Core:  Central energy producing region
  Thermonuclear fusion occurs here. Energy
  transport by radiation.
  Temperature: ~16 x 106 K
  Density: ~160 g/cm3

  Intermediate Interior: large region of
  interior photons scattered on the way to the solar
  Temperature: ~8 x 106 K
  Density: ~20 g/cm3

Convective Zone: Transition region between interior
  and surface. Energy transport by convection.
  Temperature: ~5 x 105 K
  Density: ~0.01 g/cm3
  Photosphere: Visible surface of the Sun.
  Convective "bubbles" from below visible
  Granulation. Location of Sunspots.
  Temperature: ~5.8 x 103 K
  Density: ~4 x 10-7 g/cm3

Chromosphere: Beginning of solar "atmosphere".
  Location of prominences and solar flares.
  Temperature: ~5 x 104 K
  Density: ~8 x 10-8 g/cm3
  Transition Region: Region between corona and chromosphere.
  Temperature changes from ~50,000 K in chromosphere to ~2 x 106 K in corona over
  a few tens of kilometers!!!!
  Corona: Hot, thin, large solar "atmosphere". Coronal holes are source of the solar
  Temperature: ~2 x 106 K
  Density: ~1 x 10-14 g/cm3

Granulation:   Provide evidence of convection below solar surface .

                                              Diagram of convection.


       Regions of intense magnetic fields.
       Temperature ~ 4200 K cooler than photosphere, which makes them appear as dark spots.
       Can be used to measure solar rotation (25 days at equator, 31 days at poles).

  Chromosphere contains many dark, brush-like spikes that protrude upward.  These are called spicules.They are "jets" of gas surging away from the sun at 20 km/s.
They attain heights of ~7000 km .


  Prominences/Solar Flares:
  Huge, arching columns of gas often appearing above sunspots. Prominences are regions along a magnetic field line where conditions are right for light to be emitted.


Occasionally, kinks and stresses occur on magnetic field lines discharging amounts  of energy (knownas Solar Flares). The amount of energy released is equivalent to a 2 billion megaton bomb.
Flares release large numbers of particles into the corona.


 Energy Transport:
  There are three ways in which energy can be transported from one place to another:
     1.Radiative Transport
       Energy is transported by photons.
       Energy loss by scattering and absorption by atoms.
       Very efficient mechanism in stars (also efficient on cloudless evenings very chilly nights).
     2.Convective Transport
       Energy transported by bulk motion of mass.
       Important mechanism in stars.
       Very poorly understood process.


    3.Conductive Transport
       Energy transport by atoms colliding with one another, gaining and releasing energy.
       Efficient mechanism in solids, but not in stars.

  Examples of Energies:
  Energy to lift a sheet of paper 1 cm: ~ 1 erg
  Chemical energy in a barrel of oil: ~ 1017 ergs
  Nuclear energy in a gram of water: ~ 7 x 1019 ergs
  Total energy emitted by Sun in its lifetime: ~ 1050 ergs
  Total energy emitted by single supernova: ~ 1051 ergs

  There are 3 types of energy of concern to us here:
     1.Gravitational Energy:
  Energy resulting from the motion of matter when interacting with other matter.
     2.Chemical Energy:
  Energy resulting from, a change in the electron structure of atoms when bonding with other atoms. "Burning".
    3.Nuclear Energy:
  Energy resulting from changes in the nuclear structure of atoms.

  Gravitational Energy As A Source Of Solar Energy:


  Total gravitational energy ÷ Luminosity = How long sun could be powered.
  (1 × 1048 ergs) ÷ (4 × 1033 ergs) = Could power the sun for about 100 million years.
  Chemical Burning As A Source Of Solar Energy:
  Total chemical energy ÷ Luminosity = How long sun could be powered.
  (1 × 1046 ergs) ÷ (4 × 1033 ergs) = Could power the sun for about 10,000 years.
  Until the late 1800s, gravitational energy was thought to be sufficient, since it was not thought that the Universe was that old.

  However, for two reasons it was later thought that something more than gravitational energy powered the sun:
Darwin (1809-1882): Theories of evolution required much longer than 108 years for evolution to
proceed.Geological dating of rocks indicated that the Earth was several x 109 years old.

  So What Powers The Sun?
  Einstein, getting energy from hydrogen fusion, came up with the following equation that explains it all:

 E = mc2
 E = Energy,    m = mass,   c = speed of light

Fundamental Structure Of The Atom:

    The Nucleons:
     P+   Proton  (charge: +,  mass: 1.67 × 10-24 g)
    n    Neutron (charge: Ø, mass: 1.67 × 10-24 g)
    The Electron:
    e-   Electron (charge: -, mass: 9 × 10-28 g)
  The Proton-Proton Chain:

  Nuclear Fusion:
  High Temperatures Are Needed For Fusion. Why? Recall that like charge repel.

High temperatures (energy) are required in order to overcome this repulsion.Actual details require descriptions provided by Quantum Mechanics.

  Temperature required for the p-p chain:    T ~ 107 K

Temperature is found in the Sun's core, where density is ~ 160 g/cm3 .
       Even at this density, reaction probability is very low.
       Single proton takes ~ 7 x 109 years before reacting.
       Large number of protons make up for low probability.

  Hydrostatic Equilibrium:
   Why doesn't the Sun collapse under its own weight, or blow it
              self apart because of nuclear

Core nuclear processes produce outward pressure.
           Gravitational forces produce inward pressure.
          When the two pressures are equal (but opposite
         directions) the star (Sun) is stable against collapse -
                  Hydrostatic Equilibrium.

  Energy Output Of The Sun:
 The total amount of energy, in a given time, over a given area above the Earth's atmosphere,
 that reaches us from the Sun is called: Solar Constant.
       Numerically it is ~ 2 calories/cm2/minute.
       Models suggest that a 1% change will result in a 1º-2º change in Earth's temperature.
       During the last ice age (~20,000 years ago), Earth was ~5º cooler than today.
       A 10% (~10º) change would result in an ice covered Earth.
   Is The Solar Constant, Constant?
  The Sunspot Cycle:
  The number of sunspots observed on the Sun varies in an 11 year cycle called the Sunspot Cycle.

  The 11 year cycle is actually part of a 22 year magnetic sunspot cycle.

     1.Differential rotation winds up the magnetic field.
     2.Field becomes tangled and breaks through the surface sunspot
     3.After 11 years, field decays.
     4.Differential rotation winds up the magnetic field.
     5.Newly organized field is reversed.

  Magnetic Field Lines:

     1.Magnetic field: term that describes regions of space modified by
       moving electrical charges.
     2.Magnetic fields around planets/Sun resemble those around bar
     3.Lines indicating direction of the magnetic field are called magnetic
       field lines.
     4.Planetary and solar magnetic fields are thought to arise in a fluid,
       conducting interior.
     5.Magnetic fields arise from circulating electrical currents generated by
       the object's rotation.

  The Sunspot Cycles:
  One theory that attempts to explain the cycle is the
  Dynamo Theory.

       Differential rotation and convection twist interior magnetic fields and
       bring them to the surface.
       Magnetic fields appear as sunspot pairs.
       Magnetic fields decay and switch polarity.
       Charged particles move along magnetic field lines - they cannot
       cross them.

  Coronal Holes:

Coronal holes are regions where magnetic field lines are open, that is, they do not return to Sun's surface. Charged particles can, therefore, escape the Sun in the form of Solar Wind.

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