (by about
  7 )
  
• Every second the sun converts around 600 million
  tons of H into 596 million tons of He
  
• Extra 4 tons goes into energy (98 photons, 2
  neutrino)

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Solar Thermostat

• The sun is in a balancing act between
• Gravity
• Nuclear Fusion
• So if sun grows the Fusion rate decreases
• If sun shrinks fusion rate increases
• As sun burns more H, the core shrinks making it
  hotter and increasing rate of fusion
  
• See Link

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Solar Thermostat
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How do we know this much?

• Computer Models of Sun
• Good but still have unknown quantities
• Look at sun quakes
• vibrations on surface of sun
• Gives hints as to what is beneath surface
• seen by Doppler shift
• use information gain as input in models

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Solar Neutrino Problem

• Neutrinos can pass through a light year of lead
  without interacting
  
• Just because they interact rarely does not mean
  that they never interact
  
• Observations have seen that not enough neutrinos
  are coming out of the sun if the current solar
  model is correct
  
• This may be solved, recent experiment (within
  last year or so) have shown that the neutrino has
  mass. If this is true then there is no problem
  because neutrinos may change types.

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Super-Kamiokande
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Solar photons

• Most photons are created at core
• photons random walk to surface (see 14-11)
• along way exchange energy with surroundings
• causes hot plasma to rise to surface called
  convection

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Solar Flares

• solar flares fling charge out into space (100
  million Kelvin)

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Solar Promances

• Caused by changing Magnetic Field
• Dark Spots - filiments
• Regions of zero magnetic field

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Sunspots

• dark regions on sun, cooler then other regions
• (only 4000K rather then 6000 K)
• Q- what traps heat out?
• A- Huge Magnetic fields trap plasma (think of
  elastic bands forcing the ions to flow in a
  direction)
  
• Magnetic Fields generated by rotating charge

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Sunspots and other effects of Sun

• sunspot cycle- 15-7 years between solar Max and
  min
  
• also solar wind causes aurora borealis and aurora
  austalis (northern and southern lights)
  
• Earth protected from comic rays by its Magnetic
  field
  
• damage to satellites in orbit from charged
  particles
  
• Open Question- Does this have any effect on
  weather on Earth

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Blackbody Radiation

• Remember a hot object gives off photon
  proportional to its temperature
  
• see Link

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Spectra of stars

• Classified by spectral type
• O B A F G K M
• (oh be a fine girl kiss me)
• O- Blue hottest M-red coolest

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Meaning of Spectra of stars

• This classification sceme invented by Edward
  Pickering at Harvard College Obsevatory. Used
  women (radical idea at time) to observe stars.
  He called them computers.
• Wiliamina Fleming organized stars based on
  strength of Hydrogen line.
  
• Anna Cannon built on other womens observations
  and reordered into order used today.
  
• Payne Gaposhkin recognized that these differences
  were due to changes in ionization levels of
  emmitting atoms. This is proportional to the
  stellar temperature. This was her 1925 Ph.D.
  thesis.

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Binary Stars

• Most stars are in binary systems
• May use Newtons Version of Keplers third law to
  measure mass
  
• Where P Period and alength of semi-major axis
• Types of Binary Systems
• Visual
• Eclipsing
• Spectroscopic
• But still a difficult process since must measure
  the semi-major axis to find Masses
  

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Hertzsprung-Russell Diagram

• Plot of surface temperature (x-axis) of star vs.
  Luminosity (y-axis)
  
• See fig 11-13 in text p 227
• Hotter stars (O t ype) on left
• Cooler stars (M type) on right
• Most stars are on band known as Main Sequence.
• As they grow older they leave the Main Sequence
  and become red-giants
  

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Hertzsprung-Russell Diagram
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Lives of Stars

• Stars burn Hydrogen as fuel
• The more massive a star is the more blue it is
• Also the more massive a star is the shorter its
  lifetime because it must use much more fuel to
  maintain its size
  
• example our sun has a main sequence lifetime of
  10 billion year, but a star of 30 Msun only has a
  lifetime of 1/30 that of our sun
  
• As stars reach end of lifetime they leave the
  main sequence and grow in size (some as much as
  1000 x original size) (upper right)
  
• Eventually run out of fuel and die and become
  white dwarf, neutron stars (lower left)

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Cepheid Variable Stasrs

• Discovered in 1912 at Harvard by Henrietta
  Leavitt
  
• found that the brightness of these stars vary as
  a function of time
  
• Star is changing size
• There is a relation between the brightness of the
  star and its luminosity
  
• use to measure distances (more Later)

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Star Clusters

• Groups of stars
• 2 types
• open fairly modest (example Pleiades)
• closed more -densely packed
• Interesting for two reasons
• All stars in cluster are roughly same distance
  from Earth
  
• Stars formed around same time (within a few
  million years
  
• May look at stars which have left main sequence
  and use this to date the cluster
  
• Must use HR diagram